LARRY W. ESPOSITO

PUBLICATIONS LIST BY YEAR WITH ABSTRACTSUpdated May 8, 2023

Esposito, L. W., and E. R. Harrison. 1975. Properties of the Hulse-Taylor binary pulsar system. Astrophys. J. (Letters) 196, Ll–L2. LASP reprint 901.

Abstract: Various elementary calculations are summarized concerning the close-binary-system pulsar PSR 1913+16, discovered by Hulse and Taylor. The results are approximate and indicate only the relative importance of various relativistic and nonrelativistic effects. 1975 American Astronomical Society.

Esposito, L. W. 1976. Computation of the polarization of light scattered by the solar corona in Balmer Alpha, in Fellowship Program in Scientific Computing; Internship Program for Minority Students, ed. J. C. Adams and R. K. Rew, pp. 49–59. National Center for Atmospheric Research Technical Note. NCAR/TN/114 + PROC. LASP reprint 902.

Abstract: A discussion is developed of a computer program that calculates the polarization in hydrogen alpha from light scattered at a given point in the solar corona. The program allows for the integration of the scattering function over the solar disk, but does not consider radiative equilibrium or multiple scattering, nor the integration of the scattered light along the line of sight through the corona. The statistical equilibrium among the magnetic levels of hydrogen is calculated by the formalism of a Markov chain. Some sample results are given for various altitudes above the solar limb.

Esposito, L. W. 1977. The two-stream approximation (or Shuster-Schwarzschild method), in Standard Procedures to Compute Atmospheric Radiative Transfer in a Scattering Atmosphere, ed. J. Lenoble, pp. 52–54. Boulder, Colorado: NCAR, Radiation Commission of the International Association of Meteorology and Atmospheric Physics (IAMAP). LASP reprint 903.

Abstract: The two-stream approximation has been used by many authors to achieve a quick approximate solution to the equation of transfer by decomposing the radiation field into two opposing streams. A comparison of various methods is presented.

Esposito, L. W., and K. Lumme. 1977. The tilt effect for Saturn's rings. Icarus 31, 157–167. LASP reprint 407.

Abstract: Multiple-scattering computations are carried out to explain the variation of the observed brightness of the A and B rings of Saturn with declination of the Earth and Sun. These computations are performed by a doubling scheme for a homogeneous plane-parallel scattering medium. We test a range of choices for the phase function, albedo for single scattering, and optical depth of both the rings. Isotropic scattering and several other simple phase functions are ruled out, and we find that the phase function must be moderately peaked in both the forward and backward directions. The tilt effect can be explained by multiple scattering in a homogeneous layer, but, for ring B, this requires a single-scattering albedo in excess of 0.8. The brightest part of ring B must have an optical depth greater than 0.9. We find that the tilt effect for ring A can be reproduced by particles having the same properties as those in ring B, with the optical depth for the A ring in the range 0.4 to 0.6. 1977 Academic Press, Inc.

Lumme, K., L. W. Esposito, W. M. Irvine, and W. A. Baum. 1977. Azimuthal brightness variations of Saturn's rings, II: Observations at an intermediate tilt angle. Astrophys. J. (Letters) 216, L123–L126. LASP reprint 904.

Abstract: The brightness variation in Saturn’s ring A with orbital phase of the ring particles increases in amplitude as the declination of the Earth B decreases from 26 to 16. The amplitude of this azimuthal effect also appears to diminish at opposition. There is an indication that the effect decreases with decreasing wavelength, and hence with decreasing particle albedo. 1977 American Astronomical Society.

Esposito, L. W. 1978. Light scattering from Saturn's rings calculated by a Markov chain formalism, Ph.D. diss., Astronomy Department, University of Massachusetts, Amherst. Contribution from the Five College Observatories, Number 261.

Abstract: The theory of Markov chains is used to formulate the radiative transfer problem in a general way by modeling the successive interactions of a photon as a stochastic process. Under the minimal requirement that the stochastic process is a Markov chain, the determination of the diffuse reflection or transmission from a scattering atmosphere is equivalent to the solution of a system of linear equations. This treatment is mathematically equivalent to, and thus has many of the advantages of Monte Carlo methods, but may be considerably more rapid than Monte Carlo algorithms for numerical calculations in particular applications.The speed and accuracy of this formalism have been verified for the standard problem of finding the intensity of scattered light from a homogeneous plane-parallel atmosphere with an arbitrary phase function for scattering. Accurate results over a wide range of parameters were obtained with computation times comparable to those of a standard “doubling” routine. The generality of this formalism may thus allow fast, direct solutions to problems previously soluble only by Monte Carlo methods.

Esposito, L. W., and L. L. House. 1978. Radiative transfer calculated by a Markov chain formalism. Astrophys. J. 219, 1058–1067. LASP reprint 905.

Abstract: The theory of Markov chains is used to formulate the radiative transport problem in a general way by modeling the successive interactions of a photon as a stochastic process. Under the minimal requirement that the stochastic process is a Markov chain, the determination of the diffuse reflection or transmission from a scattering atmosphere is equivalent to the solution of a system of linear equations. This treatment is mathematically equivalent to, and thus has many of the advantages of, Monte Carlo methods, but can be considerably more rapid than Monte Carlo algorithms for numerical calculations in particular applications. We have verified the speed and accuracy of this formalism for the standard problem of finding the intensity of scattered light from a homogeneous plane-parallel atmosphere with an arbitrary phase function for scattering. Accurate results over a wide range of parameters were obtained with computation times comparable to those of a standard “doubling” routine. The generality of this formalism thus allows fast, direct solutions to problems that were previously soluble only by Monte Carlo methods. Some comparisons are made with respect to integral equation methods. 1978, American Astronomical Society.

Esposito, L. W., W. M. Irvine, K. Lumme, and W. A. Baum. 1978. Azimuthal brightness variations of Saturn's rings, in Proceedings of the Symposium on Planetary Atmospheres, ed. A. Vallance-Jones, pp. 89–91. Ottawa, Ontario: National Research Council of Canada.LASP reprint 906.

Abstract: It is now well established that near maximum inclination and in the visual band, Saturn’s A ring exhibits brightness variations with orbital phase of the ring particles. New constraints on possible models to explain this behavior are provided by the variation in amplitude of this effect with changing declination B of the Earth with respect to the ring plane, by the variation of the effect with solar phase angle, and by its dependence on wavelength. In particular, the amplitude of the azimuthal effect increases as B decreases from 26 to 16; the effect diminishes at opposition and decreases with decreasing wavelength (and hence with decreasing particle albedo).

Harding, A. K., E. Tademaru, and L. W. Esposito. 1978. A curvature-radiation-pair-production model for gamma-ray pulsars. Astrophys. J. 225, 226–236. LASP reprint 907.

Abstract: Assuming that pulsar -rays are produced via curvature radiation of primary electrons near the neutron star and that they are attenuated only through the pair production process in strong electric and magnetic fields, we use a detailed model of the magnetosphere to calculate the resulting optical depths and pulse shapes. We consider both the effect of E B = 0 and E B 0 on the attenuation of the photons, and the effect of photon energy, rotation rate, and obliquity on the pulse shapes. It is found that pulsar rotation tends to increase pair production, causing large optical depths for the shortest period pulsars. The calculations allso predict high-energy cutoffs and spectra for the observable -ray emission. 1978 American Astronomical Society.

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Esposito, L. W. 1979. An “adding” algorithm for the Markov chain formalism for radiation transfer. Astrophys. J. 233, 661–663. LASP reprint 69.

Abstract: The Markov chain radiative transfer method of Esposito and House (1978, Ap. J. 219, 1058, Paper I) has been shown to be both efficient and accurate for calculation of the diffuse reflection from a homogeneous scattering planetary atmosphere. The use of a new algorithm similar to the “adding” formula of Hansen and Travis (1974, Space Sci. Rev. 16, 527) extends the application of this formalism to an arbitrarily deep atmosphere. The basic idea for this algorithm is to consider a preceding calculation as a single state of a new Markov chain. Successive application of this procedure makes calculation possible for any optical depth without increasing the size of the linear system used. The time required for the algorithm is comparable to that for a doubling calculation for a homogeneous atmosphere, but for a non-homogeneous atmosphere, the new method is considerably faster than the standard adding routine. As with the standard adding method, the information on the internal radiation field is lost during the calculation. This method retains the advantage of the earlier Markov chain method that the time required is relatively insensitive to the number of illumination angles or observation angles for which the diffuse reflection is calculated. A technical write-up giving fuller details of the algorithm and a sample code are available from the author. 1979 American Astronomical Society.

Esposito, L. W. 1979. Extensions to the classical calculation of the effect of mutual shadowing in diffuse reflection. Icarus 39, 69-80. LASP reprint 71.

Abstract: The classical method for accounting for the mutual shadowing among closely packed particles in multiple scattering calculations is extended in the following ways: (1) By modeling the particle distribution by a Poisson process with a varying density parameter, a “Van der Waals” type approximation allows extension to a greater fractional volume density, D. In this case, it is only required that D2 <<1 instead of D<<1. (2) In the case that the particle distribution is not uniform, the classical calculation may be weighted by the pair correlation function of the distribution. (3) The use of the Markov chain formalism for radiative transfer allows inclusion of the effect of shadowing for two orders of scattering. For conditions such as might apply in Saturn’s rings, the inclusion of this effect makes less than 0.1% difference in the calculated phase curves compared to previous calculations that have included shadowing only in the first scattering. The latter are thus shown to be quite accurate. 1979 Academic Press, Inc.

Esposito, L. W., K. Lumme, W. D. Benton, L. J. Martin, H. M. Ferguson, D. T. Thompson, and S. E. Jones. 1979. International planetary patrol observations of Saturn's rings, II: Four color phase curves and their analysis. Astron. J. 84, 1408–1415. LASP reprint 68.

Abstract: New phase curves for Saturn’s rings at an intermediate tilt angle B ~ 17 are presented. Quantitative results for each of the A and B rings are reported in terms of the opposition effect, phase coefficient, and best logarithmic fit to the phase curve. In each of the four colors, there is no significant difference between the shape of the phase curves for the two rings, as was also found in B and V by Franklin and Cook (1965, Astron. J. 70, 704). The shape of the green phase curves is well determined and consistent with earlier observations taken at a maximum tilt angle. A simple, four-parameter multiple scattering model of the rings (including the effect of mutual shadowing) is shown to be consistent with the observations. The derived parameters are also consistent with the detailed analyses of the phase curves at maximum tilt angle (Kawata and Irvine, 1975, Icarus 24, 472) and of the variation of brightness of the rings as a function of tilt angle (Esposito and Lumme, 1977, Icarus 31, 157). In this multiple scattering model, the difference in the phase curves for different colors can be explained by a variation in the single scattering albedo with wavelength. The observations allow the particles to have the same composition in both the A and B rings, so that their different photometric behavior is explained by differences in optical depth and volume density in the two rings. In conjunction with earlier studies, these results provide strong support for the classical model of the rings as a layer many particles thick. 1979 American Astronomical Society.

Esposito, L. W., J. R. Winick, and A. I. Stewart. 1979. Sulfur dioxide in the Venus atmosphere: Distribution and implications. Geophys. Res. Lett. 6, 601–604. LASP reprint 70.

Abstract: The Pioneer Venus orbiter ultraviolet spectrometer sees variable disk brightness features similar to the well-known “UV markings” seen at longer wavelengths. The bright features are consistent with a homogeneous cloud of H2SO4 aerosols. The darker features show the presence of a broad-band absorber, which is at some depth in the cloud layer. Additional contrast arises from SO2 absorption. The observed strength of the SO2 absorption as a function of wavelength rules out a uniform mixing ratio for the SO2. The

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data are well fitted by an inhomogeneous light scattering model in which the SO2 scale height is one-fifth of the CO2 scale height, and the mixing ratio of SO2 at 40 mb is 1 x 10-7. A model of the oxidation of sulfur dioxide in the upper cloud reproduces the observed vertical distribution of SO2 and indicates that SO2 alone is sufficient to produce the observed amount of H2SO4 in this region. 1979 American Geophysical Union.

Lumme, K., L. W. Esposito, W. D. Benton, and W. A. Baum. 1979. International planetary patrol observations of Saturn's rings, I: Observations and data reduction. Astron. J. 84, 1402–1407.LASP reprint 908.

Abstract: The following is a description of the reduction procedure used for observations of Saturn’s rings taken with the International Planetary Patrol Network in 1977. Corrections for atmospheric smearing are determined using an assumed ring model and performed by a two-dimensional Wiener filter developed at JPL’s Image Processing Laboratory. These smearing corrections are shown to be consistent with the radial profile of the rings, not just on the major axis of the ring system, but over a range of azimuthal angles. 1979 American Astronomical Society.

Pollack, J. B., B. Ragent, R. Boese, M. G. Tomasko, J. Blamont, R. G. Knollenberg, L. W. Esposito, A. I. Stewart, and L. Travis. 1979. Nature of the Ultraviolet absorber in the Venus clouds: Inferences based on Pioneer Venus data. Science 205, 76–79. LASP reprint 909.

Abstract: Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small (~1 micrometer) or large (10 micrometer) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising. 1979 American Association for the Advancement of Science.

Stewart, A. I., D. E. Anderson, L. W. Esposito, and C. A. Barth. 1979. Ultraviolet spectroscopy of Venus: Initial results from the Pioneer Venus orbiter. Science 203, 777-779. LASP reprint 336.

Abstract: Ultraviolet spectroscopy of the Venus cloud tops reveals absorption features attributed to sulfur dioxide in the atmosphere above the cloud tops. Measurements of scattered sunlight at 2663 angstroms show evidence for horizontal and vertical inhomogeneities in cloud structure. Images of the planet at SO2 absorption wavelengths show albedo features similar to those seen at 3650 angstroms from Mariner 10. Airglow emissions are consistent with an exospheric temperature of ~275 K, and a night airglow emission has been detected, indicating the precipitation of energy into the dark thermosphere. 1979 American Association for the Advancement of Science.

Esposito, L. W. 1980. Ultraviolet contrasts and the absorbers near the Venus cloud tops. J. Geophys. Res. 85, 8151–8157. LASP reprint 324.

Abstract: The absorbers in the Venus atmosphere are not merely markers for the atmospheric motions; their absorption of incident sunlight determines the deposition of the energy which drives these motions. Measurements of the ultraviolet contrasts constrain the location, altitude, and identity of these absorbers near the Venus cloud tops. Spin-scan images from the Pioneer Venus orbiter UV spectrometer (UVS) and the cloud photopolarimeter (CPP) provide a set of planetary contrast measurements in the wavelength range from 1990 Å to 3650 Å and phase angles from 33 to 130. The planet is darkest at the point where the UVS line of sight penetrates perpendicular to the cloud tops: thus the absorbing material responsible must be deep in the atmosphere. Sulfur dioxide absorption can explain the amount of contrast seen between 2000 Å and 3200 Å. At the longer wavelengths (where the SO2 cross section has dropped by 2 orders of magnitude), the persistence of contrast requires another absorber which is deeper in the atmosphere than the SO2 (at about the 75-mbar level). This additional absorption is strongly (but not identically) associated with the location of the SO2; we find the short and long wavelength images are remarkably similar. Part of the observed contrast is due to the high-lying haze discovered from Pioneer Venus polarimetry, most importantly at the shortest wavelengths where CO2 is strongly absorbing. The correlation between planetary contrast and polarization

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does not require large-scale clearing or major vertical motions of the cloud tops as the sole cause of the observed contrast. However a scheme in which absorbers subject to photochemical destruction are mixed upward into the cloud-top region provides a consistent explanation for the origin of these markings. 1980 American Geophysical Union.

Esposito, L. W., J. P. Dilley, and J. W. Fountain. 1980. Photometry and polarimetry of Saturn's rings from Pioneer Saturn. J. Geophys. Res. 85, 5948–5956. LASP reprint 72.

Abstract: We present a profile of the average normal optical depth for Saturn’s rings between 1.22 and 2.35 RS. In the A and B rings, horizontal inhomogeneities make these values deceptive. A thinner component of the B ring with optical depth less than 0.08 covers 1/2% to 4% of its surface area. In the A ring, the more transparent component has optical depth S > 0.10 and covers more than 7% of its area. These thinner parts of the rings would rarely be apparent from earth-based observations. The particles in the C ring are larger than 15 and different from those in the B and A rings. The C ring is either homogeneous with high albedo (in the red) and forward scattering phase functions or shows a gradient in albedo with distance from Saturn. In the latter case, the red albedo ranges from at the inner edge of ring C to at the outer edge, and the phase function is both moderately forward and backward scattering. Polarimetry of Saturn’s rings provides only an upper limit for the B ring polarization, p < 15%, which is consistent with predictions from ground-based data. In the outer A ring, the polarization is negative p (blue) = –7 2%; p (red) = –1 2%. Azimuthal variations are seen on the darkside of the A ring with a peak-to-trough ratio of 1.25. The position angle of the maximum is close to that seen from earth. No variations are seen in the B and C rings with an upper limit of 6%. The measured color of the A ring supports the many-particle-thick model of Saturn’s rings. 1980 American Geophysical Union.

Gehrels, T., L. R. Baker, E. Beshore, C. Blenman, J. J. Burke, N. D. Castillo, B. DaCosta, J. Degewij, L. R. Doose, J. W. Fountain, J. Gotobed, C. E. KenKnight, R. Kingston, G. McLaughlin, R. McMillan, R. Murphy, P. H. Smith, C. P. Stoll, R. N. Strickland, M. G. Tomasko, M. P. Wijesinghe, D. L. Coffeen, and L. W. Esposito. 1980. Imaging photopolarimeter on Pioneer Saturn. Science 207, 434–439. LASP reprint 910.

Abstract: An imaging photopolarimeter aboard Pioneer 11, including a 2.5-centimeter telescope, was used for 2 weeks continuously in August and September 1979 for imaging, photometry, and polarimetry observations of Saturn, its rings, and Titan. A new ring of optical depth < 2 x 10 -3 was discovered at 2.33 Saturn radii and is provisionally named the F ring; it is separated from the A ring by the provisionally named Pioneer division. A division between the B and C rings, a gap near the center of the Cassini division, and detail in the A, B, and C rings have been seen; the nomenclature of divisions and gaps is redefined. The width of the Encke gap is 876 ± 35 kilometers. The intensity profile and colors are given for the light transmitted by the rings. A mean particle size 15 meters is indicated; this estimate is model-dependent. The D ring was not seen in any viewing geometry and its existence is doubtful. A satellite, 1979 S 1, was found at 2.53 ± 0.01 Saturn radii; the same object was observed ~ 16 hours later by other experiments on Pioneer 11. The equatorial radius of Saturn is 60,000 ± 500 kilometers, and the ratio of the polar to the equatorial radius is 0.912 ± 0.006. A sample of polarimetric data is compared with models of the vertical structure of Saturn’s atmosphere. The variation of the polarization from the center of the disk to the limb in blue light at 88º phase indicates that the density of cloud particles decreases as a function of altitude with a scale height about one-fourth that of the gas. The pressure level at which an optical depth of 1 is reached in the clouds depends on the single-scattering polarizing properties of the clouds; a value similar to that found for the Jovian clouds yields an optical depth of 1 at about 750 millibars.  1980 American Association for the Advancement of Science.

Knollenberg, R., L. Travis, M. Tomasko, P. Smith, B. Ragent, L. W. Esposito, D. McCleese, J. Martonchik, and R. Beer. 1980. The clouds of Venus: A synthesis report. J. Geophys. Res. 85, 8059–8081. LASP reprint 911.

Abstract: The results presented represent a synthesis of data from those Pioneer Venus experiments directed toward studying cloud problems. These orbiter and multiprobe experiments show the cloud system to consist of three altitude regions populated by cloud particles and smaller haze particles which extend above and below as well as coexist with the cloud particles. The optical properties derived are only consistent with the largest particles, having platelike morphology. The smallest particles are shown to require changes in

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chemical composition to explain observed behavior. The medium-sized H2So4 droplets of 2 diameter appear to be the least volatile and are the best understood. The role of the cloud particles in precipitation dynamical processes, lightning, and radiation are all discussed. 1980 American Geophysical Union.

Pollack, J. B., O. B. Toon, R. C. Whitten, R. Boese, B. Ragent, M. Tomasko, L. W. Esposito, L. Travis, and D. Wiedman. 1980. Distribution and source of the UV absorption in Venus' atmosphere. J. Geophys. Res. 85, No. A13, 8141-8150. LASP reprint 912.

Abstract: Information about the nature of the UV absorbing agents in Venus’ atmosphere has been obtained from a comparison of model predictions with a variety of photometric data obtained from the Pioneer Venus probes and orbiter as well as from the earth. The theoretical simulations were performed with numerically accurate radiative transfer codes that incorporated spacecraft constraints on the properties of aerosols and gases in Venus’ atmosphere. We find that at least two UV absorbers are needed: gaseous SO2, which is a major source of opacity at wavelengths shortward of 0.32 m, and a second absorber, which dominates above 0.32 m. The concentration of the second absorber increases sharply with increasing optical depth within the first few optical depths and then has a relatively flat profile throughout most of the remainder of the upper main cloud layer, but it is largely absent from the middle and lower cloud regions. Several mechanisms appear to be responsible for producing UV contrast features. These include changes in the optical depth of the upper haze layer, which leads to brightness differences between equatorial and polar regions, and changes in the depth over which the second UV absorber is depleted in the uppermost part of the main clouds, which creates UV features at equatorial latitudes. Atmospheric dynamics act as a driving force for the above variations in the radiative properties of the clouds through dynamically induced temperature changes and changes in vertical mixing. Many UV-absorbing materials, including sulfur, various iron compounds, and nitrogen dioxide gas have spectral absorption properties that are in serious disagreement with those deduced for the second UV absorber. One possibly tenable candidate for the second absorber is gaseous Cl2, whose spectral absorption characteristics closely match the empirically derived ones and whose expected altitude profile is crudely consistent with the sharp decreases in the concentration of the second absorber near the cloud tops and bottom of the upper clouds. Approximately 1 ppm of Cl2 is required. Whether this amount of Cl2 can be generated from the photodissociation of HCl is not clear, with the peak Cl2 mixing ratio being strongly dependent on the exact amount of HCl and other gases that are present in the upper clouds. 1980 American Geophysical Union.

Esposito, L. W. 1981. Absorbers seen near the Venus cloud tops from Pioneer Venus. Adv. Space Res. 1, 163–166. LASP reprint 340.

Abstract: Spin-scan images from the Pioneer Venus orbiter UV spectrometer and the cloud photopolarimeter provide a set of planetary contrast measurements in the wavelength range 1990 Å to 3650 Å and phase angles from 33º to 130º. The planet is darkest at the point where the UVS line of sight penetrates perpendicular to the cloud tops: thus the absorbing material responsible must be deep in the atmosphere. Sulfur dioxide absorption can explain the amount of contrast seen between 2000 Å and 3200 Å. At the longer wavelengths, the persistence of contrast requires another absorber which is deeper in the atmosphere and strongly associated with the location of the SO2. Part of the observed contrast is due to the high-lying haze discovered from Pioneer Venus polarimetry. The correlation between planetary contrast and polarization does not support large-scale clearing or major vertical motions of the cloud tops as the sole cause of the observed contrast. However, a scheme in which absorbers subject to photochemical destruction are mixed upward into the cloud top region provides a consistent explanation for the origin of these markings. 1981 COSPAR.

Gehrels, T., and L. W. Esposito. 1981. Pioneer fly-by of Saturn and its rings. Adv. Space Res. 1, 67–71. LASP reprint 913.

Abstract: We report results from analysis of data from Pioneer Saturn’s imaging photopolarimeter. These include the discovery of a new ring and satellite, the structure of the atmosphere of Saturn and Titan, the inhomogeneous nature of Saturn’s rings, and a model for the rings’ formation and bimodal particle size distribution. 1981 COSPAR.

Thompson, W. T., K. Lumme, W. M. Irvine, W. A. Baum, and L. W. Esposito. 1981. Saturn's rings: Azimuthal variations, phase curves, and radial profiles in four colors. Icarus 46, 187–200. LASP reprint 319.

Abstract: Four-color photographic photometry of Saturn for the 1977–1979 apparitions has been analyzed to determine the dependence of ring brightness on wavelength, solar phase angle, ring-particle orbital phase

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angle (azimuthal effect), declination of the Earth relative to the ring plane (tilt angle), and radial distance from Saturn. Azimuthal brightness variations up to 20% relative to the ansae are clearly apparent for the maximum of ring A but are not detectable for ring B or the outer portion of ring A. The shape of the intensity (I) versus orbital phase angle ( ) curve varies with ring tilt (B) and probably with wavelength, and shows 180 symmetry. As characterized by its slope near the ansae, this curve suggests that the azimuthal effect increases as B decreases from 26 to ~11. The phase curves I () for the ansae show very little dependence on ring tile (26 > B > 6), on wavelength, or on radial distance from Saturn; possibly the curves are somewhat steeper at the smallest tilt angles and for ring A relative to ring B. The radial profile of both rings becomes flatter with decreasing tilt angle and with decreasing wavelength. The latter effect is a natural result of the classical, many-particle-thick ring model. 1981 Academic Press, Inc.

Esposito, L. W., and L. D. Travis. 1982. Polarization studies of the Venus UV contrasts: Cloud height and haze variability. Icarus 51, 374–390. LASP reprint 332.

Abstract: Polarimetry is able to show direct evidence for compositional differences in the Venus clouds. We present observations (collected during 2 1/2 Venus years by the Pioneer Venus orbiter) of the polarization in four colors of the bright and dark ultraviolet features. We find that the polarization is significantly different between the bright and dark areas. The data show that the “null” model of L. W. Esposito (1980, J. Geophys. Res. 85, 8151–8157) and the “overlying haze” model of J. B. Pollack et al. (1980, J. Geophys. Res. 85, 8223–8231) are insufficient. Exact calculations of the polarization, including multiple scattering and vertical inhomogeneity near the Venus cloud tops, are able to match the observations. Our results give a straightforward interpretation of the polarization differences in terms of known constituents of the Venus atmosphere. The submicron haze and UV absorbers are anticorrelated: for haze properties as given by K. Kawabata et al. (1980, J. Geophys. Res. 85, 8129–8140), the excess haze depth at 9350 Å over the bright regions is h = 0.03 ± 0.02. The cloud top is slightly lower in the dark features: the extra optical depth at 2700 Å in Rayleigh scattering above the darker areas is r = 0.010 ± 0.005. This corresponds to a height difference of 1.2 ± 0.6 km at the cloud tops. The calculated polarization that matches our data also explains the relative polarization of bright and dark features observed by Mariner 10. The observed differential polarization cannot be explained by differential distribution of haze, if the haze aerosols have an effective size of 0.49 m, as determined by K. D. Kawabata et al. (1982, submitted) for the aerosols overlying the Venus equator. We propose two models for the UV contrasts consistent with our results. In a physical model, the dark UV regions are locations of vertical convergence and horizontal divergence. In a chemical model, we propose that the photochemistry is limited by local variations in water vapor and molecular oxygen. The portions of the atmosphere where these constituents are depleted at the cloud tops are the dark UV features. Strong support for this chemical explanation is the observation that the number of sulfur atoms above the cloud tops is equal over both the bright and dark areas. The mass budget of sulfur at these altitudes is balanced between excess sulfuric acid haze over the bright regions and excess SO2 in the dark regions. 1982 Academic Press, Inc.

Lane, A. L., C. W. Hord, R. A. West, L. W. Esposito, D. L. Coffeen, M. Sato, K. E. Simmons, R. B. Pomphrey, and R. B. Morris. 1982. Photopolarimetry from Voyager 2: Preliminary results on Saturn, Titan, and the rings. Science 215, 537–543. LASP reprint 914.

Abstract: The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturn’s atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65ºN. Titan’s opaque atmosphere shows strong positive polarization at all phase angles (2.7º to 154º), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring. 1982 American Association for the Advancement of Science.

Lane, A. L., R. B. Pomphrey, and L. W. Esposito. 1982. Probing the fine structure of Saturn's rings. Planetary Report 2, 7. No reprint. 10/07 PS will look for one and send copy if they find it.

Online at: Not available online. Abstract: No abstract available.

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Esposito, L. W., N. Borderies, P. Goldreich, J. N. Cuzzi, J. B. Holdberg, A. L. Lane, R. B. Pomphrey, R. J. Terrile, J. J. Lissauer, E. A. Marouf, and G. L. Tyler. 1983. Eccentric ringlet in the Maxwell gap at 1.45 Saturn radii: Multi-instrument Voyager observations. Science 222, 57–60. LASP reprint 278.

Abstract: The Voyager spacecraft observed a narrow, eccentric ringlet in the Maxwell gap (1.45 Saturn radii) in Saturn’s rings. Intercomparison of the Voyager imaging, photopolarimeter, ultraviolet spectrometer, and radio science observations yields results not available from individual observations. The width of the ringlet varies from about 30 to about 100 kilometers; its edges are sharp on a radial scale < 1 kilometer, and its opacity exhibits a double peak near the center. The shape and width of the ringlet are consistent with a set of uniformly precessing, confocal ellipses with foci at Saturn’s center of mass. The ringlet precesses as a unit at a rate consistent with the known dynamical oblateness of Saturn; the lack of differential precession across the ringlet yields a ringlet mass of about 5 x 1018 grams. The ratio of surface mass density to particle cross-sectional area is about five times smaller than values obtained elsewhere in the Saturn ring system, indicating a relatively larger fraction of small particles. Also, comparison of the measured transmission of the ringlet at radio, visible, and ultraviolet wavelengths indicates that about half of the total extinction is due to particles smaller than 1 centimeter in radius, in contrast even with nearby regions of the C ring. However, the color and brightness of the ringlet material are not measurably different from those of nearby C ring particles. We find this ringlet is similar to several of the rings of Uranus.  American Association for the Advancement of Science.

Esposito, L. W., R. G. Knollenberg, M. Y. Marov, O. B. Toon, and R. P. Turco. 1983. The clouds and hazes of Venus. In Venus, ed. D. M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz, pp. 484–564. Tucson: Univ. of Arizona Press.

Abstract: Clouds totally enshroud Venus horizontally and have an enormous vertical extent, > 50 km. They are relevant to problems in Venus meteorology, geochemistry and evolution. With the data from Pioneer Venus and the Venera series, we have recently made remarkable progress in understanding the clouds. The Venus clouds consist of a main cloud deck at 45–70 km altitude, with thinner hazes above and below. Even the densest parts of the clouds are very tenuous, with visibility of several km. The microphysical properties of the main cloud allow further subdivision into a middle, upper, and lower cloud. Much of the cloud shows a multimodal particle size distribution. The mode most visible from the Earth is H2SO4 droplets with 2–3 m diameter. At all altitudes we find small particles, though not necessarily of the same composition. The largest particles range up to 35 m; these may either be nonspherical, incorporating crystalline cores, or merely be the tail of the H2SO4 droplet distribution. The upper region of the cloud visible from Earth shows both short and long-term variations. Contrasts visible in the ultraviolet have time scales of hours to months. The haze above the main cloud varies on a time scale of years. However, we do not have a detailed enough record of the deeper regions of the clouds to describe their variation. Despite variations, the vertical structure of the clouds shows persistent features at sites separated by years and great distances. Recent models are able to explain the dominant chemical and growth processes in the clouds. Growth models can yield multimodal distributions consistent with the observations, if contaminants are invoked. The production of cloud droplets by photochemical oxidation of SO2 at the cloud tops and condensation of H2SO4 near the cloud base seems plausible. Upward flowing SO2 can explain the observed ultraviolet contrasts and planetary spectral reflectivity, at least at wavelengths < 320 nm. Another absorber is needed to explain the observations at longer wavelengths; amorphous sulfur and molecular chlorine are two candidates. The clouds are more strongly influencd by radiation than by latent heat release. The small particle size and weak convective activity in the observed clouds seem incompatible with lightning of cloud origin. Further advances may be expected from the long time base provided by the Pioneer Venus orbiter, future in-situ measurements, and more detailed comparison and synthesis of data already in hand. 1983 Arizona Board of Regents.

Esposito, L. W., M. O'Callaghan, and R. A. West. 1983. The structure of Saturn's rings: Implications from the Voyager stellar occultation. Icarus 56, 439. LASP reprint 205.

Abstract: The Voyager 2 photopolarimeter observed the star Scorpii as it was occulted by Saturn’s rings, giving ~100 m resolution in the radial direction across the entire ring system. Radial structure can been seen down to the resolution limit, and considerable new structure is seen. In the outer B ring, none of this structure is due to imbedded large particles (moonlets) in the ring system. An automatic search with finite Fourier transforms located 13 density waves excited by resonances with Saturn’s satellites. More waves were found in searches by eye of predicted resonance locations. Although strong density waves can be located quite easily, no waves have been identified having predicted torques per surface mass density less than 4 x 10 16 cm4/sec2. This puts a limit of less than 100 waves likely to be found in Saturn’s rings, mostly in the A ring.

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Unresolved or overlapping waves do not play a major role in creating the most obvious radial structure. The total mass of Saturn’s rings is estimated as 5 x 10-8 of the mass of Saturn. Measurements of wave damping imply a ring thickness of ~30 m in the outer A ring. The power spectrum for the rings shows no dominant individual wavelengths: a preferred size for radial structure in the rings is not seen. At wavelengths less than ~15 km, the ring power spectrum drops below the power of noise in the data. The majority of the variance in the ring system is characterized by distance scales greater than 20 km. It is concluded that the majority of ring structures and the majority of variance in ring optical depth are not explained by currently proposed physical mechanisms. 1983 Academic Press, Inc.

Esposito, L. W., M. O'Callaghan, K. E. Simmons, C. W. Hord, R. A. West, A. L. Lane, R. B. Pomphrey, D. L. Coffeen, and M. Sato. 1983. Voyager photopolarimeter stellar occultation of Saturn's rings. J. Geophys. Res. 88, 8643–8649. LASP reprint 208.

Abstract: On August 25, 1981, the Voyager 2 photopolarimeter system observed a stellar occultation by Saturn’s rings. We present a brief description of this experiment along with details of the data reduction. The occultation results are given in tabular and graphical form at a resolution of 60 km. Histograms of the frequency of optical depth show dominantly unimodal distributions in each of the classical ring elements. The frequency distribution of the entire ring system shows three modes at 0.08, 0.5, and 2.50. 1983 American Geophysical Union.

Lumme, K., W. M. Irvine, and L. W. Esposito. 1983. Theoretical interpretation of the ground-based photometry of Saturn's B ring. Icarus 53, 174–184. LASP reprint 370.

Abstract: New photographic photometry at small tilt angles during the 1979 and 1981 apparitions is combined with earlier data to yield several physical parameters for Saturn’s B ring in red and blue colors. Phase curves are obtained for a mean tilt angle B 6. The value of the volume density D is 0.020 0.004 with no indication of dependence on either the color or the tilt angle for 6 <B <26. This conclusion is not altered significantly if the individual ring particles have a phase function similar to the phase curves of bright solar system objects. For the geometric albedo of a single particle, we derive 0.61 0.04 (red) and 0.41 0.03 (blue), which are superior to earlier estimates because of the additional data now available. These values and the derived amount of multiple scattering as a function of tilt angle constrain the particle phase function in the red to be moderately backscattering. Inferred values of the particle single-scattering albedo are 0.7 ≤ (red) ≤ 0.92 and 0.5 ≤ (blue) ≤ 0.7, depending on the choice of phase function. No indication was found that the particle photometric properties might depend on the vertical distance from the central plane. Our results show that the ground-based photometry is entirely consistent with the classical, many-particle-thick ring model. 1983 Academic Press, Inc.

West, R. A., C. W. Hord, K. E. Simmons, H. Hart, L. W. Esposito, A. L. Lane, R. B. Pomphrey, R. B. Morris, M. Sato, and D. Coffeen. 1983. Voyager photopolarimeter observations of Saturn and Titan. Adv. Space Res. 3, 45–48. LASP reprint 56.

Abstract: The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 Å and in the near-IR at 7500 Å. Measurements of Saturn’s limb brightening and polarization at several phase angles up to 70º indicate that a significant optical depth of UV absorbers is present in the top 100 mbar of Saturn’s atmosphere in the equatorial zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation. The whole-body polarization of Titan is strongly positive in both the UV and near-IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith (1982, Icarus 51, 65-95), characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties. 1983 COSPAR.

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West, R. A., A. L. Lane, H. Hart, K. E. Simmons, C. W. Hord, D. L. Coffeen, L. W. Esposito, M. Sato, and R. B. Pomphrey. 1983. Voyager 2 photopolarimeter observations of Titan.J. Geophys. Res. 88, 8699–8708. LASP reprint 239.

Abstract: Observations of Titan’s whole disk polarization at 2460 Å and 7500 Å are presented and analyzed in terms of model scattering atmospheres.. If the Titan aerosols are spherical or nearly spherical, no single combination of refractive index and size distribution is able to fit data at both wavelengths. However, a vertically inhomogeneous distribution suggested by Tomasko and Smith (1982, Icarus 51, 65–95), characterized by a size gradient with altitude, fits the data at 2640 Å moderately well but must be modified at intermediate and large optical depths to fit the 7500 Å data. Results for synthetic phase functions indicate that the single scattering polarization must be 70% or larger in the UV and 78% or larger in the near-IR at 90 phase angle, depending on the phase function. If the correct phase function is similar to that for 0.5-m-radius spheres, the UV single-scattered polarization must be 84% and the near-IR single-scattered polarization must be over 90%. Such large polarizations are impossible for 0.5-m-radius spheres but may be possible for nonspherical particles with effective radii near 0.5 m, although the existence of nonspherical particles with the scattering properties required by these and other observations has not been demonstrated. 1983 American Geophysical Union.

West, R. A., M. Sato, H. Hart, A. L. Lane, C. W. Hord, K. E. Simmons, L. W. Esposito, D. L. Coffeen, and R. B Pomphrey. 1983. Photometry and polarimetry of Saturn at 2640 Å and 7500 Å. J. Geophys. Res. 88, 8679–8697. LASP reprint 279.

Abstract: We have reduced and tabulated photometry and polarimetry data at 2640 and 7500 Å observed by the Voyager 2 photopolarimeter experiment. Spatially resolved limb-to-terminator scans across Saturn’s equatorial zone from 12 to 68 phase angle provide information on the altitude distribution of UV absorbing hazes and the phase function and polarizing properties of stratospheric and tropospheric aerosols. Limb-to-terminator scans across the northern hemisphere at 10 phase angle are used to study altitude variations of the tropospheric cloud at several latitudes. For the equatorial zone, we find that (1) the UV photometry and polarimetry are best fit by Rayleigh’s phase matrix; (2) a stratospheric haze of small particles is allowed as long as the optical depth is near unity or less and the center of the haze layer is in the 30- to 70-mbar region. A diffuse haze fits better than a thin layer, and the aerosol/gas mixing ratio diminishes above 10 mbar. The vertical distribution and optical depth of the haze differ significantly from models proposed by others. To be in agreement with ground-based and other spacecraft data, the haze optical depth is about 0.4 at 2640 Å and decreases by a factor of 10 or more at 6400 Å. If the haze aerosol scattering properties are similar to those for spheres with mean radius 0.1 m, their imaginary refractive index is 0.4 or larger at 2640 Å, and the total column density above the tropopause is 10 cm-2; (3) UV contrasts between belts and zones are interpreted as altitude variations in the top of the tropospheric cloud. The altitudes derived here for three latitudes agree with altitudes derived from ground-based methane band studies and analyses of polarization from Pioneer 11. A high altitude absorber is abundant in the polar regions; and (4) at 7500 Å, the phase function of tropospheric aerosols in the equatorial zone is described by a synthetic two-term Henyey-Greenstein function with g1 = 0.54 0.11, g2 = –0.47 0.08, f = 0.87 0.03, and = 0.967. The equatorial zone tropospheric aerosols are positively polarizing at all the phase angles of our observations. 1983 American Geophysical Union.

Cuzzi, J. N., J. J. Lissauer, L. W. Esposito, J. B. Holberg, E. A. Marouf, G. L. Tyler, B. Pederson, and A. Boischot. 1984. The rings of Saturn. In Planetary Rings, ed.A. Brahic. Tucson: Univ. of Arizona Press.

Abstract: The structural and particle properties of Saturn’s rings are described, with particular emphasis on spacecraft observations. Properties are discussed generically rather than regionally, and we attempt in all cases to relate observed properties to favored causative processes. The ring particles are primarily icy, but there is evidence for albedo, and therefore possibly compositional, variation on both local and regional scales Most of the particles are in the 1-cm to 5-m radius range, but there is reason to suspect the existence of some particles of all sizes up to 10 km in radius. Ring structure is, in general, dominated by gravitational and collisional dynamics. Orbital resonances with various satellites drive spiral density and bending waves, and define the sharp outer edges of the A and B rings. From such features, the ring mass density and local vertical thickness may be determined. The structure of kinky ringlets in the F ring and Encke gap are probably a manifestation of gravitational perturbations by local shepherding moonlets. Electromagnetic processes are evident in the spokes, which are radially elongated regions enhanced in fine dust relative to thheir surroundings. However, the observed abundance of irregular, fine-scale structure is not well understood. It is

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also not yet clear what process creates and maintains the well defined elliptical ringlets lying in several empty gaps. Several problems of a more global scale are also outstanding, including the morphology of the inner edges of the A and B rings, and the short ring evolutionary time scale associated with the transfer of angular momentum in density waves. 1983 Arizona Board of Regents.

Esposito, L. W. 1984. Structure and dynamics of Saturn's rings as seen by the Voyager stellar occultation. Proceedings of IAU Colloquium #75. Toulouse, France: CNES.

Esposito, L. W. 1984. Sulfur dioxide: Episodic injection shows evidence for active Venus volcanism. Science 223, 1072–1074. LASP reprint 204.

Abstract: Pioneer Venus ultraviolet spectra from the first 5 years of operation show a decline (by more than a factor of 10) in sulfur dioxide abundance at the cloud tops and in the amount of submicron haze above the clouds. At the time of the Pioneer Venus encounter, the values for both parameters greatly exceeded earlier upper limits. However, Venus had a similar appearance in the late 1950s, implying the episodic injection of sulfur dioxide possibly caused by episodic volcanism. The amount of haze in the Venus middle atmosphere is about ten times that found in Earth’s stratosphere after the most recent major volcanic eruptions, and the thermal energy required for this injection on Venus is greater by about an order of magnitude than the largest of these recent Earth eruptions and about as large as the Krakatoa eruption of 1883. The episodic behavior of sulfur dioxide implies that steady-state models of the chemistry and dynamics of cloud-top regions may be of limited use. 1984 American Association for the Advancement of Science.

Esposito, L. W., J. N. Cuzzi, J. B. Holberg, E. A. Marouf, G. L. Tyler, and C. C. Porco. 1984. Saturn’s rings: Structure, dynamics, and particle properties. In Saturn, ed. T. Gehrels and M. S. Matthews, pp. 463–545. Tucson: Univ. of Arizona Press.

Abstract: The rings of Saturn are not only beautiful, but also a laboratory for particle dynamics and a likely relic of the early solar system. Recent spacecraft results have brought a manifold increase in our knowledge and understanding of the rings. Spacecraft measurements have a resolution as good as 100 m on the rings, revealing unexpected microstructure down to the limit of resolution. Density waves and bending waves propagate through the rings. Analysis of these waves gives the mass, viscosity, and thickness of the rings. The mass of the rings is ~5 x 10-8 MS, about the same as Mimas’. The rings are thinner than 100 m, but thicker than a single particle. The ring particles have a broad size distribution, ranging from 1 cm to 5 m, with some areas containing particles as small as several microns. Moonlets (particles as large as 10 km) are quite rare. The ring particles are bright and strongly backscattering, consistent with rough, icy blocks. Recently observed features in the rings include dark, nearly radial lines (spokes) in the B ring and noncircular ringlets and ring edges. Several regions in the rings contain fine dust, which allows a coupling between the magnetosphere and the observable rings. The important questions of the origin and evolution of Saturn’s rings are still unresolved. 1984 Arizona Board of Regents.

Esposito, L. W. 1985. Long term changes in Venus sulfur dioxide. Adv. Space Res. 5, 85–90. LASP reprint 61.

Abstract: Pioneer Venus data from the first 5 years of operation show a decline by more than a factor of ten in SO2 at the cloud tops. A consistent decline has also recurred in the amount of sub-micron haze above the clouds. The correlation between these two observables is 0.8 over this period. A plausible explanation is injection of SO2 from episodic volcanism. The episodic behavior implies that steady state models of the Venus cloud chemistry and dynamics may be of limited use. 1985 COSPAR.

Esposito, L. W. 1986. Structure and evolution of Saturn's rings. Icarus 67, 345–357. LASP reprint 387.

Abstract: Abundant new information on the structure and evolution of Saturn’s rings comes from the recent Voyager encounters. Much of the newly observed structure still lacks a satisfactory explanation. The particles in the rings are easily broken: smaller particles are seen at density wave locations, in an eccentric ringlet, and the outer part of the A ring. The ring thickness ranges from 1 m or less in the C ring to 1–5 m in the B ring and 5–30 m in the A ring, with the largest thickness furthest from Saturn. A possible solution to the apparent youth of the rings is that they are young. A less radical solution is that only ring A is young. It may have been recently created by the demolition of a small moon near the rings. 1986 Academic Press, Inc.

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Lane, A. L., C. W. Hord, L. W. Esposito, K. E. Simmons, A. L. Graps, W. R. Pryor, R. A. West, R. M. Nelson, B. D. Wallis, B. J. Buratti, and L. J. Horn. 1986. Photometry from Voyager 2: Initial results from the Uranian atmosphere, satellites, and rings. Science 233, 65–70. LASP reprint 377.

Abstract: The Voyager 2 photopolarimeter successfully completed the Uranus encounter, acquiring new data on the planet’s atmosphere, its principal satellites, and its ring system. Spatially resolved photometry of the atmosphere at 0.27 micrometers shows no enhancement in absorption toward the pole, unlike the case for Jupiter and Saturn. Stellar occultation measurements indicate the temperature at the 1-millibar level over the north pole is near 90 kelvins. The geometric albedos of the five large satellites of Uranus were measured at 0.27 and 0.75 micrometers and indicate the presence of low-albedo, spectrally flat absorbing material. Titania seems to have a fluffy surface, as indicated by its phase curve. The nine ground-based rings were detected and their internal structure, optical depths, and positions were determined. The sharp edges of the ring made it possible to measure its edge thickness (less than 150 meters) and particle sizes (less than 30 meters); little or no dust was detected. New narrow rings and partial rings (arcs) were measured, and the narrow component of the ring was found to be discontinuous. 1986 American Association for the Advancement of Science.

Ragent, B., L. W. Esposito, M. G. Tomasko, M. Y. Marov, V. P. Shari, and V. N. Lebedev. 1986. Particulate matter in the Venus atmosphere. In Venus International Reference Atmosphere, ed. G. M. Keating. Elsevier Science.

Abstract: The paper presents a summary of the data currently available (June 1984) describing the planet-enshrouding particulate matter in the Venus atmosphere. A description and discussion of the state of knowledge of the Venus clouds and hazes precedes the tables and plots. The tabular material includes a precis of upper haze and cloud-top properties, parameters for model-size distributions for particles and particulate layers, and columnar masses and mass loadings.

Showalter, M. R., J. N. Cuzzi, E. A. Marouf, and L. W. Esposito. 1986. Satellite “wakes” and the orbit of the Encke gap moonlet. Icarus 66, 297–323. LASP reprint 366.

Abstract: Quasiperiodic optical depth variations have been observed in the Voyager stellar (PPS) and radio occultation profiles near the Encke gap of Saturn’s rings and have also been detected in one Voyager image. These fluctuations are believed to be the gravitational “wakes” of a moonlet orbiting within the gap. The existence of such a body had already been proposed by J. N. Cuzzi and J. D. Scargle (1985, Astrophys. J. 292, 276–290), based on radial “wavy edges” visible in numerous Voyager images of the gap. We develop a general model for these wakes and use the results to estimate the moonlet’s orbit and mass from the occultation data; this model may have broader applications to planetary rings. The moonlet longitude is best determined from the PPS scan interior to the gap and is estimated to trail the observed profile by 32. Considering the uncertainty caused by our neglect of particle collisions and self-gravity, the longitude is consistent with other estimates obtained from a second portion of the PPS scan exterior to the gap, and also from the radio profile. The moonlet orbits close to the center of the gap at an estimated semimajor axis of 133,603 10 km; it has a mass of 5–10 x 10-12 Saturn masses, which corresponds to a radius of ~10 km if it is composed primarily of water ice. The consistency of the orbit parameters inferred from the PPS, radio, and wavy-edge data virtually guarantees that a single dominant moonlet orbits within the gap. 1986 Academic Press, Inc.

Cuzzi, J. N., and L. W. Esposito. 1987. The rings of Uranus. Sci. Amer. 255, 52–54, 63–66. LASP reprint 398.

Abstract: Why are the rings of Uranus so narrow and dark? We are beginning to suspect that the austere Uranian ring system may have had a violent and chaotic past. Findings from the Voyager 2 encounter suggest that Uranus’ present system—consisting of 10 narrow rings, numerous dusty bands, some narrow ring arcs, and a bevy of moonlets—may be only a fragment of its former self and merely one more passing stage in an ongoing process of creation and loss from which future rings of Uranus will arise. 1987 Scientific American, Inc.

Esposito, L. W. 1987. The changing shape of planetary rings. Astronomy 15, 6–17. LASP reprint 915.

Abstract: Rings are not the ancient, smooth objects they appeared to be just a decade ago. With the discovery of new ring systems, it is becoming clear that planetary rings are young objects that grow and decay.

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Esposito, L. W. 1987. Venus. In McGraw-Hill 1987 Yearbook of Science and Technology, pp. 495–497. New York: McGraw-Hill. LASP reprint 916.Abstract: An overview of evidence for volcanic activity on Venus.

Esposito, L. W., C. C. Harris, and K. E. Simmons. 1987. Features in Saturn's rings. Astrophys. J. Supp. 63, 749–770. LASP reprint 396.

Abstract: We present tabular information and figures for 216 significant features in Saturn’s rings found by the Voyager Photopolarimeter stellar occultation on 25 August, 1981. 1987 The American Astronomical Society.

Miner, E. D., A. P. Ingersoll, W. Kurth, L. W. Esposito, and T. V. Johnson. 1987. Science objectives and preliminary sequence designs for the Voyager Neptune encounter. Jet Propulsion Laboratory, JPL TM D–4607.

Nelson, R. M., B. J. Buratti, B. D. Wallis, A. L. Lane, R. A. West, K. E. Simmons, C. W. Hord, and L. W. Esposito. 1987. Voyager 2 photopolarimeter observations of the Uranian satellites. J. Geophys. Res. 92, 14905–14910. LASP reprint 917.

Abstract: We report the ultraviolet (0.25 m) and infrared (0.75 m) geometric albedos, phase curves, and phase coefficients for the large Uranian satellites using full-disk photometric observations obtained by the photopolarimeter subsystem during the Voyager 2 encounter with the Uranian system. The phase coefficients and phase curves of the Uranian satellites are consistent with surfaces that have a loosely packed regolith with a heavily cratered terrain. Ariel, Titania, and Oberon exhibit a reddening with increasing phase angle of observation. We do not observe this effect for Umbriel. We have determined preliminary phase integrals for Ariel, Umbriel, Titania, and Oberon, and we have used this information and the geometric albedos reported herein and previously reported by the Voyager imaging subsystem to derive Bond albedos for the satellites. These are 0.22 0.1, 0.07 0.05, 0.16 0.12, and 0.19 0.22 for Ariel, Umbriel, Titania, and Oberon, respectively. We have compared the geometric albedos and color ratios of the Uranian satellites with comparable sets of data from the literature of other large satellites in the solar system, and we find that the Uranian satellites as a class are separate and distinct from the large Jovian and Saturnian satellites. This indicates a compositional difference between the three classes of large icy satellites. This may be due to different conditions at the time of formation and/or differences in the subsequent evolution of the surfaces of these objects. This finding is consistent with the hypothesis that a common surficial modification process exists for all the satellites in the Uranian system that is different from the processes that modify the surfaces of the Jovian and Saturnian satellites.  1987 American Geophysical Union.

West, R. A., A. L. Lane, C. W. Hord, L. W. Esposito, K. E. Simmons, R. M. Nelson, and B. D. Wallis. 1987. Temperature and aerosol structure of the nightside Uranian stratosphere from Voyager 2 photopolarimeter stellar occultation measurements. J. Geophys. Res. 92, No. A13, 15030–15036. LASP reprint 918.

Abstract: The Voyager 2 photopolarimeter experiment measured the ultraviolet flux from the star Pegasi as the star emerged from behind the Uranus limb at planetocentric latitude 68.9ºN on the nightside. We have compared these measurements to model atmospheres with, and without, an aerosol component. If the atmosphere is free of aerosols, the temperature is 85 ± 2.3 K at a pressure of 2.7 mbar, 90 ± 6 K at 1 mbar, and 96 ± 13 K at 0.37 mbar, and the radius of the 1-mbar level is 25,219 ± 6.3 km. The radius value is close to the inferred radius (25,225 km) of the 1-mbar level at latitude 69º in the southern hemisphere (G. Lindal, private communication, 1987). This agreement argues for hemispheric symmetry in the shape of the 1-mbar geopotential and, therefore, in the latitude profile of the zonal wind velocity which was not measured in the northern hemisphere. Our measurement, when coupled with a 1-mbar equatorial radius of 25,734 km (G. Lindal, private communication, 1986), leads to an effective planetary oblateness of = 0.0230 ± 0.004, slightly smaller than the value (0.024) determined from earlier work. We place upper limits to the amount of aerosol that can form a discrete layer or haze-top boundary; the aerosol extinction coefficient, k ext ≤ ~10-4 km-

1, if the layer is at an altitude equal to or higher than the 1-mbar altitude. Our ability to sense haze boundaries diminishes rapidly below 3 mbar. A well mixed haze may be present. Candidates for this haze include dust from meteor and ring particle infall, photochemically formed diacetylene condensing at temperatures below

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103 K, and other photochemical species that condense at temperatures higher than about 80 K. We show that dust from the rings and meteors contributes no more than a few tenths of a percent to the extinction profile. Current predictions for the diacetylene abundance reveal that diacetylene aerosols may contribute as much as a few tens of percent to the extinction profile and can be accommodated by our observations. More detailed photochemical transport aerosol models are needed to explore these questions further. 1987 American Geophysical Union.

Esposito, L. W., M. Copley, R. Eckert, L. Gates, A. I. F. Stewart, and H. Worden. 1988. Sulfur dioxide at the Venus cloud tops, 1978–1986. J. Geophys. Res. 93, 5267–5276. LASP reprint 435.

Abstract: Ultraviolet spectroscopy from the Pioneer Venus orbiter shows a decline in the cloud top abundance of SO2 from about 100 ppb to about 10 ppb in the period 1978–1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10%, which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO2 abundance to twice the inferred values in later orbits. Tracking of SO2 features and power spectral analysis give rotation periods for the longer-lived features of 3.6–5.2 days, consistent with cloud-tracked winds observed at other wavelengths. The behavior of SO2 and polar haze can be plausibly explained by episodic injection of SO2 into the cloud top regions, for example, by active volcanism. 1988 American Geophysical Union.

Horn, L. J., P. A. Yanamandra-Fisher, L. W. Esposito, and A. L. Lane. 1988. Physical properties of the Uranian delta ring from a possible density wave. Icarus 76, 485–492. LASP reprint 444.

Abstract: The Voyager PPS stellar occultation observations of the Uranian ring show evidence for a moonlet interior to the ring, which excites a density wave at 48,299.6 0.4 km. The identification of a density wave is from the wavelength and amplitude behavior and the morphology of the observed feature. Sixty-five discrete locations are possible for the orbit of this unseen moonlet. Allowing for these 65 possible locations, we find the surface mass density of the ring 5 g/cm2, the viscosity 10 40 cm2/sec,

and the local ring height 7 h 20 m. These values are comparable to some parts of Saturn’s rings. All of the inner and outer first-order Lindblad resonances were calculated for the 65 possible moonlet locations. The 65 locations for the moonlet are labeled by azimuth number m of the resonance associated with each location that would excite the density wave in the ring. Moonlet 101, located at 47,984.1 0.4 km, has resonances that can also shepherd the inner edge of the ring and the outer edge of the ring. 1988 Academic Press, Inc.

Stern, S. A., L. W. Esposito, and E. S. Barker. 1988. Ground-based CCD observations of Venus SO2: 1986–87. Laboratory for Atmospheric and Space Physics, LASP Technical Report and reprint 425.

Abstract: We obtained ground-based near-UV monochromatic images of Venus in 1986 and 1987 at McDonald Observatory using a small telescope equipped with a CCD camera. Our objective was to study the abundance and spatial/temporal variations of SO2 in the Venus atmosphere. Analysis indicates that seeing and atmospheric opacity limitations prevent meaningful scientific results to be obtained from the present data set. The observations, data analysis procedures, and data quality are reviewed here. Recommendations are made on the basis of our findings with regard to future ground-based and space-based Venus SO2 absorption imaging efforts.

Brophy, T. G., and L. W. Esposito. 1989. Simulation of collisional transport processes and the stability of planetary rings. Icarus 78, 181–205. LASP reprint 446.

Abstract: A model kinetic equation is solved numerically for a flattened Keplerian disk using a phase-space fluid method which models the particle transport process as a Markov process. Ringlets composed of single-sized particles and ringlets composed of two different-sized particles are investigated. The accuracy of the new computational method is verified by close agreement with the analytical results of F. H. Shu and G. R. Stewart (1985, Icarus 62, 360–383), P. Goldreich and S. Tremaine (1978, Icarus 34, 227–239), W. R. Ward (1981, Geophys. Res. Lett. 8, No. 6, 641–643), and D. N. C. Lin and P. Bodenheimer (1981, Astrophys. J. 248, L83–L86), and the simulations of D. Spaute and R. Greenberg (1987, Icarus 70, 289–302) and J. M.

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Petit and M. Henon (1988, Astron. Astrophys. 199, 343–356). The generally accepted theory of viscous spreading (e.g., N. Borderies, P. Goldreich, and S. Tremaine) is verified for the special case of ringlets where all the particles are the same size. Ringlets composed of two components of particle size evolve in such a manner that the lighter particles are confined by the heavier particles. Some implications are that (1) a natural mechanism may sharpen the optical depth profile of edges even in the absence of an external forcing mechanism under some conditions, and (2) in some cases, intermediate optical depths are dynamically preferred. Simulation of the dominant features of observed planetary rings (sharp edges, microstructure) may have been excluded by some of the simplifying assumptions of analytic theories, such as single-sized particles and the existence of a simple scalar viscosity.  1989 Academic Press, Inc.

Esposito, L. W., and J. E. Colwell. 1989. Creation of the Uranus rings and dust bands, Nature 339, 605–607. LASP reprint 455.

Abstract: Voyager observations of the extended hydrogen exosphere of Uranus and of the ring and its shepherds set an upper limit to the age of the ring of 6 x 108 years. Unless we are seeing Uranus at a special time in its history, this requires a continuing process to create the ring material. We propose that the moons, rings, and dust now visible in the Uranus system are created by the diminution of larger objects by meteoroid impacts. A Monte Carlo calculation shows that the largest surviving fragments of the original precursor satellites are in the size range of the new satellites observed by Voyager near the rings. The current bombardment rate is sufficient to create all the observed rings and dust bands. The observed and inferred components of the Uranus ring system fit a power-law size distribution with index of ~3. We predict that the material in Neptune’s ring system has a similar size distribution. 1989 Macmillan Magazines Ltd.

Lane, A. L., R. A. West, C. W. Hord, R. M. Nelson, K. E. Simmons, W. R. Pryor, L. W. Esposito, L. J. Horn, B. D. Wallis, B. J. Buratti, T. G. Brophy, P. Yanamandra-Fisher, J. E. Colwell, D. A. Bliss, M. J. Mayo, and W. D. Smythe. 1989. Photometry from Voyager 2: Initial results from the Neptunian atmosphere, satellites, and rings. Science 246, 1450–1454. LASP reprint 919.

Abstract: The Voyager photopolarimeter successfully accomplished its objectives for the Neptune encounter, performing measurements on the planet, several of its satellites, and its ring system. A photometric map of Neptune at 0.26 micrometer (m) shows the planet to be bland, with no obvious contrast features. No polar haze was observed. At 0.75 m, contrast features are observed, with the Great Dark Spot appearing as a low-albedo region and the bright companion as being substantially brighter than its surroundings, implying that it is at a higher altitude than the Great Dark Spot. Triton’s linear phase coefficients of 0.011 magnitudes per degree at 0.26 m and 0.013 magnitudes per degree at 0.75 m are consistent with a solid-surface object possessing high reflectivity. Preliminary geometric albedos for Triton, Nereid, and 1989N2 were obtained at 0.26 and 0.75 m. Triton’s rotational phase curve shows evidence of two major compositional units on its surface. A single stellar occultation of the Neptune ring system elucidated an internal structure in 1989N1R, in the ~50-kilometer region of modest optical depth. 1989N2R may have been detected. The deficiency of material in the Neptune ring system, when compared to Uranus’, may imply the lack of a “recent” moon-shattering event. 1989 American Association for the Advancement of Science.

Brophy, T. G., G. R. Stewart, and L. W. Esposito. 1990. A phase-space fluid simulation of a two-component narrow planetary ring: Particle size segregation, edge formation, and spreading rates. Icarus 83, 133–155. LASP reprint 467.

Abstract: A two-component kinetic equation is solved numerically for a flattened narrow planetary ring. The Krook kinetic equation for single-sized identical planetary ring particles (see T. G. Brophy and L. W. Esposito, 1989, Icarus 78, 181–205, or F. H. Shu and G. R. Stewart, 1985, Icarus 62, 360–383) is generalized for the case of two-component systems. The generalized two-component equations are solved numerically using the phase-space fluid computational method developed in Brophy and Esposito (1989). The results of a simulation of a two-component narrow ring, in which the large particles are eight times as massive as the small particles, are presented in detail. The dynamics of the unconstrained edges of this ring are resolved by the simulation and show sharpening not expected for single-component rings. This is due to a tendency for small particles to accumulate at the edges of the ring, creating a flatter and sharper-edged equivalent optical-depth profile than that for a single-component ring. This is caused by a tendency for equipartition of random kinetic energies, which retards the large particle evolution and quickens the small particle evolution, and by torques exerted by each component on the other at edges, due to each component having a different asymmetric drift speed. The tendency for equipartition causes damping of the large particle dispersion

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velocity, which results in a slower overall spreading rate for the ring than that for a single-component ring of similar equivalent optical depth. 1990 Academic Press, Inc.

Colwell, J. E., and L. W. Esposito. 1990. A model of dust production in the Neptune ring system. Geophys. Res. Lett. 17, 1741–1744. LASP reprint 503.

Abstract: Voyager 2 images of the Neptunian ring system show it to be relatively dusty with the fraction of micron-sized dust particles in the rings between about 0.2 and 0.7 (Smith et al. 1989). We apply a numerical model developed for the Uranus dust bands (Colwell and Esposito, Icarus, 86, 530–560) to the Neptune system. Our results show that 1) the suggested bimodal distribution of dust fractions in the rings is not related to ring optical depth and may be due to different ring particle size distributions; 2) collision velocities between the macroscopic ring particles must be ~1 m/sec if they are to produce dust fractions above 1/2; 3) the small moons of Neptune must have regoliths for meteoroid excavation to produce the dust population at Voyager ring plane crossing; and 4) dust optical depths interior to 1989N3R are negligible unless there are unseen macroscopic source bodies in that region. 1990 American Geophysical Union.

Colwell, J. E., and L. W. Esposito. 1990. A numerical model of the Uranian dust rings. Icarus 86, 530–560. LASP reprint 494.

Abstract: The tenuous dust rings of Uranus ( ) discovered by Voyager 2 are not readily associated with any of the other observed components of the Uranus ring system. Their short lifetime due to exospheric drag and other disruptive processes requires continuous production of the dust particles. Our numerical simulations of the Uranus rings show that the dust rings could be the visible component of low optical depth moonlet belts. These belts would be made up of objects from 10 to 105 cm that serve as sources of the dust via micrometeoroid bombardment and collisional release of ejecta. Dust is created within the main rings of Uranus through the same processes. The dust bands cannot be the transient remnants of large impacts onto unseen moons because the impacts occur too infrequently since the dust band lifetimes are very short (≤ 100 years). We propose that the dust bands are the result of continuous ejection and reabsorption of micron-sized regolith material within hypothesized moonlet belts ( ). Some of the ejecta is removed from the moonlet belt giving rise to a continuous sheet of micron-sized dust particles. Numerical simulations of such a system in a Markov chain formalism reproduce the observed characteristics of the Uranian dust rings. The broad sheet of dust which extends approximately from the ring to Cordelia could be dust ejected from Cordelia by continuous meteoroid erosion. This dust would then radially evolve until accreting onto ring particles. Our simulations show that the surface of Cordelia would have to be relatively hard, or regolith-free, for enough dust to be excavated to account for the observed optical depth. In addition, we would expect a similar sheet between Ophelia and the ring, and none is observed. We therefore propose that this sheet is also the visible component of one or more particularly broad moonlet belts, with Cordelia the largest member of the moonlet belt distribution. 1990 Academic Press, Inc.

Colwell, J. E., L. J. Horn, A. L. Lane, L. W. Esposito, P. A. Yanamandra-Fisher, S. H. Pilorz, K. E. Simmons, M. D. Morrison, C. W. Hord, R. M. Nelson, B. D. Wallis, R. A. West, and B. J. Buratti. 1990. Voyager photopolarimeter observations of Uranian ring occultations. Icarus 83, 102–125. LASP reprint 473.

Abstract: The Voyager 2 photopolarimeter (PPS) observed the stars Sagitarii and Persei as they were occulted by the Uranian ring system on January 26, 1986 (A. L. Lane et al., 1986, Science 233, 65–70). Both occultations passed through the ring system so that orbital radii were sampled at two different longitudes in the ring plane for each occultation. The Sagitarii occultation was a grazing occultation and only , , and 1986U1R occulted the star. Persei was occulted by each of the nine classical rings twice, and each of these rings was identified on ingress and egress, although the 6 ring in Persei ingress is a marginal detection, and the ring in Persei ingress was not detected. The orbits of the nine classical rings have been successfully modeled by numerous Earth-based stellar occultations (R. G. French et al., 1988, Icarus 73, 349–378), and Voyager occultation profiles of the rings from the radio science occultation (RSS, D. L. Gresh et al. 1989, Icarus 78, 131–168) and the ultraviolet spectrometer occultations (UVS, J. B. Holberg et al. 1987, Astron. J. 94, 178–188) have been published. The PPS occultations provide high resolution profiles of the rings (~10 m/point for Sgr, ~100 m/point for Persei), which contain information on waves (L. J. Horn et al., 1988, Icarus 76, 485–492), edge sharpness, vertical thickness, and azimuthal variations in radial structure within the rings. The PPS profiles of the ring show structure similar to that seen in the radio, UVS, and Earth-based occultations, although the magnitude of peaks and valleys in optical depth changes with azimuth. 1986U1R, , , , and 6 all show varying degrees of azimuthal inhomogeneity. The ring is not present in the Persei

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ingress data although it shows up clearly in egress, and the ring is only 0.6 km wide at Persei ingress and is opaque to the starlight, while on the egress cut it is 2.6 km wide. We also present the results of a rigorous statistical search of the data for previously undetected rings. Several features were identified in the Persei data that were more statistically significant than some of the known ring profiles (6 and in Persei ingress), but the low signal-to-noise ratio for this occultation prevents the identification of any new rings.  1990 Academic Press, Inc.

Esposito, L. W. 1990. Does Venus have active volcanos? Astronomy 18, 42–47. LASP reprint 921.

Abstract: A hot controversy in Venusian geology is whether or not the planet has active volcanos. This article presents Earth-based and spacecraft observations suggesting that Venus is a volcanically active planet. Magellan, soon to arrive at Venus, may provide an answer to this question.

Esposito, L. W. 1990. Sulfur dioxide and its variations. In Middle Atmosphere of Venus. Akademie der Wissenschaften der DDR. LASP reprint 514.

Abstract: Ultraviolet spectroscopy from the Pioneer Venus orbiter shows a decline in the cloud top abundance of SO2 from about 100 ppb to about 10 ppb in the period 1978–1986. A consistent decline in polar haze has occurred over the same period with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10%, which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO2 abundance to twice the inferred values in later orbits. Tracking of SO2 features and power spectral analysis give rotation periods for the longer lived features of 3.6–5.2 days, consistent with cloud tracked winds observed at other wavelengths. The behavior of SO2 and polar haze can be plausibly explained by episodic injection of SO2 into the cloud top regions, for example, by active volcanism.

Esposito, L. W. 1990. Variation of the sulfur dioxide concentration in the atmosphere of Venus and the concept of active volcanism. Translated from Astronomicheskii Vestnik 24, 57–58.LASP reprint 920.

Abstract: The experimental data on the concentration of sulfur dioxide above the clouds in the atmosphere of Venus cannot be explained by atmospheric dynamics. The only hypothesis agreeing with the observed variations of the SO2 concentration is the thermal effects of tremendous volcanic eruptions on scales ten times greater than the eruption of El Chichon. 1990 Plenum Publishing Corporation.

Na, C. Y., L. W. Esposito, and T. E. Skinner. 1990. International Ultraviolet Explorer observation of Venus SO2 and SO. J. Geophys. Res. 95, 7485–7491. LASP reprint 485.

Abstract: Results of recent International Ultraviolet Explorer (IUE) observations of Venus made on January 20, 1987 and April 2 and 3, 1988, along with a reanalysis of the 1979 observations (R. R. Conway et al., 1979, Geophys. Res. Lett. 6, 629–631) are presented. The observations indicate that the amount of sulfur dioxide at the cloud tops of Venus declined by a factor of 8 ± 4 from 380 ± 70 ppb in 1979 to 50 ± 20 ppb in 1987 and 1988. These values are consistent with the Pioneer Venus results (1997, L. W. Esposito, et al., in Venus II, pp. 415–458, Tucson: Univ. of Arizona Press). A recalibration of the solar flux for Pioneer Venus results and a comparison of existing SO2 measurements on Venus, unpublished manuscript, 1989). We identify absorption features of sulfur monoxide for the first time and estimate the SO mixing ratio above the cloud level is 20 ± 10 ppb for 1979. This is consistent with photochemical models by J. R. Winick and A. I. F. Stewart (1980, J. Geophys. Res. 85, 7849–7860) and Y. K. Yung and W. B. DeMore (1982, Icarus 51, 199–247) and with the upper limit from W. J. Wilson et al. (1981, Icarus 45, 624–637). 1990 American Geophysical Union.

Esposito, L. W. 1991. Planetary rings: Ever decreasing circles. Nature 354, 107. LASP reprint 574.

Abstract: The origin of the rings surrounding the planets has, if anything, become a deeper puzzle over the years. In part, this is because, since 1977, rings have been discovered around each of the giant planets Jupiter, Uranus and Neptune, so that Saturn is no longer unique. Moreover, detailed measurements by the Pioneer and Voyager spacecraft have overthrown the old prevailing idea that the rings were formed at the same time as the planets. The rings are so shortlived that they must be continually replaced for them to still be

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visible. One possible explanation is that rings are created whenever a small moon is destroyed in a collision with a comet or asteroid, but this does not explain how Saturn’s rings originated. Saturn’s rings may have been formed by tidal disruption of a comet that passed too close to the planet. 191 Macmillan Magazines Ltd.

Esposito, L. W., A. Brahic, J. Burns, and E. A. Marouf. 1991. Particle properties and processes in Uranus’ rings. In Uranus, ed. J. Bergstralh, E. D. Miner, and M. Matthews, pp. 410–465. Tucson: Univ. of Arizona Press.

Abstract: The Uranian rings provide some confirmations but also many new challenges for ring physics. The Voyager observations complement ground-based data in defining the details of the ring environment, in the discovery of small moons and dust among the rings, and in providing strong indications that the rings are younger than the solar system. Current analytic and numerical models can explain many details, but must be extended to be fully realistic. Occultation observations of the ring are interpreted to yield an effective size of the ring particles that exceeds 70 cm, a surface mass density that exceeds 80 g cm -2, and a ring vertical thickness greater than tens of meters for solid ice particles. The particles forming the classical rings are dark and gray with albedo 0 = 0.014 ± 0.004. The small amount of dust ( < 2 x 10-3) that exists in the classical rings and between the rings in bands ( 1.5 x 10-5) is likely created by erosion of ring particles and unseen satellites resulting from collisions and micrometeoroid bombardment. As proposed for regions of the other known ring systems, new ring material can be continually created by the destruction of small moons near the rings. This phenomenon may explain the youthful appearance of the Uranian rings. Otherwise, the confinement of the rings over the age of the solar system requires some strong mechanism that is yet to be understood. 1991 Arizona Board of Regents.

Moreau, D., L. W. Esposito, and G. Brasseur. 1991. The chemical composition of the dust-free Martian atmosphere: Preliminary results of a two-dimensional model. J. Geophys. Res. 96, 7933–7945. LASP reprint 573.

Abstract: This paper describes a two-dimensional model of the Martial atmosphere in which chemical, radiative and dynamical processes ae treated interactively. The model is developed for a carbon dioxide-hydrogen-oxygen-nitrogen atmosphere and provides estimates of concentrations for 19 chemical species. The dynamical equations are expressed in the transformed Eulerian coordinates. The wave driving and eddy mixing coefficients resulting from gravity and Rossby wave absorption are computed consistently with the evolving distribution of the mean zonal wind. The net diabatic heating/cooling rate is derived from a detailed radiative scheme including the contributions of CO2, O3, H2O and O2, and is computed consistently with the calculated distribution of temperature and trace species quantities. The computed temperature field as well as the meridional and seasonal variations of ozone column abundance are in good agreement with the distributions observed by Mariner 9 and Viking spacecraft and the results obtained by previous studes. The present version of the model does not include the effects of dust, clouds and polar hood; only the chemistry in a dust-free atmosphere is considered. 1991 American Geophysical Union.

Moroz, V. I., E. V. Petrova, L. V. Ksanfomality, O. F. Ganpantzerova, N. V. Goroshkova, A. V. Zharkov, G. E. Nikitin, L. Esposito, J.-P Bibring, M. Combes, and A. Soufflot. 1991. Characteristics of aerosol phenomena in the Martian atmosphere from KRFM experiment data. Plan. Space Sci. 39, 199–207. LASP reprint 523.

Abstract: Photometric limb-to-limb profiles of Mars were obtained in eight narrow bands between 315 and 550 nm from the Phobos 2 spacecraft.. Tentative results of the analysis are presented in terms of optical properties of the atmosphere and surface.. The imaginary part of the refraction index. from 0.01 to 0.03 for 315 nm and from 0.005 to 0.01 for 550 m was estimated for the “constant” dust haze, using Mie theory for spherical particles. These values of are a few times higher than obtained by laboratoryy tests of terrestrial analogues, including basalt, andesit, and montmorillonite. Two explanations are possible: influence of irregular shape of particles and/or the presence of some more absorbing substances (such as goethite). Particle sizes of a few tenths of micrometres having a refraction index. of 1.55 are compatible with the discussed model of the “constant” haze. The full shape of the near equatorial photometric profile on 550 nm can be explained by the slightly absorbed atmosphere (with optical depth 0.4 and imaginary part of the refraction index 0.0075) above the moonlike (roughness factor q 0) surface.Icy particles with the same average sizes as in the haze (rm = 0.4 m) but with more narrow size distribution can explain the bright spot above Arsia Mons. Optical depth 0.1 and column mass density 7 10-5 g cm-2 of the icy clouds were evaluated.

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Brophy, T. G., L. W. Esposito, G. R. Stewart, and P. A. Rosen. 1992. Numerical simulation of satellite-ring interactions: Resonances and satellite-ring torques. Icarus 100, 412–433. LASP reprint 554.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-470F45N-8F&_coverDate=12%2F31%2F1992&_alid=415801008&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=52bfb1f688e682390f87254149dfe691

Abstract: A kinetic equation is solved numerically for a flattened planetary ring which is perturbed gravitationally by a nearby satelllite. The Krook kinetic equation for planetary rings is solved in two spatial dimensions and in time, with interparticle collisions, with satellite forcing, and without self-gravity. The phase-space fluid numerical method is extended to two-dimensional systems by ignoring negligible high-order velocity moments. In simulations of satellite-induced wakes, we verify the role of local shear reversal and angular momentum flux decrease as described by Borderies et al. (1983, Icarus 55, 124–132). A new result is that the amplitude of wakes is limited by purely kinematic effects, even in the absence of collisions. The results of a simulation of an inner Lindblad resonance location, as the distribution approaches steady-state, are presented in detail. The surface mass density, pressure tensor components, and mean velocities are calculated during the simulation. The detailed mechanisms of local torque balance and force eccentricity “damping” at resonance are illuminated. The angular momentum flux perturbations play the critical role of balancing the satellite torque on the surface mass density asymmetry. The eccentricities very close to resonance are limited by collisional phase mixing. The resulting near steady-state distribution feels a net radially integrated torque of essentially zero from the satellite to the first-order accuracy intended by the method, though there are large negative and positive torques on the regions just inside of and outside of the exact resonance. We show that previous analytic calculations of the torque (which show nonzero net torque) were the result of small second-order deviations of the surface density on top of the large first-order structure which our simulations reproduce, while there are second-order effects not accounted for in those calculations. Our simulations do not show an increase of the velocity dispersion in the resonance zone, so that energy conservation considerations do not require a net torque due to conservation of the Jacobi constant of the planar restricted three-body problem. We therefore argue that simulations or calculations which include all second-order effects are needed in order to determine the torques which will actually occur in rings. 1992 Academic Press, Inc.

Colwell, J. E., and L. W. Esposito. 1992. Origins of the rings of Uranus and Neptune, I: Statistics of satellite disruptions. J. Geophys. Res. (Planets ) 97, 10227–10241. LASP reprint 545.

Online at: http://www.agu.org/pubs/crossref/1992/92JE00788.shtmlAbstract: Stochastic simulations of the collisional fragmentation of the small moons of Neptune and

Uranus confirm the conclusions of B. A. Smith et al. (1986, in Satellites of Jupiter, ed. D. Morrison, pp. 277–339, Univ. of Arizona Press, Tucson; and 1989, Science 246, 1422–1449) that many of these moons cannot have survived intact since the end of planetary formation. We perform two types of stochastic simulations of the collisional history of small moons. Monte Carlo simulations in which only the largest surviving fragment from each disruption is followed show bimodal probability distributions for the size of the largest fragment. Once the moon is destroyed the first time, the collisional cascade to smaller sizes proceeds relatively quickly. A Markov chain approach allows us to follow the size distribution from each disruption to arbitrarily small sizes. These simulations show that the more numerous smaller fragments can outlive the largest fragment followed in the Monte Carlo formalism. We find that the evolution of moon populations from catastrophic fragmentation is more complex than can be described by a simple break-up “time scale.” We rederive cratering rates using the mehod of Smith et al. (1986, 1989) for all the satellites of Uranus and Neptune with an improved crater scaling and modified impactor distributions These changes produced a higher number of predicted craters larger than the radii of the small moons of Uranus and Neptune than derived by Smith et al. (1986, 1989). Our more detailed simulations of the catastrophic fragmentation process also show a higher rate of disruptions than estimated by Smith et al. (1986, 1989). The smallest observed moons at Uranus and Neptune have calculated lifetimes against catastrophic fragmentation of less than 5 x 108 years. If we require that half the mass of a satellite is given escape velocity, we find lifetimes for Cordelia (1986U7), Naiad (1989N6), and Thalassa (1989N5) of 9 x 108, 2 x 109, and 4 x 109 years, respectively. We conclude that we are observing a collisionally evolved small satellite population around Neptune and Uranus and that some observed moons are gravitationally bound rubble piles that have undergone multiple disruptions. In order for the satellites to be primordial, the population of planet family

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comets in the outer solar system would have to be smaller than current estimates by at least a factor of 5. 1992 American Geophysical Union.

Esposito, L. W. 1992. Planetary rings: Running rings around modellers. Nature 360, 531–532.LASP reprint 922.

Online at: http://search.nature.com/search/?sp-a=sp1001702d&sp-sfvl-field=subject|ujournal&sp-t=results&sp-p=all&sp-q-2=Esposito&sp-x-2=uaui&sp-q-4=360&sp-x-4=uvolume&sp-d=custom&sp-start-day=01&sp-end-day=31&sp-s=date

Abstract: The extraordinary rings around Saturn are now known to have parallels around its neighbouring giant planets. The structure, origin, and diversity of these rings can be explained by a close relationship between the rings and a multitude of small moons orbiting nearby or among them. 1989 Macmillan Magazines Ltd.

Hord, C. W., W. E. McClintock, A. I. F. Stewart, C. A. Barth, L. W. Esposito, G. E. Thomas, B. R. Sandel, D. M. Hunten, A. L. Broadfoot, D. E. Shemansky, J. M. Ajello, A. L. Lane, and R. A. West. 1992. Galileo Ultraviolet Spectrometer experiment. Space Sci. Rev. 60, 503–530. LASP reprint 544.

Online at: http://adsabs.harvard.edu/abs/1992SSRv...60..503HAbstract: The Galileo ultraviolet spectrometer experiment uses data obtained by the Ultraviolet

Spectrometer (UVS) mounted on the pointed orbiter scan platform and from the Extreme Ultraviolet Spectrometer (EUVS) mounted on the spining part of the orbiter with the field of view perpendicular to the spin axis. The UVS is a Ebert-Fastie design that covers the range 113–432 nm with a wavelength resolution of 0.7 nm below 190 and 1.3 nm at longer wavelengths. The UVS spatial resolution is 0.4 deg x 0.1 deg for illuminated disc observations and 1 deg x 0.1 deg for limb geometries. The EUVS is a Voyager design objective grating spectrometer, modified to cover the wavelength range from 54 to 128 nm with wavelength resolution 3.5 nm for extended sources and 1.5 nm for point sources and spatial resolution of 0.87 deg x 0.17 deg. The EUVS instrument will follow up on the many Voyager UVS discoveries, particularly the sulfur and oxygen ion emissions in the Io torus and molecular and atomic hydrogen auroral and airglow emissions from Jupiter. The UVS will obtain spectra of emission, absorption, and scattering features in the unexplored, by spacecraft, 170–432 nm wavelength region. The UVS and EUVS instruments will provide a powerful instrument complement to investigate volatile escape and surface composition of the Galilean satellites, the Io plasma torus, micro- and macro-properties of the Jupiter clouds, and the composition structure and evolution of the Jupiter upper atmosphere. 1992 Kluwer Academic Publishers.

McClintock, W. E., G. M. Lawrence, R. A. Kohnert, and L. W. Esposito. 1992. Optical design of the ultraviolet imaging spectrograph for the Cassini mission to Saturn, from Instrumentation for Planetary and Terrestrial Atmospheric Remote Sensing. SPIE 1745, 26–38. LASP reprint 568.

Online at: http://bookstore.spie.org/index.cfm?fuseaction=DetailPaper&ProductId=60597&coden=Abstract: When the Cassini spacecraft arrives at Saturn early in the next century, it will carry an

Ultraviolet Imaging Spectrograph (UVIS) designed and built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. Observations made with the UVIS will support a broad set of scientific investigations including spectroscopy, imaging, and occultations. The UVIS consists of three spectroscopic channels covering the wavelength ranges 55–115 nm, 115–190 nm, and 160–320 nm. Each channel has an off-axis parabolic telescope followed by a toroidal grating spectrograph and an imaging microchannel plate-CODOCON detector. Mirror coatings and detector photocathode materials optimize the sensitivity of each channel for its particular wavelength range. Spectrograph entrance slit mechanisms provide four independent spectral and spatial resolution modes for each of the three channels. A fourth optical train consisting of an off-axis parabolic telescope and solar blind photomultiplier tube with a CsI photocathode provides a high sensitivity photometer mode within the UVIS. The UVIS configuration was selected as a balanced solution to a large number of engineering and scientific constraints. We describe these constraints, the optical design, and the anticipated performance of the instrument. Society of Photo-Optical Instrumentation Engineers.

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Colwell, J. E., and L. W. Esposito. 1993. Origins of the rings of Uranus and Neptune, 2: Initial conditions and ring moon populations. J. Geophys. Res. (Planets) 98, 7387–7401. LASP reprint 562.

Online (abstract only) at: http://www.agu.org/pubs/crossref/1993/93JE00329.shtmlAbstract: The smallest moons of the Jovian planets are unlikely to survive intact the flux of cometary

impactors in the outer solar system for billions of years. In paper 1 (Colwell and Esposito, 1992, J. Geophys. Res. 97, 10227–10241), we showed that the small moons of Uranus and Neptune are fragments or rubble pile agglomerations left over from some older, larger population of satellites. Catastrophic fragmentation occurs in ~108 years for these ring moons. The fate of the debris following a fragmenting impact is central to understanding the evolution of these satellites and the hypothesized origin of rings from their debris. In this extension of our earlier work, we examine the possible effects of the velocity distribution of fragments following a catastrophic fragmentation on satellite diminution via a collisional cascade. We compare our results with those presented in paper 1. Fragment velocities are critical in the evolution of the collisional cascade because of the possibility of reaccretion following disruption. Using our simulations of the collisional cascade, including the effects of the fragment velocity distribution, we estimate an unseen population of moons in the 1 to 10 km size range of ~1000 at Uranus and at Neptune. Using our model fragment velocity distribution, we calculate the initial phase space distribution of the new ring particles. This provides a physically realistic initial condition for simulations of the collisional evolution of planetary rings. We find that a narrow ring with a characteristic width of ~50 km is a natural outcome of the catastrophic disruption of satellites. 1993 American Geophysical Union.

Esposito, L. W. 1993. Understanding planetary rings. Ann. Rev. Earth Planet. Sci. 21, 487–523. LASP reprint 566.

Online at : http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.ea.21.050193.002415Abstract: Planetary rings, which were until recently thought unique to the planet Saturn, have now been

observed around all the giant planets. These rings are composed of many particles with a broad range in size. The observed ring systems are quite diverse. Jupiter’s ring is thin and composed of dust-like small particles. Saturn’s rings are broad, bright, and opaque. Uranus has narrow, dark rings among broad lanes of dust that are invisible from Earth. Neptune’s rings include incomplete arcs restricted to a small range of the circumference. The common occurrence of ring material around the planets is one of the major scientific findings of the past 15 years. We have a first order understanding of the dynamics and key processes in rings, much of it based on previous work in galactic and stellar dynamics. The rings are a kinetic system, where the deviations from perfect circular, equatorial motion can be considered as random velocities in a viscous fluid. Unfortunately, the models are often idealized and cannot yet predict many phenomena in the detail observed by spacecraft observations. This review briefly describes the rings in the solar system and the mathematical and physical approaches to understanding them. 1993 Annual Reviews, Inc.

McClintock, W. E., G. M. Lawrence, R. A. Kohnert, and L. W. Esposito. 1993. Optical design of the ultraviolet imaging spectrograph for the Cassini mission to Saturn. Opt. Eng. 32, 3038–3046. LASP reprint 612.

Online at: http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=OPEGAR000032000012003038000001&idtype=cvips&gifs=yes

Abstract: When the Cassini spacecraft arrives at Saturn early in the next century, it will carry an Ultraviolet Imaging Spectrograph (UVIS) designed and built by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. Observations made with the UVIS will support a broad set of scientific investigations including spectroscopy, imaging, and occultations. The UVIS consists of two spectroscopic channels covering the wavelength ranges 56 to 118 and 110 to 190 nm. Each channel has an off-axis parabolic telescope followed by a toroidal grating spectrograph and an imaging microchannel plate-CODACON detector. Mirror coatings and detector photocathode materials optimize the sensitivity of each channel for its particular wavelengthh range. Spectrograph entrance slit mechanisms provide three independent spectral and spatial resolution modes for each of the three channels. A third optical train consisting of a parabolic telescope and solar blind photomultiplier tube with a CsI photocathode provides a high-sensitivity photometer mode within the UVIS. The UVIS configuration was selected as a balanced solution to a large number of engineering and scientific constraints. We describe these constraints, the optical design, and the anticipated performance of the instrument. 1993 Society of Photo-Optical Instrumentation Engineers.

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Zasova, L. V., V. I. Moroz, L. W. Esposito, and C. Y. Na. 1993. SO2 in the middle atmosphere of Venus: IR measurements from Venera-15 and comparison to UV data. Icarus 105, 92–109. LASP reprint 584.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45PTG3T-22&_coverDate=09%2F30%2F1993&_alid=91678091&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=1a344a643d4f3e2eee9ff8d8255c34cb.

Abstract: We present and compare two sets of Venus SO2 measurements: IR spectra from the USSR Venera 15 orbiter and UV spectra from the US Pioneer Venus orbiter and sounding rockets. We choose the 40-mb pressure level for this comparison. At low and mid latitudes, both data sets give a mixing ratio (SO2) of tens of parts per billion and SO2 scale height (HSO2) of 1.5–2.5 km, which are in good agreement with photochemically predicted values. This confirms that photochemical processes dominate in this latitude range. Both data sets show the SO2 abundance increasing to several hundred parts per billion at high latitudes. The derived scale heights, however, are discrepant for high latitudes: IR data show great variability associated with dynamic features such as the cold collar, high diffuse clouds, the hot dipole and the polar cap; UV data show a decrease to HSO2 = 1 km. From the IR data, we derive HSO2 = 3–5 km outside the cold collar, and SO2 = 100–200 ppb in the hot dipole (increasing of SO2 = 1000 ppb in regions with high diffuse clouds). For the cold collar, the IR data yield SO2 = 1–10 ppb and HSO2 = 1 km. To explain the differences between the IR and UV results, we conclude that the SO2 scale height may decrease above 69 km altitude (P = 40 mb): the scale height differences are explained by the different viewing angles and the different opacity at UV and IR wavelengths. It is possible that temporal variations also contribute. 1993, 1999 Academic Press.

Na, C. Y., L. W. Esposito, W. E. McClintock, and C. A. Barth. 1994. Sulfur dioxide in the atmosphere of Venus, II. Modeling results. Icarus 112, 389–395. LASP reprint 646.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NK0GB-D&_coverDate=12%2F31%2F1994&_alid=91679143&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=f0ef106eea3a033ababad7686d4e52c0

Abstract: We have analyzed the results from the UV sounding rocket observations of Venus made on 15 September 1988 and 29 March 1991, which obtained high-resolution spectra of Venus clouds from 190 to 230 nm. The albedo of Venus in this wavelength range is dominated by absorption features of sulfur dioxide and sulfur monoxide. We estimate that the mixing ratio of SO2 above the clouds of Venus is 80 40 ppb for 1988 and 120 60 ppb for 1991. The scale height of SO2 at the same altitude region is 3 1 km for both 1988 and 1991. These numbers are in good agreement with both Pioneer Venus and IUE observations made around the same time period, and indicate that no large change in SO2 above the clouds occurred from 1982 to 1991. In addition, the SO mixing ratio above the clouds derived from the 1991 observations is 12 5 ppb, and the scale height of SO above the clouds is close to that of the bulk atmosphere. Our analyses indicate that the mixing ratio of SO decreases sharply below the 64-km level, which is in good agreement with the photochemical models. The mixing ratio of SO2 at the cloud top level derived from the 1988 observation ranges from 60 30 ppb at the equator to 300 150 ppb near 50S. The scale height of SO2 at the cloud top region ranges from about 3–4 km at the equator to ~2 km near 50S. Venera 15 observations show similar latitudinal variation of SO2. 1994, 1999 Academic Press, Inc.

Canup, R. M., and L. W. Esposito. 1995. Accretion in the Roche zone: Coexistence of rings and ringmoons. Icarus 113, 331–352. LASP reprint 640.

Online at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NK0GB-D&_coverDate=12%2F31%2F1994&_alid=91679143&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=f0ef106eea3a033ababad7686d4e52c0.

Abstract: Traditional accretion simulations predict rapid accumulation of ring debris into single satellites, while most theories of ring formation dismiss any accretion within the classical Roche limit. The former contradicts the continued presence of planetary rings, while the latter fails to adequately account for the many small satellites observed within ring systems. The coexistence of rings and small satellites thus challenges the premise of a strict boundary between accreting and nonaccreting regions. We have developed an accretion model designed to better examine accumulation processes in the dynamically transitional regime of outer planetary rings. We utilize “three-body” capture criteria, motivated by the work of K. Ohtsuki (1993, Icarus

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106, 228–246), to account for the effects of strong tidal forces on accretion. Our findings indicate that tidally modified accretion occurs in a relatively broad range of orbital radii surrounding the classical Roche limit. Tidally modified accretion has a very unique character: for a given particle density, only bodies which differ greatly in mass can remain gravitationally bound, as like-sized bodies overflow their mutual Hill sphere. We find that this constraint greatly limits the degree of accretional growth and prevents runaway accretion near the Roche limit. Numerical simulations show that through the course of tidally modified accretion, a fragmentation-produced debris distribution evolves into a bimodal population, with one element consisting of a swarm of small, high-velocity bodies and the other composed of a small number of large “moonlets” on fairly circular orbits. The latter are precluded from accreting with one another due to the tidal influences of the planet. Tidally modified accretion thus offers a natural explanation for the formation of systems of coexisting rings and ringmoons from disrupted parent bodies. 1995, 1999 Academic Press, Inc.

Hord, C. W., W. R. Pryor, A. I. F. Stewart, K. E. Simmons, J. J. Gebben, C. A. Barth, W. E. McClintock, L. W. Esposito, W. K. Tobiska, R. A. West, S. J. Edberg, J. M. Ajello, and K. L. Naviaux. 1995. Direct observations of the Comet Shoemaker-Levy 9 fragment G impact by Galileo UVS. Geophys. Res. Lett. 22, 1565–1568. LASP reprint 651.

Online at: http://www.agu.org/pubs/crossref/1995/95GL01414.shtmlAbstract: The Galileo Ultraviolet Spectrometer (UVS) team has detected the Shoemaker-Levy 9 fragment

G impact on Jupiter in data recently played back from the spacecraft tape recorder. A 20% brightening of the disc-integrated signal of Jupiter was detected at 292 nm during a swath across Jupiter that lasted 1.6 sec and was centered at 1994–July 18 (day 199)/07:33:31 UT (all times in this paper are corrected to be the time of the event as seen from Earth). The emission brightness, when combined with simultaneous photopolarimeter radiometer (PPR) measurements at 945 nm, is consistent with thermal radiation at a temperature of 7800 (+500, –600) K emitted over an area of 40 (+60, –25) km2. No excess signal was seen during swaths 5 1/3 sec before and after the detection swath. 1995 American Geophysical Union.

Porco, C. C., J. N. Cuzzi, L. W. Esposito, J. J. Lissauer, and P. O. Nicholson. 1995. Neptune’s ring system. In Neptune and Triton, ed. J. T. Bergstralh, E. D. Miner, and M. S. Mathews, pp. 703–804. Tucson: Univ. of Arizona Press.

Abstract: We review the current state of knowledge regarding the structure, particle properties, kinematics, dynamics, origin, and evolution of the Neptune rings derived from Earth-based and Voyager data. Neptune has a diverse system of five continuous rings—2 broad (Galle and Lassell) and 3 narrow (Adams, Le Verrier, and Arago)—plus a narrow discontinuous ring sharing the orbit of one of its ring-region satellites, Galatea. The outmost Adams ring contains the only arcs observed so far in Voyager images. These were responsible for most of the Earth-based stellar occultation detections of circum-Neptunian material in the mid-1980s. Three other Earth-based detections recorded material of some sort within Lassell and Galle, and interior to Galle. The five arcs vary in angular extent from ~1º to ~10º, and exhibit internal azimuthal structure with typical spatial scales of ~0.5º. All five lie within ~40º of longitude. Combined analysis of Earth-based occultation data and Voyager photometry yields typical optical depths of arcs ~ 0.1, and ~ 0.003 for the two narrow rings Adams and Le Verrier. Dust is present throughout the Neptune system, and measureable quantities of it were detected over Neptune’s north pole. The Adams ring (including the arcs) and the Le Verrier ring contain a significant fraction of dust, comparable to that observed in Saturn’s F ring; Lassell appears to have a different mix of particle sizes than Le Verrier or Adams. The Neptune ring particles are as dark as those in the rings of Uranus (with a single-scattering albedo ~ 0.04), are probably red, and may consist of ice “dirtied” with silicates and/or some carbon-bearing material. A kinematic model for the arcs derived from Voyager data, the arcs’ physical characteristics, and their orbital geometry and phasing are all roughly in accord with single-satellite arc shepherding by Galatea, though the presence of small kilometer-sized bodies embedded either within the arcs or placed at their Lagrange points may explain some inconsistencies with this model. Detailed numerical simulations demonstrate the plausibility of a model in which disrupted satellites provide the source of the Neptune rings and arcs. 1995 Arizona Board of Regents.

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Thiessenhusen, K. U., L. W. Esposito, J. Kurths, and F. Spahn. 1995. Detection of hidden resonances in Saturn’s B-ring. Icarus 113, 206–212. LASP reprint 923.

Online athttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NJJC1-4S&_coverDate=01%2F31%2F1995&_alid=91680444&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=dab23700872b6c0b8a54068dfc624bbe.

Abstract: The Voyager 2 Photopolarimeter experiment has yielded the highest resolved data of Saturn’s rings, exhibiting a wide variety of features. The B-ring region between 105,000 and 110,000 km from Saturn has been investigated. It has a high matter density and contains no significant features visible by eye. Analysis with statistical methods has led us to the detection of two significant events. These features are correlated with the inner 3:2 resonances of the F-ring shepherd satellites Pandora and Prometheus and may be evidence of large ring particles caught in the corotation resonances. 1995, 1999 Academic Press.

Canup, R. M., and L. W. Esposito. 1996. Accretion of the Moon from an impact-generated disk. Icarus 119, 427–446. LASP reprint 675.

Online at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45N4TKM-26&_user=918210&_handle=W-WA-A-A-AC-MsSAYVA-UUW-AUCWEDVEUU-CWCBVUZW-AC-U&_fmt=summary&_coverDate=02%2F29%2F1996&_rdoc=12&_orig=browse&_srch=%23toc%236821%231996%23998809997%23307688!&_cdi=6821&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=ba739ad3cc13757e09ba0cc0777954fb.

Abstract: We present the first published numerical calculations of accretion of an impact-generated protolunar disk into a single large moon. Our calculations are based on the model developed by R. M. Canup and L. W. Esposito (1995, Icarus 113, 331–352) to describe accretion in the Roche zones around the giant planets. Previous numerical simulations of a large impact event predict the formation of a disk of material centered near or within the Roche limit ( ). A natural expectation based on our previous results and comparison with the satellite systems of the outer planets would be for multiple small moons to arise from such a protolunar disk. Multiple moonlets could accrete to form a single moon if they evolved into crossing orbits due to tidal interaction with the Earth. This would occur if the innermost moonlet in the disk were also the most massive, so that it evolved outward at the relatively fastest rate and swept up all exterior material. Our calculations, which include both moonlet accretion and orbital evolution, demonstrate that forming massive moonlets in the inner disk near the Roche limit is extremely difficult. We conclude that an Earth system with multiple moons is the final result unless some particularly severe constraints on initial conditions in the disk are met. A disk with a lunar mass of material exterior to or an extremely steep radial surface density profile at the onset of collisional growth is required for a single, lunar-sized body to result from accretion of silicate density material in a protolunar disk. The former corresponds most closely to disks produced by impactors with nearly twice the mass of Mars and about twice the angular momentum of the current Earth/Moon system. Other processes, such as gravitational instability or primary accretion of an iron core in the inner disk, might be able to “seed” accretional growth and allow for the formation of a single moon if disk temperature and compositional requirements are met. Our analysis demonstrates the need for more detailed, higher resolution impact simulations. 1996 Academic Press.

Canup, R. M., and L. W. Esposito. 1997. Evolution of the G ring and the population of macroscopic ring particles. Icarus 126, 28–41. LASP reprint 700.

Online at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NJHJ6-M&_coverDate=03%2F31%2F1997&_alid=91681159&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=e1606f2eb11f031ae9830c15c686242f.

Abstract: An evolutionary model of the G ring incorporating theoretical results from R. M. Canup and L. W. Esposito (1995, Icarus 113, 331–352) yields a complete particle size distribution that is consistent with existing observations. Results from numerical modeling demonstrate that a G ring origin from the disruption of a 1.5–3 km progenitor satellite can match all known properties of the ring. In addition, we estimate the population of unseen macroscopic material from both observational upper limits and our theoretical model for the region surrounding the G ring, where the Cassini spacecraft will likely make its innermost passes in the

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Saturnian system. For models that fit all available data, the probability of Cassini striking a hazardous ring particle is less than 1%.  1997 Academic Press.

Esposito, L. W., J.-L. Bertaux, V. Krasnopolsky, V. I. Moroz, and L. V. Zasova. 1997. Chemistry of lower atmosphere and clouds. In Venus II, ed. S. W. Bougher, D. M. Hunten, and R. J. Phillips, pp. 415–458. Tucson: Univ. of Arizona Press.

Abstract: Venus is totally covered by clouds, up to 60 km in vertical extent. The photochemistry that creates the cloud aerosols grades into thermal chemistry that is dominant below the upper clouds. We review selected measurements from Venera, Vega, Hubble Space Telescope (HST), and ground-based telescopes that constrain the chemistry of the lower atmosphere and clouds. The in-situ Vega measurements of SO2 abundance in the deep atmosphere are at variance with earlier gas chromatograph measurements and with present chemical models. Russian and American measurements of SO2 in the upper cloud have been reconciled. SO2 continues its long decline seen since 1978 above the cloud tops.. We review chemical models of the atmosphere above the clouds and recent progress in modeling of the cloud layer and sub-cloud atmosphere. We still do not know the species responsible for Venus’ blue absorption. The existence of large and/or crystalline size modes of cloud particles is still open. Advances in understanding Venus atmospheric chemistry will require future measurements of sulfur and chlorine compounds. 1997 Arizona Board of Regents.

Na, C. Y., and L. W. Esposito. 1997. Is disulfur monoxide the second absorber on Venus? Icarus 125, 361–368. LASP reprint 695.

Online at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45M919J-11&_coverDate=02%2F28%2F1997&_alid=91681559&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=9a102ca0d0ce5015f37039b8890a8d06.

Abstract: The diminished spherical albedo of Venus from 200 to 500 nm requires at least two different absorbers in the cloud layer. SO2 has been identified as the dominant absorber between 200 and 320 nm; however, the identity of the second UV absorber which darkens the planet from 320 to 500 nm remains a mystery. We present the results from photochemical calculations of sulfur oxides in the atmosphere of Venus, which indicate that S2O is a strong candidate for the second absorber. The photochemical calculations are constrained by recent observations of SO and SO2 by a CU/LASP sounding rocket experiment carried out on 29 March 1991. We calculate that the photochemical lifetime of S2O in the middle atmosphere of Venus is much less than the dynamical mixing time scale, and this can explain the shortest time scales for changes in the dark markings. The calculated vertical profile of S2O from 58 to 96 km is consistent with constraints on the second absorber. In addition, since S2O is chemically derived from SO2, the observed spatial correlation between the second absorber and SO2 can be directly explained. The conclusion that S2O is an excellent candidate for the second absorber could be confirmed by laboratory measurements of its absorption cross section. 1977 Academic Press.

Esposito, L. W., J. E. Colwell, and W. E. McClintock. 1998. Cassini UVIS observations of Saturn’s rings. Planet. Space Sci. 46, 1221–1235. LASP reprint 753.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-3W1R5DR-F&_coverDate=10%2F09%2F1998&_alid=379977510&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5823&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=82d3f0636cddcad9928b4bd1f9334dc2

Abstract: The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of the Cassini Orbiter spacecraft. Its science objectives include investigation of the chemistry, clouds, and energy balance of the Titan and Saturn atmospheres; neutrals in the magnetosphere; D/H ratio for Titan and Saturn; and structure and evolution of Saturn’s rings. The rings of Saturn are the best studied of planetary rings and contain the majority of the ring material in the solar system. The four-year Cassini tour provides multiple observation opportunities and long time coverage.The UVIS observations include photometry, imaging, spectroscopy, and stellar occultations. This article describes the UVIS instrument and the observations it will make. 1998 Elsevier Science Ltd.

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Harri, A.-M., V. Linkin, J. Polkko, M. Marov, J.-P Pommereau, A. Lipatov, T. Siili, K. Manuilov, V. Lebedev, A. Lehto, R. Pellinen, R. Pirjola, T. Carpentier, C. Malique, V. Makarov, L. Khloustova, L. Esposito, J. Maki, G. Lawrence, and V. Lystsev. 1998. Meteorological observations on Martian surface: Met-packages of Mars—96 small stations and penetrators. Planet. Space Sci. 46, No. 6/7, 779–793. LASP reprint 744.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-3TN9M81-D&_coverDate=07%2F31%2F1998&_alid=379978455&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5823&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=da36481467f2ba4c503d8129dd9826ac

Abstract: The scientific objectives of a meterological experiment on the Martian surface are defined, and the meteorological equipment of the landing elements of the Mars-96 mission are described with emphasis on the applicability for re-use in forthcoming Mars missions. The general strategy for atmospheric surface observations is discussed. Meteorological surface observations are of utmost value in studying the Martian atmosphere. The climatological cycles and atmospheric circulations, as well as the boundary layer phenomena can be understood thoroughly only if the contributions of in situ surface measurements are amalgamated with the remote observations.  1998 Elsevier Science Ltd.

Throop, H. B., and L. W. Esposito. 1998. G ring particle sizes derived from ring plane crossing observations. Icarus 131, 152–166. LASP reprint 752.

Online athttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45K0YV5-C&_coverDate=01%2F31%2F1998&_alid=91681930&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=d02112abfb7d4c40c053bded6d6c3ca3.

Abstract: The Saturn ring plane crossings in 1995–1996 allowed observers using the Hubble Space Telescope and the W. M. Keck telescope to image the planet’s diffuse rings from 0.3 to 2.2 m at a scattering angle of . We calculate the G ring reflectance for size distributions of dust to kilometer-sized bodies derived from a physical, evolutionary model. The model tracks the evolution of the G ring from its initial formation following the disruption of a progenitor satellite (R. M. Canup and L. W. Esposito, 1997, Icarus 126, 28–41) until a steady state distribution is reached. We calculate the total particle scattering from contributions due to Mie scattering, isotropic scattering, and Lambert scattering and compare the spectra, phase curves, and RMS particle mass from our physical model to that observed by HST, Keck, and Voyager. A range of particle size distributions from the models is consistent with the observations. These distributions have a dust component that can be described by the differential power law exponent qdust, in the range 1.5–3.5. A quasi-Gaussian size distribution centered at 15 m also matches the observations, although not predicted by the evolutionary model. Distributions with , such as that proposed by M. R. Showalter and J. N. Cuzzi (1993, Icarus 103, 124–143) based on Voyager G ring photometry, are too blue to match the spectrum. In order to fit the visible optical depth, many of the models require longer particle lifetimes against plasma drag than Voyager plasma measurements imply. This may suggest that plasma densities are overestimated, that the ring has unaccounted-for dust sources, or that the ring is not in steady state and we are seeing it at a particularly bright moment. 1998 Academic Press.

Colwell, J. W., L. W. Esposito, and D. Bundy. 2000. Fragmentation rates of small satellites in the outer solar system. J. Geophys. Res. 105, 17,589–17,599. LASP reprint 806.

Online at: http://www.agu.org/pubs/crossref/2000/1999JE001209.shtmlAbstract: The narrow rings of Uranus and Neptune exist in a system of observed and hypothesized small

moons. Catastrophic fragmentation of these moons by comet impact has been proposed as the mode of origin of those rings, and earlier efforts to model the process showed that small moons are destroyed by impact on short timescales, leading to rapid collisional erosion of any primordial satellite system (J. E. Colwell and L. W. Esposito, 1992, J. Geophys. Res. 97, 10227–10241). We reexamine the question of impact fragmentation of small satellites in the light of new observational data on the population of Kuiper Belt and Centaur objects that produce the impacting flux and new theoretical and computational studies of catastrophic fragmentation. We find that the impacting flux used by Colwell and Esposito (1992) is consistent with the new observations of Juiper Belt objects and calculations of their transport into the solar system. However, new fragmentation criteria from modeling of the asteroid belt and hydrocode simulations lengthen the model collisional lifetimes of satellite systems. The observed distribution of rings, dust bands, and moons at Uranus and Neptune suggest

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a catastrophic disruption model with a relatively weak dependence on target radius. 2000 American Geophysical Union.

Throop, H. B., J. Bally, L. W. Esposito, and M. J. McCaughrean. 2001. Evidence for dust grain growth in young circumstellar disks. Science 292, 1686–1689. LASP reprint 899.

Online athttp://www.sciencemag.org/cgi/content/abstract/292/5522/1686?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Throop+H&searchid=1053022817523_8339&stored_search=&FIRSTINDEX=0&fdate=10/1/1995&tdate=5/31/2003.

Abstract: Hundreds of circumstellar disks in the Orion nebula are being rapidly destroyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form planetary systems. Nevertheless, the first stage of planet formation—the growth of dust grains into larger particles—may have begun in these systems. Observational evidence for these large particles in Orion’s disks is presented. A model of grain evolution in externally irradiated protoplanetary disks is developed and predicts rapid particle size evolution and sharp outer disk boundaries. We discuss implications for the formation rates of planetary systems.

Barbara, J. M., and L. W. Esposito. 2002. Moonlet collisions and the effects of tidally modified accretion in Saturn’s F ring. Icarus 160, 161–171. LASP reprint 864.

Online athttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45K0YV5-C&_coverDate=01%2F31%2F1998&_alid=91681930&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=d02112abfb7d4c40c053bded6d6c3ca3.

Abstract: We both test and offer an alternative to a meteoroid bombardment model (M. R. Showalter 1998, Science 282, 1099–1102) and suggest that anomalous localized brightenings in the F ring observed by Voyager result from disruptive collisions involving poorly consolidated moonlets, or “rubble piles.” This model can also explain the transient events observed during ring plane crossing. We have developed an evolutionary model that considers both the competing effects of accretion and disruption at the location of the F ring. Our numerical model is a Markov process where probabilities of mass transfer between the states of the system form a “transition matrix.” Successive multiplications of this matrix by the state vector generate expectation values of the distribution after each time step as the system approaches quasi-equilibrium. Competing effects of accretion and disruption in the F ring are found to lead to a bimodal distribution of ring particle sizes. In fact, our simulation predicts the presence of a belt of kilometer-sized moonlets in the F ring. These moonlets may continually disrupt one another and re-accrete on short time scales. We also agree with J. N. Cuzzi and J. A. Burns (1988, Icarus 74, 284–324), who suggest that the classical F ring itself may be the consequence of a relatively recent collision between two of the largest of these yet unseen objects. Cassini observations can confirm the existence of the moonlet belt by directly observing these objects or the waves they create in the rings. 2002 Elsevier Science (USA).

Blanc, M., S. Bolton, J. Bradley, M. Burton, T.E. Cravens, I. Dandouras, M.K. Dougherty, M.C. Festou, J. Feynman, R.E. Johnson, T.G. Gombosi, W.S. Kurth, P.C. Liewer, B.H. Mauk, S. Maurice, D. Mitchell, F.M. Neubauer, J.D. Richardson, D.E. Shemansky, E.C. Sittler, B.T. Tsurutani, P. Zarka, L.W. Esposito, E. Grun, D.A. Gurnett, A.J. Kliore, S.M. Krimigis, D. Southwood, J.H. Waite, and D.T. Young. 2002. Magnetospheric and plasma science with Cassini-Huygens. Space Science Reviews. 104, Issue 1-2, 253-346. LASP reprint 1057.

Online at: http://www.springerlink.com/(wp5s4355vepzfhyuvam0yz45)/app/home/contribution.asp?referrer=parent&backto=searchcitationsresults,8,16;

Abstract: Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an `induced' magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturn's `intrinsic' magnetosphere, the magnetic cavity Saturn's planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment. The flow of magnetospheric plasma around the obstacle, caused by Titan's atmosphere/ionosphere, produces an elongated

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cavity and wake, which we call an `induced magnetosphere'. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titan's ionosphere, which is the obstacle to the external plasma flow. We then study Titan's induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan. Saturn's magnetosphere, which is dynamically and chemically coupled to all other components of Saturn's environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturn's upper atmosphere, and the source of Saturn's auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them. Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other. Saturn's magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment ' extended solid surfaces, dust and gas clouds, plasma and energetic particles ' are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form. © Springer 2002.

Cuzzi, J.N., J.E. Colwell, L.W. Esposito, C.C. Porco. C.D. Murray, P.D. Nicholson, L.J. Spilker, E.A. Marouf, R.C. French, N. Rappaport, and D. Muhleman. 2002. Saturn's rings: Pre-Cassini status and mission goals. Space Science Reviews. 104. Issue 1-2, 209-251. LASP reprint 1008.

Online at: http://www.springerlink.com/(wp5s4355vepzfhyuvam0yz45)/app/home/contribution.asp?referrer=parent&backto=searchcitationsresults,1,1 ABSTRACT?

Abstract: Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized. © Springer 2002.

Esposito, L. W. 2002. Planetary rings. Rep. Prog. Phys. 65, 1741–1783. Planetary rings. LASP reprint 874.

Online at: http://stacks.iop.org/RoPP/65/1741Abstract: In our solar system, planetary rings are found around all the giant planets, showing spectacular

variety. Jupiter’s thin ring system is composed mostly of dust. Saturn’s rings are the largest and best studied, and the target of the NASA/ESA Cassini space mission that will begin orbiting Saturn in 2004. Its ring system consists of the broad A and B rings (separated by the Cassini division) and the optically thinner C and D rings. Outside the main rings are the narrow “braided” F ring and rings E and G. Uranus has 10 narrow, sometimes eccentric rings and a family of dust bands. Neptune has three distinct rings (Galle, LeVervier, and Adams); the outermost Adams ring is patchy, with the thicker segments termed “arcs.” All the ring systems have moons interspersed, which sculpt, collect, and release ring material. Moons are the likely parents of the present rings, ground down by meteorites and destroyed randomly to produce the relatively short-lived ring systems. Thus, we observe the natural stochastic results of birth and death processes when we examine the rings closely. Ring systems are relatively nearby and provide a natural laboratory for phenomena in flattened disks, including the nebula around our Sun that gave rise to the planets. Cassini will observe Saturn’s rings and the numerous physical phenomena occurring within them close-up from 2004 to 2008, refining and possibly redefining our view of ring physics. 2002 IOP Publishing Ltd.

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Esposito, L. W. 2003. Cassini Imaging at Jupiter. Science 299,1529-1530. LASP reprint 900. Online at: http://www.sciencemag.org/cgi/content/full/299/5612/1529?

ijkey=J5/90d7cDsytE&keytype=ref&siteid=sci. Abstract: The Voyager and Galileo spacecraft have provided tantalizing glimpses of Jupiter's active

atmosphere and its rings and moons. Now, on its way to Saturn, Cassini has succeeded in imaging the giant planet continuously for 6 months. In his Perspective, Esposito discusses the imaging results reported by Porco et al. The data provide unprecedented insights into atmospheric phenomena such as giant storms and into the source of Jupiter's rings.

Richardson, L. J., D. Deming, G. Wiedemann, C. Goukenleuque, D. Steyert, J. Harrington, and L. W. Esposito. 2003. Infrared observations during the secondary eclipse of HD 209458b, I. 3.6-micron occultation spectroscopy using the VLT. Astrophys. J. 584, 1053-1062. LASP reprint 998.

Online at: http://www.arXiv.org/find/astro-ph/1/au:+Richardson_L/0/1/0/all/0/1Abstract: We search for an infrared signature of the transiting extrasolar planet HD 209458b during

secondary eclipse. Our method, which we call “occultation spectroscopy,” searches for the disappearance and reappearance of weak spectral features due to the exoplanet as it passes behind the star and later reappears. We argue that at the longest infrared wavelengths, this technique becomes preferable to conventional “transit spectroscopy.” We observed the system in the wing of the strong 3 band of methane near 3.6 m during two secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution of 3300. Our analysis, which utilizes a model template spectrum, achieves sufficient precision to expect detection of the spectral structure preducted by an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium model for the exoplanet atmosphere. However, our observations show no evidence for the presence of this spectrum from the exoplanet, with the statistical significance of the non-detection depending on the timing of the secondary eclipse, which depends on the assumed value for the orbital eccentricity. Our results reject certain specific models of the atmosphere of HD 209458b as inconsistent with our observations at the 3 level, given assumptions about the stellar and planetary parameters.

Brooks, S. M., L. W. Esposito, M. R. Showalter, and H. B. Throop. 2004. The size distribution of Jupiter’s main ring from Galileo imaging and spectroscopy. Icarus. 170, 35-57. LASP reprint 973. Online at: : Brooks, S. M., L. W. Esposito, M. R. Showalter, and H. B. Throop. 2004. The size distribution of Jupiter’s main ring from Galileo imaging and spectroscopy. Icarus, 170, 35-37. LASP reprint 973.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4CB64G4-1&_user=3058215&_coverDate=07%2F31%2F2004&_alid=257618260&_rdoc=10&_fmt=full&_orig=search&_cdi=6821&_sort=d&_st=4&_docanchor=&_acct=C000047944&_version=1&_urlVersion=0&_userid=3058215&md5=265af6189d606c16892187e429634fbf

Abstract: Galileo’s Solid State Imaging (SSI) experiment obtained 36 visible wavelength images of Jupiter’s ring system at a variety of phase angles during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188–213). The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter’s main ring during orbit C3 at wavelengths from 0.7 to 5.2 um; a second observation was attempted during orbit E4. We analyze the high-phase-angle NIMS and SSI observations of the main ring, which were obtained during Galileo’s third and fourth orbits, to constrain the size distribution of the micron-sized dust population in the main ring. This portion of the population is best constrained at high phase angles since the small dust grains dominate the light scattering at these geometries, and contributions from larger parent bodies in the ring are negligible.

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Esposito, L. W., C. A. Barth, J. E. Colwell, G. M. Lawrence, W. E. McClintock, A. I. F. Stewart, H. U. Keller, A. Korth, H. Lauche, M. C. Festou, A. L. Lane, C. J. Hansen, J. N. Maki, R. A. West, H. Jahn, R. Reulke, K. Warlich, D. E. Shemansky, and Y. L. Yung. 2004. The Cassini Ultraviolet Imaging Spectrograph investigation. Space Sci. Rev. 115, 294-361. LASP reprint 999.

Online at: http://www.springerlink.com/content/q2727598rq066n86/Abstract: The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of

the Cassini Orbiter spacecraft. UVIS has two spectrographic channels that provide images and spectra covering the ranges from 56 to 118 nm and 110 to 190 nm. A third optical path with a solar blind CsI photocathode is used for high signal-to-noise-ratio stellar occultations by rings and atmospheres. A separate Hydrogen Deuterium Absorption Cell measures the relative abundance of deuterium and hydrogen from their Lyman-alpha emission. The UVIS science objectives include investigation of the chemistry, aerosols, clouds, and energy balance of the Titan and Saturn atmospheres; neutrals iin the Saturn magnetosphere; the deuterium-to-hydrogen (D/H) ratio for Titan and Saturn; icy satellite surface properties; and the structure and evolution of Saturn’s rings.

Esposito, L.W., J. E. Colwell, K. Larsen, W. E. McClintock, A. I. F. Stewart, J. Tew Hallett. D. E. Shemansky, J. M. Ajello, C. J. Hansen, A. R. Hendrix, R. A. West, H. U. Keller, A. Korth, W. R. Pryor. R. Reulke, and Y. L. Yung. 2005. Ultra-Violet Imaging Spectroscopy shows an active Saturn system. Science. 307, 1251-1255. LASP reprint 1000.

Online at http://www.sciencemag.org/cgi/content/full/307/5713/1251Abstract: Neutral oxygen in the Saturn system shows high variability, with rapid production and loss. The

total number of oxygen atoms peaks at 4 x 1034. The Saturn aurora brighten in response to solar wind forcing. Saturn’s auroral spectrum closely resembles Jupiter’s. Phoebe’s surface shows variable water ice content and the observational evidence indicates it originated in the outer solar system, although no cometary activity is detected. Saturn’s rings also show variable water abundance, with the purest ice in the outermost A ring. This radial variation is consistent with initially pure water ice bombarded by meteorites, but smaller radial structures may indicate collisional transport and recent renewal events in the last 107 – 108 years.

Ajello J.M., W. Pryor, L.W. Esposito, A.I.F. Stewart, W. McClintock, J. Gustin, D. Grodent, J.-C. Gerard, and J.T. Clarke. The Cassini Campaign Observations of the Jupiter Aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph. 2005. Icarus 178 327-345. LASP reprint 1025.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_aset=V-WA-A-W-A-MsSAYZA-UUW-U-AAVCVYUYZE-AAVBUZAZZE-DBYCYBBDZ-A-U&_origAset=V-WA-A-W-W-MsSAYWW-UUA-U-AAVCVYUYZE-AAVBUZAZZE-DBYCYAYBY-W-U&_rdoc=1&_fmt=full&_udi=B6WGF-4H2PJT3-1&_coverDate=11%2F15%2F2005&_cdi=6821&_orig=search&_st=13&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=3b787f2079c617d80919e5bc74d48ecf

Abstract: We have analyzed the previous Cassini Ultraviolet Imaging Spectrometer (UVIS) observations of the Jupiter aurora with an auroral atmosphere two-stream electron transport code. The observations of Jupiter by UVIS took place during the previous tCassini Campaign. The Cassini Campaign included support spectral and imaging observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). A major result for the UVIS observations was the identification of a large color variation between the far ultraviolet (FUV: 1100–1700 Å) and extreme ultraviolet (EUV: 800–1100 Å) spectral regions. This change probably occurs because of a large variation in the ratio of the soft electron flux (10–3000 eV) responsible for the EUV aurora to the hard electron flux (~15–22 keV) responsible for the FUV aurora. On the basis of this result a new color ratio for integrated intensities for EUV and FUV was defined (4πI1550–1620 Å /4πI1030–1150 Å) which varied by approximately a factor of 6. The FUV color ratio (4πI1550–1620 Å /4πI1230–1300 Å) was more stable with a variation of less than 50% for the observations studied. The medium resolution (0.9 Å FWHM, G140M grating) FUV observations (1295–1345 Å and 1495–1540 Å) by STIS on 13 January 2001, on the other hand, were analyzed by a spectral modeling technique using a recently developed high-spectral resolution model for the electron-excited H2 rotational lines. The STIS FUV data were analyzed with a model that considered the Lyman band spectrum (B ) as composed of an allowed direct excitation component (X ) and an optically forbidden component

(X followed by the cascade transition ). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen to emphasize the cascade component. The ratio

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of the two components is a direct measurement of the mean secondary electron energy of the aurora. The mean secondary electron energy of the aurora varies between 50 and 200 eV for the polar cap, limb and auroral oval observations. We examine a long time base of Galileo Ultraviolet Spectrometer color ratios from the standard mission (1996–1998) and compare them to previous Cassini UVIS, HST, and International Ultraviolet Explorer (IUE) observations.

Pryor, W.R., A. I. F. Stewart,. L. W. Esposito and, J. E. Colwell, A. J. Jouchoux, A. J. Steffl, D. E. Shemansky, J. M. Ajello, R. A. West, C. J. Hansen, B. T. Tsurutani, W. S. Kurth, G. B. Hospodarsky, D. A. Gurnett, K. C. Hansen, J. H. Waite, Jr., F. J. Crary, D. T. Young, N. Krupp, J. T. Clarke, D. Grodent, and M. K. Dougherty. Cassini UVIS observations of Jupiter's auroral variability. 2005. Icarus, 178, 312–326. LASP reprint 1058.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4GY878X-2&_user=918210&_coverDate=11%2F15%2F2005&_alid=379982032&_rdoc=1&_fmt=full&_orig=search&_cdi=6821&_sort=d&_st=4&_docanchor=&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=e3134a16410fbe0c80c001a94cafe350

Abstract: The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ~2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

Shemansky, D.E., A.I.F. Stewart, R.A. West, L.W. Esposito, J.T. Hallet, and X. Liu. 2005. The Cassini UVIS stellar probe of the Titan atmosphere. Science. 308, 978-982. LASP reprint 1012.

Online at: http://www.sciencemag.org/cgi/content/full/308/5724/978?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=shemansky&searchid=1141151017288_4494&FIRSTINDEX=0&journalcode=sci

Abstract: The Cassini Ultraviolet Imaging Spectrometer (UVIS) observed the extinction of photons from two stars by the atmosphere of Titan during the third Titan flyby. Six species were identified and measured: CH4, C2H2, C2H4, C2H6, C4H2, and HCN. The observations cover altitudes from 450 km to 1600 km above the surface. A mesopause is inferred from extraction of the temperature structure of CH4, located at 615 km with a temperature minimum of 114 K. The asymptotic kinetic temperature at the top of the atmosphere determined from this experiment is 151 K. The higher order hydrocarbons and HCN peak sharply in abundance and are undetectable below altitudes ranging from 750 to 600 km, leaving CH4 as the only identifiable carbonaceous molecule in this experiment below 600 km.

Esposito, L. W. 2006. Planetary Rings, Cambridge, UK: Cambridge University Press.

Colwell, J. E., L. W. Esposito, and M. Sremcevic. 2006. Gravitational Wakes in Saturn’s A ring measured by Stellar Occultations from Cassini. GRL. 33, L07201, doi:10.1029/2005GL025163. LASP reprint 1053.

Online at: http://www.agu.org/pubs/crossref/2006.../2005GL025163.shtml

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Hansen , C. J., L. W. Esposito, A. I. F. Stewart, J. Colwell, A. R. Hendrix, W. Pryor, D. E. Shemansky, and R. A. West. 2006. Enceladus’ Water Vapor Plume. Science. 311. no.5766. 1422-1425. LASP reprint 1061.

Online at: http://www.sciencemag.org/cgi/content/full/311/5766/1422?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=enceladus%27+water+vapor+plume&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

Abstract: The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

Ajello, J. M., M. H. Stevens, A. I. F. Stewart, K. Larsen, L. W. Esposito, J.E. Colwell, W. E. McClintock, G. Holsclaw, J. Gustin, W. R. Pryor. 2007. Titan Airglow Spectra from Cassini UVIS: I. EUV Analysis. Geophys. Res. Lett. 34, L24204, doi:10.1029/2007GL031555. LASP Reprint #1092.

Online at: http://www.agu.org/journals/gl/gl0724/2007GL031555/ Abstract. We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging

Spectrograph (UVIS) on Cassini. Using one spectral channel in the EUV from 561–1182 Å and one in the FUV from 1115–1913 Å, UVIS observed the disk on 13 December, 2004 at low solar activity. The EUV spectrum consists of three band systems of N2 (b 1∏u, b′ 1∑u+, c4′ 1∑u+ → X 1∑g+), while the FUV spectrum consists of one (a 1∏g → X 1∑g+). Both the EUV and FUV spectra contain many N I and N II multiplets that are produced primarily by photodissociative ionization. Spectral intensities of the N2 c4 ′ 1∑u+(v′ = 0) → X 1∑g+(v″ = 0–2) progression from 950–1010 Å are resolved for the first time. The UVIS observations reveal that the c4′ 1∑u+(0) → X 1∑g+ (0) vibrational band near 958 Å is weak and undetectable, and that N I multiplets near 953.2 and 964.5 Å are present instead. Magnetospheric particle excitation may be weak or sporadic, since the nightside EUV spectrum on this orbit shows no observable nitrogen emission features and only H Ly-β.

André, N.; M. Blanc, S. Maurice, P. Schippers, E. Pallier, T.I. Gombosi, K.C. Hansen, D.T. Young, F.J. Crary, S. Bolton, E.C. Sittler, H.T. Smith, R.E. Johnson, R. A. Baragiola, A. J. Coates, A. M. Rymer, M.K. Dougherty, N. Achilleos, C.S. Arridge, S.M. Krimigis, D.G. Mitchell, N. Krupp, D. C. Hamilton, I. Dandouras, D. A. Gurnett, W.S. Kurth, P. Louarn, R. Srama, S. Kempf, H.J. Waite, L.W. Esposito, J.T. Clarke. 2008. Identification of Saturn’s Magnetospheric Regions and Associated Plasma Processes: Synopsis of Cassini Observations During Orbit Insertion, Rev. Geophys. 46, RG4008, doi:10.1029/2007RG000238.

Online at: http://www.agu.org/journals/rg/rg0804/2007RG000238/ Abstract. Saturn's magnetosphere is currently studied from the microphysical to the global scale by the

Cassini-Huygens mission. During the first half of 2004, in the approach phase, remote sensing observations of Saturn's magnetosphere gave access to its auroral, radio, UV, energetic neutral atom, and dust emissions. Then, on 1 July 2004, Cassini Saturn orbit insertion provided us with the first in situ exploration of Saturn's magnetosphere since Voyager. To date, Saturn orbit insertion is the only Cassini orbit to have been described in common by all field and particle instruments. We use the comprehensive suite of magnetospheric and plasma science instruments to give a unified description of the large-scale structure of the magnetosphere during this particular orbit, identifying the different regions and their boundaries. These regions consist of the Saturnian ring system (region 1, within 3 Saturn radii (RS)) and the cold plasma torus (region 2, within 5–6 RS) in the inner magnetosphere, a dynamic and extended plasma sheet (region 3), and an outer high-latitude magnetosphere (region 4, beyond 12–14 RS). We compare these observations to those made at the time of the Voyager encounters. Then, we identify some of the dominant chemical characteristics and dynamical phenomena in each of these regions. The inner magnetosphere is characterized by the presence of the dominant plasma and neutral sources of the Saturnian system, giving birth to a very special magnetosphere dominated by water products. The extended plasma sheet, where the ring current resides, is a variable region with stretched magnetic field lines and contains a mixture of cold and hot plasma populations resulting from plasma transport processes. The outer high-latitude magnetosphere is characterized by a quiet magnetic field and an absence of plasma. Saturn orbit insertion observations enabled us to capture a snapshot of the large-scale structure of the Saturnian magnetosphere and of some of the main plasma processes operating in this complex environment. The analysis of the broad diversity of these interaction processes will be one of the main themes of magnetospheric and plasma science during the Cassini mission.

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Colwell, J.E., L. W.Esposito, M. Sremcevic, G. R. Stewart, and W. E. McClintock. 2007. Self-Gravity Wakes and Radial Structure of Saturn's B Ring. Icarus. 190, 127-144. LASP reprint 1089.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4NGKWDH-1&_user=918210&_coverDate=09%2F30%2F2007&_rdoc=11&_fmt=full&_orig=browse&_srch=doc-info(%23toc%236821%232007%23998099998%23665908%23FLA%23display%23Volume)&_cdi=6821&_sort=d&_docanchor=&_ct=22&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=005a51c4046dfe1462a7280eb315e317

Abstract: We analyze stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph and find large variations in the apparent normal optical depth of the B ring with viewing angle. The line-of-sight optical depth is roughly independent of the viewing angle out of the ring plane so that optical depth is independent of the path length of the line-of-sight. This suggests the ring is composed of virtually opaque clumps separated by nearly transparent gaps, with the relative abundance of clumps and gaps controlling the observed optical depth. The observations can be explained with a model of self-gravity wakes like those observed in the A ring. These trailing spiral density enhancements are due to the competing processes of self-gravitational accretion of ring particles and Kepler shear. The B ring wakes are flatter and more closely packed than their neighbors in the A ring, with height-to-width ratios <0.1 for most of the ring. The self-gravity wakes are seen in all regions of the B ring that are not opaque. The observed variation in total B ring optical depth is explained by the amount of relatively empty space between the self-gravity wakes. Wakes are more tightly packed in regions where the apparent normal optical depth is high, and the wakes are more widely spaced in lower optical depth regions. The normal optical depth of the gaps between the wakes is typically less than 0.5 and shows no correlation with position or overall optical depth in the ring. The wake height-to-width ratio varies with the overall optical depth, with flatter, more tightly packed wakes as the overall optical depth increases. The highly flattened profile of the wakes suggests that the self-gravity wakes in Saturn's B ring correspond to a monolayer of the largest particles in the ring. The wakes are canted to the orbital direction in the trailing sense, with a trend of decreasing cant angle with increasing orbital radius in the B ring. We present self-gravity wake properties across the B ring that can be used in radiative transfer modeling of the ring. A high radial resolution (not, vert, similar10 m) scan of one part of the B ring during a grazing occultation shows a dominant wavelength of 160 m due to structures that have zero cant angle. These structures are seen at the same radial wavelength on both ingress and egress, but the individual peaks and troughs in optical depth do not match between ingress and egress. The structures are therefore not continuous ringlets and may be a manifestation of viscous overstability.

Esposito, L.W., E.R. Stofan, T. Cravens. 2007. Exploring Venus. Introductory chapter to “Exploring Venus as a terrestrial planet”, AGU Monograph Series, Volume 176, 1-6. No LASP Reprint, book. NON-UVIS PAPEROnline at: http://www.agu.org/cgi-bin/agubooks?book=SEGM1764412

Abstract: As the search for planets in other solar systems picks up speed and more and more dedicated scientists turn their eyes toward the heavens, observations of our mysterious, cloud-covered “sister planet” Venus become more and more important. Venus’s evolution and geology are very similar to Earth’s, yet its acid clouds and lead-melting surface temperatures make it extremely unlikely as a habitat for any form of life as we know it. A deeper understanding of Venus’s geophysical and atmospheric characteristics could help guide planetary scientists and astronomers in their quest for earth-like planets. At the same time, Venus’s notorious “runaway greenhouse effect” may offer lessons for curbing our own potentially disastrous interference with our own planet’s climate and ensuring the own future prosperity of life on Earth. This book discusses how the study of Venus will aid our understanding of terrestrial and extra-solar planet evolution, with particular reference to surface and interior processes, atmospheric circulation, chemistry, and aeronomy. Incorporating results from the recent European Venus Express mission, Exploring Venus as a Terrestrial Planet examines the open questions and relates them to Earth and other terrestrial planets. This book is bound to stimulate thinking about those broader issues as the new Venus data arrive.

Markiewicz, W.J. , D.V. Titov,, N. Ignatiev, H.U. Keller, D. Crisp, S.S. Limaye, R. Jaumann, R. Moissl, N. Thomas, L. Esposito, S. Watanabe, B. Fiethe, T. Behnke, I. Szemerey, H. Michalik, H. Perplies, M. Wedemeier, I. Sebastian, W. Boogaerts, S.F. Hviid, C. Dierker, B. Osterloh, W. Boker, M. Koch, H. Michaelis, D. Belyaev, A. Dannenberg, M. Tschimmel, P. Russo, T. Roatsch, K.D. Matz. Venus Monitoring Camera for Venus Express. 2007. Planetary and Space Science. 55, 1701-1711. LASP reprint 1091.

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Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-4MWXPX5-2&_user=918210&_coverDate=10%2F31%2F2007&_rdoc=5&_fmt=full&_orig=browse&_srch=doc-info(%23toc%235823%232007%23999449987%23669897%23FLA%23display%23Volume)&_cdi=5823&_sort=d&_docanchor=&_ct=15&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=8e6a8f75889f4fc1fdee1f2dd2b5f688

Abstract. The Venus Express mission will focus on a global investigation of the Venus atmosphere and plasma environment, while additionally measuring some surface properties from orbit. The instruments PFS and SPICAV inherited from the Mars Express mission and VIRTIS from Rosetta form a powerful spectrometric and spectro-imaging payload suite. Venus Monitoring Camera (VMC)—a miniature wide-angle camera with 17.5° field of view—was specifically designed and built to complement these experiments and provide imaging context for the whole mission. VMC will take images of Venus in four narrow band filters (365, 513, 965, and 1000 nm) all sharing one CCD. Spatial resolution on the cloud tops will range from 0.2 km/px at pericentre to 45 km/px at apocentre when the full Venus disc will be in the field of view. VMC will fulfill the following science goals: (1) study of the distribution and nature of the unknown UV absorber; (2) determination of the wind field at the cloud tops (70 km) by tracking the UV features; (3) thermal mapping of the surface in the 1 μm transparency “window” on the night side; (4) determination of the global wind field in the main cloud deck (50 km) by tracking near-IR features; (5) study of the lapse rate and H2O content in the lower 6–10 km; (6) mapping O2 night-glow and its variability.

Mills, F.P., L.W. Esposito, Y.L. Yung. 2007. Atmpsoheric composition, chemistry and clouds. A chapter in “Exploring Venus as a terrestrial planet”, L.W. Esposito, E.R. Stofan, T. Cravens, Eds. AGU Monograph Series, Volume 176, 73-100. No LASP Reprint, book.Online at: https://www.agu.org/cgi-bin/agubookstore?book=SEGM1764412

Abstract: As the search for planets in other solar systems picks up speed and more and more dedicated scientists turn their eyes toward the heavens, observations of our mysterious, cloud-covered “sister planet” Venus become more and more important. Venus’s evolution and geology are very similar to Earth’s, yet its acid clouds and lead-melting surface temperatures make it extremely unlikely as a habitat for any form of life as we know it. A deeper understanding of Venus’s geophysical and atmospheric characteristics could help guide planetary scientists and astronomers in their quest for earth-like planets. At the same time, Venus’s notorious “runaway greenhouse effect” may offer lessons for curbing our own potentially disastrous interference with our own planet’s climate and ensuring the own future prosperity of life on Earth. This book discusses how the study of Venus will aid our understanding of terrestrial and extra-solar planet evolution, with particular reference to surface and interior processes, atmospheric circulation, chemistry, and aeronomy. Incorporating results from the recent European Venus Express mission, Exploring Venus as a Terrestrial Planet examines the open questions and relates them to Earth and other terrestrial planets. This book is bound to stimulate thinking about those broader issues as the new Venus data arrive.

Tian, F., A.I.F. Stewart, O. B. Toon, K. Larsen, L. W. Esposito. 2007. Monte Carlo Simulations of the water vapor plume on Enceladus. Icarus 188, 154–161. LASP reprint 1087.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4MV1B19-3&_user=918210&_coverDate=05%2F31%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=a4118fb96857dde15c1badd8e2655076

Abstract: Monte Carlo simulations are used to model the July 14, 2005 UVIS stellar occultation observations of the water vapor plumes on Enceladus. These simulations indicate that the observations can be best fit if the water molecules ejected along the Tiger Stripes in the South Polar region of Enceladus have a vertical surface velocity of 300–500 m/s at the surface. The high surface velocity suggests that the plumes on Enceladus originate from some depth beneath the surface. The total escape rate of water molecules is 4–6×1027 s-1, or 120–180 kg/s, consistent with previous works, and more than 100 times the estimated mass escape rate for ice particles. The average deposition rate in the South Polar region is on the order of 1011 cm-2 s-1, yielding a resurfacing rate as high as 3×10-4 cm/yr. The globally averaged deposition rate of water molecules is about one order of magnitude lower.

Ajello, J, J. Gustin, A.I.F. Stewart, K. Larsen, L.W. Esposito, W. Pryor, W. McClintock, M. H. Stevens, C.P. Malone and D. Dziczek. 2008. Titan airglow spectra from the Cassini Ultraviolet Imaging Spectrograph: FUV disk analysis. Geophys. Res. Lett. 35, L06102, doi:10.1029/2007GL032315. LASP Reprint #1106.

Online at: http://www.agu.org/pubs/crossref/2008/2007GL032315.shtmlAbstract: We present a spectral analysis of the far ultraviolet (FUV: 1150–1900 Å) disk airglow

observations of Titan's atmosphere by the Cassini Ultraviolet Imaging Spectrograph (UVIS). The FUV

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spectrum consists of emissions from the Lyman-Birge-Hopfield (LBH) band system of N2 excited by photoelectrons (a 1Πg → X 1Σg +), N I multiplets from solar photodissociative excitation of N2, resonantly scattered solar H Ly-α and sunlight reflected by N2 in the mesosphere-stratosphere and modified by aerosols (e.g., tholins) and hydrocarbon absorption. Below 1450 Å, the strongest emissions arise from H Ly-α with an intensity of 208 Rayleighs (R), LBH bands with an intensity of 43 R, and the N I multiplets with a combined intensity of 16 R. Above 1450 Å, most of the UVIS signal is due to reflected sunlight. Mixing ratios of tholins, C2H2, C2H4 and C4H2 have been derived from the reflected sunlight using a Rayleigh scattering model. The derived mixing ratios are in good agreement with Voyager infrared observations and with FUV photochemical models, assuming solar energy deposition above 1450 Å occurs near 250 km (Wilson and Atreya, 2004). We also present the first geometric albedo measurement of Titan from 1500–1900 Å.

Esposito, L.W., B. K. Meinke, J.E. Colwell, P.D. Nicholson, M.M. Hedman. 2008. Moonlets and Clumps in Saturn’s F Ring. Icarus. Vol 194/1, 278-289. LASP reprint 1102.

Online at: http://dx.doi.org/10.1016/j.icarus.2007.10.001 Abstract. Cassini UVIS star occultations by the F ring detect 13 events ranging from 27 m to 9 km in

width. We interpret these structures as likely temporary aggregations of multiple smaller objects, which result from the balance between fragmentation and accretion processes. One of these features was simultaneously observed by VIMS. There is evidence that this feature is elongated in azimuth. Some features show sharp edges. At least one F ring object is opaque and may be a “moonlet.” This possible moonlet provides evidence for larger objects embedded in Saturn's F ring, which were predicted as the sources of the F ring material by Cuzzi and Burns [Cuzzi, J.N., Burns, J.A., 1988. Icarus 74, 284–324], and as an outcome of tidally modified accretion by Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161–171]. We see too few events to confirm the bi-modal distribution which Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161–171] predict. These F ring structures and other youthful features detected by Cassini may result from ongoing destruction of small parent bodies in the rings and subsequent aggregation of the fragments. If so, the temporary aggregates are 10 times more abundant than the solid objects. If recycling by re-accretion is significant, the rings could be quite ancient, and likely to persist far into the future.

Hansen, C. J., Esposito, L.W., Stewart, A.I.F., Meinke, B., Wallis, B., Colwell, J., Hendrix, A.R., Larsen, K., Pryor, W., Tian, F. 2008. Water Vapor Jets in Enceladus’ Plume. Nature. 456, 477-479.

Online at: http://www.nature.com/nature/journal/v456/n7221/abs/nature07542.htmlAbstract: A plume of water vapour escapes from fissures crossing the south polar region of

the Saturnian moon Enceladus1–6. Tidal deformation of a thin surface crust above an internal ocean could result intensile and compressive stresses that would affect the width of the fissures7; therefore, the quantity of water vapour released at different locations in Enceladus’ eccentric orbit is a crucial measurement of tidal control of venting. Here we report observations of an occultation of a star by the plume on 24 October 2007 that revealed four high-density gas jets superimposed on the background plume. The gas jet positions coincide with those of dust jets reported elsewhere8 inside the plume. The maximum water column density in the plume is about twice the density reported earlier2. The density ratio does not agree with predictions7—we should have seen less water than was observed in 2005. The ratio of the jets’ bulk vertical velocities to their thermal velocities is 1.560.2, which supports the hypothesis that the source of the plume is liquid water, with gas accelerated to supersonic velocity in nozzle-like channels9.

Pryor, W., P. Gangopadhyay, B. Sandel, T. Forrester, E. Quemerais, E. Moebius, L. Esposito, I. Stewart, B. McClintock, A. Jouchoux, J. Colwell, V. Izmodenov, Y. Malama, K. Tobiska, D. Shemansky, J. Ajello, C.Hansen, and M. Bzowski. 2008. Radiation transport of heliospheric Lyman-alpha from combined Cassini and Voyager data sets. Astronomy and Astrophysics. A&A 491, 21-28.

Online at: http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/abs/2008/43/aa8862-07/aa8862-07.html

Abstract: Aims. Heliospheric neutral hydrogen scatters solar Lyman- radiation from the Sun with “27-day” intensity modulations observed near Earth due to the Sun’s rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. Method. This paper presents Cassini data from 2003-2004 obtained

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downwind near Saturn at ~10 AU that at times show undamped "27-day" waves in good agreement with the single-scattering models of Pryor et al., 1992. Simultaneous Voyager 1 data from 2003-2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of ~0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al., 2006) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov and Malama 1993, Izmodenov et al., 2001, Baranov and Izmodenov, 2006). Results. We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm-3 and 0.095 cm-3. This work generally agrees with earlier discussions of Voyager data in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Colwell, J. E., Cooney, J. H., Esposito, L. W., Sremcevic, M. 2009. Density Waves in Cassini UVIS Stellar Occultations 1. The Cassini Division. Icarus. 200, 574-580.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4VB01WD-5&_user=918210&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=edf43d17133f5a8d1f37a1fb01119190

Abstract: We analyze density waves in the Cassini Division of Saturn’s rings revealed by multiple stellar occultations by Saturn’s rings observed with the Cassini Ultraviolet Imaging Spectrograph. The dispersion and damping of density waves provide information on the local ring surface mass density and viscosity. Several waves in the Cassini Division are on gradients in the background optical depth, and we find that the dispersion of the wave reflects a change in the underlying surface mass density. We find that over most of the Cassini Division the ring opacity (the ratio of optical depth to surface mass density) is nearly constant and is ~5 times higher than the opacity in the A ring where most density waves are found. However, the Cassini Division ramp, a 1100-km-wide, nearly featureless region of low optical depth that connects the Cassini Division to the inner edge of the A ring, has an opacity like that of the A ring and significantly less than that in the rest of the Cassini Division. This is consistent with particles in the ramp originating in the A ring and being transported into the Cassini Division through ballistic transport processes. Damping of the waves in the Cassini Division suggests a vertical thickness of 5-6 m. Using a mean opacity of 0.1 cm2/g we find the mass of the Cassini Division, excluding the ramp, is 3.1°¡1016 kg while the mass of the Cassini Division ramp, with an opacity of 0.015 cm2/g, is 1.1°¡1017 kg. Assuming a power-law size distribution for the ring particles, the larger opacity of the main Cassini Division is consistent with the largest ring particles there being ~5 times smaller than the largest particles in the ramp and A ring.

Dougherty, M.K., Esposito, L. W. and Krimigis, S.M., Eds. 2009. Saturn from Cassini-Huygens. Dordrecht, Netherlands: Springer-Verlag.Online at: http://www.springerlink.com/content/978-1-4020-9216-9?sortorder=asc&p_o=10

Preface: This book is one of two volumes meant to capture, to the extent practical, the scientific legacy of the Cassini-Huygens prime mission, a landmark in the history of planetary exploration. As the most ambitious and interdisciplinary planetary exploration mission flown to date, it has extended our knowledge of the Saturn system to levels of detail at least an order of magnitude beyond that gained from all previous missions to Saturn. Nestled in the brilliant light of the new and deep understanding of the Saturn planetary system is the shiny nugget that is the spectacularly successful collaboration of individuals, organizations and governments in the achievement of Cassini-Huygens. In some ways the partnerships formed and lessons learnedmay be themost enduring legacy of Cassini-Huygens. The broad, international coalition that is Cassini-Huygens is now conducting the Cassini Equinox Mission and planning the Cassini Solstice Mission, and in a major expansion of those fruitful efforts, has extended the collaboration to the study of new flagship missions to both Jupiter and Saturn. Such ventures have and will continue to enrich us all, and evoke a very optimistic vision of the future of international collaboration in planetary exploration. The two volumes in the series Saturn from Cassini-Huygens and Titan from Cassini-Huygens are the direct products of the efforts of over 200 authors and co-authors. Though each book has a different set of three editors, the group of six editors for the two volumes has worked together through every step of the process to ensure that these two volumes are a set. The books are scholarly works

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accessible at a graduate-student level that capture the approximate state of knowledge of the Saturn system after the first 4 years of Cassini’s tenure in Saturn orbit. The topics covered in each volume range from the state of knowledge of Saturn and Titan before Cassini-Huygens to the ongoing planning for a return to the system with vastly more capable spacecraft. In something of a departure from the norm for works such as these, we have included an appendix in each of the books featuring the people of Cassini-Huygens who are truly responsible for its success – the people behind the scientific scenes who ensure that everything works as flawlessly as it has. We dedicate the Cassini-Huygens volumes to them and to those who started the journey with us but could not finish it. We hope that all who read the books will share in the new knowledge and gain a deeper appreciation for the tireless efforts of those who made possible its attainment.

Dougherty, M.K., Esposito, L. W. and Krimigis, S.M., Eds. 2009. Overview. In Saturn from Cassini-Huygens. M. Dougherty et al. Eds. pp.1-8. Dordrecht, Netherlands: Springer-Verlag.

Online at: http://www.springerlink.com/content/978-1-4020-9216-9?sortorder=asc&p_o=0Introduction: This book is one of two volumes whose aim is to capture the main

scientific results of the Cassini-Huygens prime mission from orbit around the Saturn system, covering observations from the first four years of orbital tour and incorporating data from July 2004 to June 2008. The second book, Titan from Cassini-Huygens, contains the material pertinent to Saturn’s largest moon Titan; its surface, atmosphere and interaction with Saturn’s magnetosphere. This book, Saturn from Cassini-Huygens, focuses on the new results from Saturn, its satellites (excluding Titan), rings and magnetosphere. Details of the Cassini orbiter spacecraft and the Huygens probe, and their respective science instruments and investigations, as well as the overall design of the Cassini-Huygens mission can be found in a 3-volume series of books written prior to the arrival of Cassini-Huygens at the Saturn system. We do not attempt to reproduce any of that material here; instead the reader is referred to the relevant volumes (The Cassini-Huygens Mission 2002, 2004a, b). In this book we detail in the various chapters, information concerning the orbital tour of Cassini-Huygens, as executed during the four year prime mission, the extended Cassini Equinox mission and plans for a further extension after that, the Cassini Solstice mission.

Charnoz, S., L. Dones, L.W. Esposito, P.R. Estrada, M.M. Hedman. 2009. Origin and evolution of Saturn’s ring system, A chapter in the book Saturn From Cassini-Huygens. M. Dougherty et al. Eds. 17, 537-575. Dordrecht, Netherlands: Springer-Verlag.

Online at: http://www.springerlink.com/content/978-1-4020-9216-9?sortorder=asc&p_o=10Abstract: The origin and long-term evolution of Saturn’s rings is still an unsolved

problem in modern planetary science. In this chapter we review the current state of our knowledge on this long-standing question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn’s rings are not older than 108 years. In addition, Saturn’s large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn’s satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the “propellers” in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus’ plume provide new constraints on evolutionary processes in Saturn’s rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings’ fine scale structure, and progress in our understanding of the formation of the solar system provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn’s rings. They strengthen the idea that Saturn’s rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings’ origin are

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reviewed in the light of these recent advances.

Spencer, J.R., A. C. Barr, L.W. Esposito, P. Helfenstein, A.P. Ingersoll, R. Jaumann, C.P. McKay, F. Nimmo, C.C. Porco, J.H. Waite. 2009. Enceladus: An Active Cryovolcanic Satellite. A chapter in the book Saturn From Cassini-Huygens. Springer-Verlag. M. Dougherty et al. Eds. 21, 683-724

Online at: http://www.springerlink.com/content/978-1-4020-9216-9?sortorder=asc&p_o=20Abstract: Enceladus is one of the most remarkable satellites in the solar system, as

revealed by Cassini’s detection of active plumes erupting from warm fractures near its south pole. This discovery makes Enceladus the only icy satellite known to exhibit ongoing internally driven geological activity. The activity is presumably powered by tidal heating maintained by Enceladus’ 2:1 mean-motion resonance with Dione, but many questions remain. For instance, it appears difficult or impossible to maintain the currently observed radiated power (probably at least 6GW) in steady state. It is also not clear how Enceladus first entered its current self– maintaining warm and dissipative state- initial heating from non-tidal sources is probably required. There are also many unanswered questions about Enceladus’ interior. The silicate fraction inferred from its density of 1:68 g cm 2 is probably differentiated into a core, though we have not direct evidence for differentiation. Above the core there is probably a global or regional liquid water layer, inferred from several models of tidal heating, and an ice shell thick enough to support the 1 km amplitude topography seen on Enceladus. It is possible that dissipation is largely localized beneath the south polar region. Enceladus’ surface geology, ranging from moderately cratered terrain to the virtually crater-free active south polar region, is highly diverse, tectonically complex, and remarkably symmetrical about the rotation axis and the direction to Saturn. South polar activity is concentrated along the four “tiger stripe” fractures, which radiate heat at temperatures up to at least 167K and are the source of multiple plumes ejecting 200 kg s 1 of H2O vapor along with significant N2 (or C2H4), CO2, CH4, NH3, and higher-mass hydrocarbons. The escaping gas maintains Saturn’s neutral gas torus, and the plumes also eject a large number of micron- sized H2O ice grains that populate Saturn’s E-ring. The mechanism that powers the plumes is not well understood, and whether liquid water is involved is a subject of active debate. Enceladus provides perhaps the most promising potential habitat for life in the outer solar system, and the active plumes allow the unique opportunity for direct sampling of that zone. Enceladus is thus a prime target for Cassini’s continued exploration of the Saturn system, and will be a tempting target for future missions.

Bradley, E. T., Colwell, J.E., Esposito, L.W., Cuzzi, J.N., Tollerud, H., Chambers, L., 2010. Far Ultraviolet Spectral Properties of Saturn’s Rings from Cassini UVIS. Icarus. 206, 458-466

Online at: http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%236821%232010%23997939997%231783459%23FLA%23&_cdi=6821&_pubType=J&view=c&_auth=y&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=6bb66254536798e7be60d6705936036aAbstract: Spectra taken by the Cassini Ultraviolet Imaging Spectrograph (UVIS) of Saturn’s C ring, B ring, Cassini Division, and A ring have been analyzed in order to characterize ring particle surface properties, and water ice abundance in the rings. UVIS spectra sense the outer few microns of the ring particles. Spectra of the normalized reflectance (I/F) in all four regions show a characteristic water ice absorption feature near 165 nm. Our analysis shows that the fractional abundance of surface water ice is largest in the outer B ring and decreases by over a factor of 2 across the inner C ring. We calculate the mean path length of UV photons through icy ring particle regolith and the scattering asymmetry parameter using a Hapke reflectance model and a Shkuratov reflectance model to match the location of the water ice absorption edge in the data. Both models give similar

Esposito, L.W., A.R. Hendrix, Eds. 2010. Introduction to Special Issue. Saturn’s Rings and Icy Satellites from Cassini, Icarus. 206, 381

Online at: http://dx.doi.org/10.1016/j.icarus.2010.02.001

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No abstract, Intro: This special section includes contributed papers from the international conference, Saturn from Cassini–Huygens, held at Imperial College, London, 28 July–1 August 2008. The review papers from that conference have been published in the book, Saturn from Cassini–Huygens, Dougherty et al., 2009 In: M.K. Dougherty, L.W. Esposito and S.M. Krimigis, Editors, Saturn from Cassini–Huygens, Springer-Verlag, Dordrecht, Netherlands (2009).Dougherty et al. (2009). The editors of this special section solicited contributions, which then followed the regular submission and review procedures for Icarus. This special section of Icarus includes the contributed and other recent papers related to Saturn’s rings and icy satellites. The articles cover a broad range of topics, both observational and theoretical, including satellite, ring and magnetosphere interactions. The editors thank the authors and reviewers who contributed to this issue. The detailed papers published here complement and expand the topics in the chapters of the book Saturn from Cassini–Huygens. We thank the Cassini Project for its support.

retrieved values of the photon mean, however the retrieved asymmetry (g values) are different. The photon mean path lengths are nearly uniform across the B and A rings. Shortward of 165 nm the rings exhibit a slope that turns up towards shorter wavelengths, while the UV slope of 180/150 nm (reflectance outside the water absorption ratioed to that inside the absorption band) tracks I/F with maxima in the outer B ring and in the central A ring. Retrieved values of the scattering asymmetry parameter show the regolith grains to be highly backscattering in the FUV spectral regime.

Colwell, J. E., L. W. Esposito, R. G. Jerousek, M. Sremcevic, D. Pettis, E. T. Bradley. 2010. Cassini UVIS Stellar Occultation Observations of Saturn’s Rings. Astron. J. 140 1569

Online at: http://iopscience.iop.org/1538-3881/140/6/1569/ Abstract: The Cassini spacecraft's Ultraviolet Imaging Spectrograph (UVIS) includes

a high-speed photometer (HSP) that has observed more than 100 stellar occultations by Saturn's rings. Here, we document a standardized technique applied to the UVIS-HSP ring occultation datasets delivered to the Planetary Data System as higher level data products. These observations provide measurements of ring structure that approaches the scale of the largest common ring particles (~5 m). The combination of multiple occultations at different viewing geometries enables reconstruction of the three-dimensional structure of the rings. This inversion of the occultation data depends on accurate calibration of the data so that occultations of different stars taken at different times and under different viewing conditions can be combined to retrieve ring structure. We provide examples of the structure of the rings as seen from several occultations at different incidence angles to the rings, illustrating changes in the apparent structure with viewing geometry.

Cuzzi, J. N., J. A. Burns, S. Charnoz, R. N. Clark, J. E. Colwell, L. Dones, L. W. Esposito, G. Filacchione, R. G. French, M. M. Hedman, S. Kempf, E. A. Marouf, C. D. Murray, P. D. Nicholson, C. C. Porco, J. Schmidt, M. R. Showalter, L. J. Spilker, J. N. Spitale, R. Srama, M. Sremcevic, M. S. Tiscareno, J. Weiss. 2010. An Evolving View of Saturn’s Dynamic Rings. Science, 327, 1470-1475, doi:10.1126/science.1179118.

Abstract: The rings of Saturn are composed mostly of water ice but contain smallamounts of an unknown reddish contaminant. They exhibit a wide range of structure acrossmany spatial scales. Some of this structure can be traced to the interplay of the fluid nature and the self-gravity of innumerable orbiting cm-m size particles, and the effects of a tribe ofperipheral and embedded moonlets, but much remains unexplained. It is actively debatedwhether the vigorous evolutionary processes to which the rings are subject mean they are much younger than the solar system. Observations of the rings by the Cassini spacecraft over the past five years, combined with key observations from Earth and even the Voyager flybys of decades ago, reveal the dynamic, constantly changing nature of many aspects of ring structure on timescales as short as years, months, and even days. Many processes on view have parallels in protoplanetary disks.

Esposito, L.W. 2010. Composition, Structure, Dynamics and Evolution of Saturn’s rings. Annual Review of Earth & Planetary Sciences, 38, 383-410.

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Online at : http://arjournals.annualreviews.org/eprint/MFjJv6VV7W43PR4wvNUN/full/10.1146/annurev-earth-040809-152339?cookieSet=1

Abstract   : Cassini observations confirm that Saturn’s rings are predominantly water ice. The particles in Saturn’s rings cover a range of sizes from dust to small moons. Occultation results show the particles form temporary elongated aggregates tens of meters across. Some ring structure is created by moons, others by various instabilities. Data from future Cassini measurements can help to decide if the rings are remnants of the Saturn nebula or fragments of a destroyed moon or comet.

Gérard, J.-C., B. Hubert, J. Gustin, V. I. Shematovich, D. Bisikalo, L. Esposito, A.I. Stewart, 2010. EUV spectroscopy of the Venus dayglow with UVIS on Cassini. Icarus. 211, 1, 70-80.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-5161PFV-2&_user=918210&_coverDate=01%2F31%2F2011&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=94529dd4456ad61289c1e8f4f2ccede3&searchtype=a

Abstract: We analyze EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level. We present the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 115.2 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield- Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X bands measured along the track with those predicted by an airglow model. This model includes the treatment of multiple scattering for the optically thick OI and NI multiplets. It is found that the calculated intensity of the OII emission at 83.4 nm is larger and the limb brightening more pronounced than predicted by the model. An increase in the ion density by a factor of 5 to 10 brings the observations and the modeled values into agreement. The calculated intensity variation of the OI 98.9 nm and CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The calculated brightness of the NI 120 nm multiplet is larger by a factor of ~2-3 than the observations.

Gustin, J., I. Stewart, J-C. Gerard, L.W. Esposito. 2010. Characteristics of Saturn’s FUV airglow from limb viewing spectra obtained with Cassini-UVIS. Icarus. 210 (2010), 270-283.

Online at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-50DW3XP- 6&_user=918210&_coverDate=06%2F30%2F2010&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=18b4ee405af7c379aed608a6f3a4cae8

Abstract : This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from –1200 to 4000 km. The FUV limb emission consists of three main contributions: 1) H Ly-α peaking at 1100 km with 0.7 kilo-Rayleights (kR), 2) reflected sunlight longward of 1550 Å which maximizes at –950 km with 16.5 kR and 3) H2 bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 4.6 kR. A vertical profile of the local volume emission rate (VER) has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. The vertical VER is well matched by a Chapman function, characterized by a maximum value of 4.3 photons cm-3 s-1 in the total H2 UV bandwidth (800-1650 Å), peaking at 1020 km. Comparisons between the observed and synthetic airglow H2 spectra in the 1150-1650 Å bandwidth show that the H2 bands are accounted for in spectral shape by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H2 fluorescence lines and 20 eV electron impact spectra, the latter contributing ~88% of the total H2 airglow.

Hedelt, P., Y. Ito, H.U. Keller, R. Reulke, P. Wurz, H. Lammer, H. Rauer, L. Esposito. 2010.

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Titan’s atomic hydrogen corona. Icarus. Volume 210, Issue 1, p. 424-435.http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-50DW3XP-1&_user=918210&_coverDate=11%2F30%2F2010&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=8c03429098e4986eac9ce19a6d8c4b94&searchtype=a

Abstract: Based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini orbiter, Titan’s atomic hydrogen exosphere is investigated. Data obtained during the T9 encounter is used in order to infer the distribution of atomic hydrogen throughout Titan’s exosphere, as well as the exospheric temperature. The measurements performed during the flyby are modeled by performing Monte Carlo radiative transfer calculations of solar Lyman-α radiation, which is resonantly scattered on atomic hydrogen in Titan’s exosphere. Two different atomic hydrogen distribution models are applied in order to determine the best fitting profile. One model is a static model which uses the Chamberlain formalism in order to calculate the distribution of atomic hydrogen throughout the exosphere, whereas the second model is a dynamic model, based on the Monte Carlo method. The density distributions provided by both models are able to fit the measurements although both models yield their main differences at the exobase: Best fitting exobase atomic hydrogen densities of nH = (2± 0.5) · 104 cm−3 and nH = (9 ±1) · 104 cm−3 were found using the density distribution provided by both models, respectively. This is based on the fact that during the encounter, HDAC was sensitive to altitudes above about 3,000 km, hence well above the exobase at about 1,500 km. Above this altitude the absolute densities provided by both models are comparable within the uncertainties of the measurement. Due to a strong noise pattern in the HDAC data and partly undersampled data, a decision about which model distribution better fits the data measured is not possible. The inferred exobase density using the Chamberlain profile is a factor of two lower that obtained from Voyager 1 measurements and much lower than the value inferred from photochemical models. Using the density profile provided by the dynamical model, the best fitting exobase density is a factor of two higher than the Voyager measurements, but within its uncertainty comparable to the upper limit of densities obtained from photochemical models. Furthermore a best fitting exospheric temperature of T = (175 ± 25)K was obtained, assuming an isothermal exosphere for the calculations. It is within the temperature range that has been determined by different instruments aboard Cassini. This temperature is close to the critical temperature above which hydrodynamic escape from Titan’s exosphere occurs.

Elliott, J.P. and L.W. Esposito. 2011. Regolith Depth Growth on an Icy Body Orbiting Saturn and Evolution of Bidirectional Reflectance due to Surface Composition Changes. Icarus. 212, 268–274.

Online at : http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-51NVRF7-1&_user=918210&_coverDate=12%2F11%2F2010&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000047944&_version=1&_urlVersion=0&_userid=918210&md5=4f23f2868bed25276cf547c03458d180&searchtype=a

Abstract: Using a Markov chain model, we consider the regolith growth on a small body in orbit around Saturn, subject to meteoritic bombardment, and assuming all impact ejecta are re-collected. We calculate the growth of regolith and the fractional pollution, assuming an initial pure ice body and amorphous carbon as a pollutant. We extend the meteorite flux of Cuzzi and Estrada (Cuzzi, J., Estrada, P. [1998]. Icarus 132, 1–35) to larger sizes to consider the effect of disruption of the moonlet on other moonlets in the ensemble. This is a relatively small effect, completely negligible for moonlets of 1 m radius. For the given impact model, fractional pollution reaches 22% for 1 m bodies, but only 3% for 10 m bodies, 1.7% for 20 m bodies, and 1% for 30 m bodies after 4 byr. By considering an ensemble of moonlets, which have identical cross-sections for releasing and capturing ejecta, this analysis can be extended to a model of particles in Saturn’s rings, where the calculated spectra can be compared to observed ring spectra. The measured spectral reflectance of Saturn’s rings from Cassini observations therefore constrains the size and age of the ring particles. The comparison between 1 m, 10 m, 20 m, and 30 m particles confirms that for larger ring mass, the current rings would be less polluted; for the largest particles, we expect negligible changes in the UV spectrum after 4 byr of meteoritic bombardment. We consider two end members for mixing of the meteoritic material: areal and intimate. Given the uncertainties in the actual mixing of the meteoritic infall and in its composition (as a worst case, we assume the meteoritic material is 100% amorphous carbon, intimately mixed) initially pure ice 30 m ring particles would darken after 4 byr of exposure by 15%.

Xu, F, A. B. Davis, R. A. West and L. W. Esposito. 2011. Markov chain formalism for polarized light transfer in plane-parallel atmospheres, with numerical comparison to the Monte Carlo method. Optics Express. Vol. 19, 2, 946-967.

Online at : http://vjbo.osa.org/virtual_issue.cfm

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Abstract. Building on the Markov chain formalism for scalar (intensity only) radiative transfer, this paper formulates the solution to polarized diffuse reflection from and transmission through a vertically inhomogeneous atmosphere. For verification, numerical results are compared to those obtained by the Monte Carlo method, showing deviations less than 1% when 90 streams are used to compute the radiation from two types of atmospheres, pure Rayleigh and Rayleigh plus aerosol, when they are divided into sublayers of optical thicknesses of less than 0.03.

Pryor W.R., A.M. Rymer, D.G. Mitchell, T.W. Hill, D.T. Young, J. Saur, G.H. Jones, S. Jacobsen, S.W.H. Cowley, B.H. Mauk, A.J. Coates, J. Gustin, D. Grodent, J.-C. Gérard, L. Lamy, J. D. Nichols, S.M. Krimigis, L.W. Esposito, M.K. Dougherty, A.. Jouchoux, A.I.F. Stewart, W.E. McClintock, G.M. Holsclaw, J.M. Ajello, J.E. Colwell, A.R. Hendrix, F.J. Crary, J.T. Clarke, X. Zhou. 2011. The Enceladus auroral footprint at Saturn. Nature. 472, 331-333. doi:10.1038/nature09928.

Online at: http://www.nature.com/nature/journal/v472/n7343/full/nature09928.htmlAbstract: Although there are substantial differences between the magnetospheric populations ofJupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus1-9 could lead to

electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with its volcanically active moon Io. Powerful field aligned electron beams associated with the Io–Jupiter coupling create an ultraviolet auroral footprint in Jupiter’s ionosphere10. Auroral UV emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kR11, about an order of magnitude dimmer than the Io footprint and below the threshold of the Hubble Space Telescope (HST), consistent with its non-detection12. Here we report the detection by the Cassini spacecraft of magnetic field-aligned ion and electron beams with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in 7 of 317 Cassini Ultraviolet Imaging Spectrograph (UVIS) scans of the moon’s footprint. In addition to the intermittent nature of the emission, other notable differences between the Enceladus-Saturn coupling and the Io–Jupiter coupling, include 1) the beams are offset several moon radii co-rotationally downstream from Enceladus; 2) co-aligned energetic ion and electron beams are observed; and 3) electron beams in the wake of Enceladus flicker in energy by at least an order of magnitude on timescales of a few minutes.

Stevens, M.H., J. Gustin, J.M. Ajello, J.S. Evans, R.R. Meier, A. J. Kochenash, A.W. Stephan, A.I.F. Stewart, L.W. Esposito, W.E. McClintock, G. Holsclaw, E. T. Bradley, B.R. Lewis. 2011. The Production of Titan’s Far Ultraviolet Nitrogen Airglow. JGR. 116, A05304, doi:10.1029/2010JA016284.

Online at: http://www.agu.org/pubs/crossref/2011/2010JA016284.shtml Abstract: The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed Titan’s dayside limb in the

extreme ultraviolet (EUV) and far ultraviolet (FUV) on 22 June, 2009 from a distance of only 23 Titan radii. These high quality observations reveal the same FUV emissions arising from photoelectron excitation and photofragmentation of molecular nitrogen on Earth. We investigate both of these solar driven processes with a terrestrial airglow model adapted to Titan and find that total predicted radiances for the two brightest N2 band systems agree with the observed peak radiances to within 2%. The altitude of the observed limb peak is between 900-1000 km and also consistent with model predictions in the FUV and EUV. Weaker EUV Carroll-Yoshino N2 bands within the CY(v=3,4,6) progressions between 870-1010 Å are underpredicted by a factor of three while the CY(0,1) band near 980 Å is overpredicted by a factor of three. We find no evidence for C I emissions in Titan’s FUV airglow in contrast to previous Titan airglow studies and identify several vibrational bands from the N2 Vegard-Kaplan system arising from photoelectron impact in their place instead.

Hansen, C. J., D. E. Shemansky, L. W. Esposito, A. I. F. Stewart, B. R. Lewis, J. E. Colwell, A. R. Hendrix, R. A. West. 2011. The Composition and Structure of the Enceladus Plume. GRL. 38, L11202, doi:10.1029/2011GL047415.

Online at : http://www.agu.org/pubs/crossref/2011/2011GL047415.shtml Abstract. The Cassini Ultraviolet Imaging Spectrograph (UVIS) has been used to observe an occultation

of the Sun by the water vapor plume at the south polar region of Saturn’s moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O, with a line-of-sight column density of 0.9 ± 0.23 x 1016 cm-2. The upper-limit for N2 is 5 x 1013 cm-2, or < 0.5% in the plume; the lack of N2 has significant implications for models of the geochemistry in Enceladus’ interior. The inferred rate of water vapor injection into Saturn’s magnetosphere is ~200 kg/sec. The calculated values of H2O flux from three occultations observed by UVIS have a standard deviation of 30 kg/sec (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume, with Mach numbers of 5 - 8

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thus vertical gas velocities exceeding 1000 m/sec. UVIS results pertain to the gas in the plume, and are complementary to the data described in the Postberg et al. companion paper, which addresses the solid particles in the plume. Both papers support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface.

Esposito, L.S., N. Albers, B. K. Meinke, M. Sremcevic, Pr. Madhusudhanan, J. Colwell, R. G. Jerousek. 2012. A Predator-Prey Model for Moon-triggered Clumping in Saturn’s Rings. Icarus. 217, 1, 103-114.http://dx.doi.org/10.1016/j.icarus.2011.09.029

Abstract: UVIS occultation data show clumping in Saturn’s F ring and at the B ring outer edge, indicating aggregation and disaggregation at these locations that are perturbed by Prometheus and by Mimas. The inferred timescales range from hours to months. Occultation profiles of the edge show wide variability, indicating perturbations by local mass aggregations. Structure near the B ring edge is seen in power spectral analysis at scales 200–2000 m. Similar structure is also seen at the strongest density waves, with significance increasing with resonance strength. For the B ring outer edge, the strongest structure is seen at longitudes 90 and 270 relative to Mimas. This indicates a direct relation between the moon and the ring clumping. We propose that the collective behavior of the ring particles resembles a predator–prey system: the mean aggregate size is the prey, which feeds the velocity dispersion; conversely, increasing dispersion breaks up the aggregates. Moons may trigger clumping by streamline crowding, which reduces the relative velocity, leading to more aggregation and more clumping. Disaggregation may follow from disruptive collisions or tidal shedding as the clumps stir the relative velocity. For realistic values of the parameters this yields a limit cycle behavior, as for the ecology of foxes and hares or the ‘‘boom-bust’’ economic cycle. Solving for the long-term behavior of this forced system gives a periodic response at the perturbing frequency, with a phase lag roughly consistent with the UVIS occultation measurements. We conclude that the agitation by the moons in the F ring and at the B ring outer edge drives aggregation and disaggregation in the forcing frame. This agitation of the ring material may also allow fortuitous formation of solid objects from the temporary clumps, via stochastic processes like compaction, adhesion, sintering or reorganization that drives the denser parts of the aggregate to the center or ejects the lighter elements. Any of these more persistent objects would then orbit at the Kepler rate. We would also expect the formation of clumps and some more permanent objects at the other perturbed regions in the rings. . . including satellite resonances, shepherded ring edges, and near embedded objects like Pan and Daphnis (where the aggregation/disaggregation cycles are forced similar to Prometheus forcing of the F ring).

Baillié, K., J. E. Colwell, J. J. Lissauer, L. W. Esposito, M. Sremcevic 2011. Waves in Cassini UVIS Stellar Occultations 2. Waves in the C Ring. Icarus. Volume 216, Issue 1, Pages 292–308. doi: 10.1016/j.icarus.2011.05.019. http://www.sciencedirect.com/science/article/pii/S0019103511001928

Abstract: We performed a complete wavelet analysis of Saturn's C ring on 62 stellar occultation pro les provided by Cassini Ultraviolet Imaging Spectrograph High Speed Photometer using a WWZ power transform. With a co-adding process, we found evidence of the 10 waves reported by Rosen et al. 1991, Icarus 93, 3-24 and Rosen et al. 1991, Icarus 93, 25-44, the 12 additional waves reported by Colwell et al. 2009, Rings structure, In: Dougherty M. K., Esposito L. W., and Krimigis S. M, Saturn from Cassini-Huygens, Berlin: Springer, 2009, pp. 375-412., together with 16 previously unreported wavelike features. Fifteen of these appear to be propagating waves (wavelength changing systematically with distance from Saturn), including a 52-km-long wave in a plateau at 86397 km, the longest new wavetrain in the C ring. We produced a complete map of resonances with external satellites and possible structures rotating with Saturn's rotation period up to order eight, allowing us to associate a previously observed wave with the Atlas 2:1 inner Lindblad resonance (ILR) and newly detected waves with the Mimas 6:2 ILR and the Pandora 4:2 ILR. We derived surface

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mass densities and mass extinction coeffcients: we find = 0:22 g:cmcc for the Atlas 2:1 ILR (feature 33), = 1:31 g:cm-2 for the Mimas 6:2 ILR (feature 36), and = 1:42 g:cm-2 for the Pandora 4:2 ILR (feature 37). Knowing the optical depth over the whole C ring, we were able to determine a range of mass extinction coe cients ( = /) for the waves associated to a resonance (0.13 - 0.28 cm2 g-1), which is higher than the reported values for the A ring (0.01 - 0.02 cm2 g-1) and the Cassini Division (0.07 - 0.12 cm2 g-1 from Colwell et al. 2009, Icarus 200, 574-580). We also note that the mass extinction coeffcient is probably not constant along the C ring (in contrast to the A ring and the Cassini Division): it is systematically higher in the plateaus than elsewhere, suggesting smaller particles in the plateaus. We present the results of our analysis of these new waves in the C ring and estimate the mass of the C ring to be between 5:8 1015 kg and 1.3 1016 kg (equivalent to an icy satellite of radius between 15.2 km and 19.6 km with a density of 400 kg m1-3 close to that of Pan or Atlas). Using the ring viscosity derived from the wave damping length, we also estimate the vertical thickness of the C ring between 1.9 m and 5.6 m, comparable to the vertical thickness of the Cassini Division.

Gerard, J.-C., J. Gustin, B, Hubert, G.R. Gladstone and L.W. Esposito. 2011. Measurements of the Helium 584‐Å airglow during the Cassini flyby of Venus. Planetary Space Sci. Volume 59, Issue 13, Pages 1524–1528 http://www.sciencedirect.com/science/article/pii/S0032063311002042

Abstract   : The HeI resonance line at 584 Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme UltraViolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ~320 R and the bright limb reached ~700 R. These values are slightly higher than those determined from previous observations. The sensitivity of the 584‐Å intensity to the helium abundance is analyzed using recent cross sections and solar irradiance measurements at 584 Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8x106 cm‐3 at the level of where the CO2 is 2x1010 cm‐3. If the solar flux by Woods and Rottman (2002) is used, the best agreement with the observations is obtained for a helium density about twice the VTS3 model density.

Jerousek, R. G., Colwell, J. E., Esposito, L. W. 2011. Morphology and Variability of The Titan Ringlet and Huygens Ringlet Edges. Icarus. Volume 216, Issue 1, Pages 280–291.  http://www.sciencedirect.com/science/article/pii/S0019103511003460

Abstract: We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges.

More than 100 stellar occultations of the Huygens Ringlet and Titan Ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness (h>1 km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model.

Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ~300 meters in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the 1 weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ.

Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern.

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Albers, N, M. Sremcevic, J.E. Colwell, and L.W. Esposito. 2012. Saturn’s F Ring as seen by Cassini UVIS: Kinematics and Statistics. Icarus. Albers, N, M. Sremcevic, J.E. Colwell, and L.W. Esposito. 2009. Saturn’s F Ring as seen by Cassini UVIS: Kinematics and Statistics. Icarus. Volume 217, p. 367-388, doi:10.1016/j.icarus.2011.11.016.Online at: http://www.sciencedirect.com/science/article/pii/S001910351100443X

Abstract: We present a new orbit model of Saturn’s F ring based on 93 stellar occultations recorded by the Ultraviolet Infrared Spectrograph (UVIS) High Speed Photometer (HSP) onboard the Cassini spacecraft between 2005-139T and 2008-332T and the Voyager PPS stellar and RSS radio occultation.We demonstrate that the F ring is well described by the orbit of a precessing ellipse in an inclined ring plane. Our results are in good agreement with Earth-based and HST occultations as well as Voyager ISS imaging. Fairly large residuals with a RMS of 24 km are genuine and inherent to the F ring, representing its well-known kinks. We find the residuals to increase in time, thus reflecting the ever-stronger perturbations by the shepherd moon Prometheus during the approach of anti-apse alignment between moon and F ring core predicted to occur in December 2009.

The HSP observations further allow us to identify the individual structures of the F ring as the F ring core, multiple inner and outer strands, and a secondary core that seemingly is neither core nor strand. Photometric properties such as peak optical depth, full-width-at-half-maximum and equivalent width are used to characterize each of these structures. A nearly linear correlation between these is revealed and interpreted as “self-containment”, where material seems radially redistributed, and not just geometrically spread or concentrated. With an average peak optical depth of (τmax) = 0.3 and a fullwidth- at-half-maximum of (F) = 15 km the F ring core is a much denser and narrower structure than the surrounding strands with (τmax) = 0.03 and(F) = 70 – 100 km. In a few cases the core exhibits optical depths exceeding τmax > 1 with a single case of τmax > 3. Yet in many cases we find a narrow (~ few km), optically thick (τmax > 0.5) component that potentially is the actual F ring core surrounded by an envelope of more diffuse material. The secondary core, observed since the beginning of 2008, shares the primary’s morphology and, with an average equivalent width a fourth of that of the primary, carries a significant amount of the F ring’s mass. Its photometric properties and location, however, make it more similar to an inner strand, directly pointing to their common physical origin. We suggest that it is an ephemeral, but frequently occurring phenomenon, that provides the most striking example of violent strand creation and demonstrates the dynamical evolution of the F ring. Given time, this secondary core will be re-absorbed by the primary due to a difference in precession rates and mean motion, most likely triggering multiple strand creations.

Titov, D.V., W. J. Markiewicz, N. I. Ignatiev, L. Song, S. S. Limaye, A. Sanchez-Lavega, J. Hesemann, M. Almeida, T. Roatsch, K.-D. Matz, F. Scholten, D. Crisp, L. W. Esposito, S. F. Hviid, R. Jaumann, H. U. Keller, R. Moissl. 2012. Icarus. 217 (2012) 682–701. Morphology of the cloud tops as observed by the Venus Express Monitoring Camera (Published February 2012). http://www.sciencedirect.com/science/article/pii/S0019103511002375

Abstract: Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3 years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with spatial resolution from 50 km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like brigh ‘‘lace clouds’’ and cloud columns at the low latitudes, dark polar oval and narrow circular and spira ‘‘grooves’’ in the polar regions, different types of waves at the high latitudes. The VMC observation revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the

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‘‘polar cap’’. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with hig temporal resolution that allowed one to follow changes in the cloud morphology at various scales We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At 50 latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60 S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow ( 300 km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local ‘‘brightening events’’ when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80 latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops.

Molaverdikhani, K., K. McGouldrick, L. W. Esposito. 2012. The abundance and distribution of the unknown ultraviolet absorber in the Venusian atmosphere. Icarus. 217, 2, 648–660http://www.sciencedirect.com/science/article/pii/S0019103511003174

Abstract: Observations of Venus using the ultraviolet filter of the Venus Monitoring Camera (VMC) on ESA's Venus Express Spacecraft provide the best opportunity for study of the spatial and temporal distribution of the Venusian unknown ultraviolet absorber since the Pioneer Venus mission. We compare the results of 125 radiative transfer models of the upper atmosphere of Venus to each pixel in a subset of VMC UV channel images. Using a best fit criterion based upon the notion that the distribution of the unknown absorber should be independent of the geometric parameters, we find that the unknown absorber is well-mixed with the upper atmosphere of Venus, mostly with a scale height larger than previously measured SO2 and aerosol scale heights. Our analysis indicates that the unknown absorber is located mostly at altitudes between the top of the atmosphere and the top of the upper cloud deck, and exhibits a scale height that is consistent with, and sometimes greater than, the atmospheric scale height. We find that the average abundance of unknown absorber in the equatorial region is 0.20±0.05 optical depth and it decreases in the polar region to 0.05±0.05 optical depth.

Meinke, B.K., L. W Esposito, N. Albers, M. Sremcevic, C. Murray. 2012. Classification of F ring features observed in Cassini UVIS occultations. Icarus. 218, 1, p.545-554, doi:10.1016/j.icarus.2011.12.020

Abstract: The Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant F ring features in 101 occultations since July 2004. This work nearly doubles the number of features reported by Esposito et al. (2008). As the number of statistically significant features has grown, it has become useful to classify them for the purposes of cataloging. We define three classes: Moonlet, Icicle, and Core, which visually classify the shapes of features seen to date in the occultation profiles of the F ring. Two features fall into the Moonlet class. Each is opaque in its occultation, which makes them candidates for solid objects. A majority of features are classified as Icicles, which partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Finally, the variety of core region shapes displays how even the general shape of the F ring is ever-changing. The core region of the F ring (typically ~10 km wide) usually has a smooth U-shape to it, but the core region takes the shape of Ws and Vs in some occultation profiles. Our lengthy observing campaign reveals that Icicles are likely transient clumps, moonlets are possible solid objects, and cores show the variety of F ring morphology Icicles may evolve into moonlets, which are an order of magnitude less abundant.

Ajello, J.M, M.H. Stevens, J. Gustin, K. Larsen, A.I.F. Stewart, L.W. Esposito, W.E. McClintock, G.M. Holsclaw, E.T. Bradley. 2012. Cassini UVIS Observations of Titan Nightglow Spectra. JGR. VOL. 117, A12315, doi:10.1029/2012JA017888http://www.agu.org/pubs/crossref/2012/2012JA017888.shtml

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Abstract: The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan’s day and night limb-airglow and disk-airglow on multiple occasions, including during an eclipse observation. The 71 airglow observations analyzed in this paper show EUV (600–1150 Å) and FUV (1150–1900 Å) atomic multiplet lines and band emissions arising from either photoelectron induced fluorescence and solar photo-fragmentation of molecular nitrogen (N2) or excitation by magnetosphere plasma. The one year set of observations, occurring mostly in 2009, allow varied geometry e.g. the spacecraft distances to Titan change from 104 to 106 km, the limb tangent heights vary from 0 to 7,000 km, the solar zenith angle varies from 50 to 150o and the target center phase angles change from 10 to 170o. These spectral measurements help us define the best geometry for airglow observations, which help determine the energy input into the upper atmosphere of Titan. The altitude of the peak UV emissions on the limb during daylight occurred inside the thermosphere at the altitude of the topside ionosphere (near 1000 km altitude). However, at night on the limb, a subset of emission features, much weaker in intensity, arise in the atmosphere with two different geometries. First, there is a twilight photoelectron-excited glow that persists with solar depression angle up to 25-30 degrees past the terminator, until the solar XUV shadow height passes the altitude of the topside ionosphere (1000-1200 km). The UV twilight glow spectrum is similar to the dayglow but weaker in intensity. Second, beyond 120o solar zenith angle, when the upper atmosphere of Titan is in total XUV darkness, there is indication of weak and sporadic night side UV airglow emission from magnetosphere plasma, with similar N2 excited features as above in the daylight or twilight glow over an extended altitude range from 500-1200 km minimum ray height. In this paper we present the first night side EUV and FUV airglow limb spectra of Titan showing molecular emissions. The most intense UV limb night side spectrum was obtained in the southern hemisphere near the location of the radio occultation measurement of the sporadic low altitude ionosphere layer.

Esposito, L.W. 2013. Rising sulphur on Venus. Nature Geoscience, Vol 6, 20-21. doi:10.1038/ngeo1675http://www.nature.com/ngeo/journal/v6/n1/full/ngeo1675.htmlNo Abstract – News and Views

Baillié, K., Colwell, J. E., Esposito, L. W. 2013. Meter-sized moonlet population in Saturn’s C ring and Cassini Division. Astron. J, 145, 171. http://iopscience.iop.org/1538-3881/145/6/171Abstract: Stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph reveal the presence of transparent holes a few meters to a few tens of meters in radial extent in otherwise optically thick regions of the C ring and the Cassini Division. We attribute the holes to gravitational disturbances generated by a population of ~10 m boulders in the rings that is intermediate in size between the background ring particle size distribution and the previously observed ~100 m propeller moonlets in the A ring. The size distribution of these boulders is described by a shallower power-law than the one that describes the ring particle size distribution. The number and size distribution of these boulders could be explained by limited accretion processes deep within Saturn's Roche zone.

Torres, P.J., Madhusudhanan, P., Esposito, L.W. 2013. Mathematical analysis of a model for moon-triggered clumping in Saturn's rings. Physica D. 259, 55-62. http://www.sciencedirect.com/science/article/pii/S0167278913001607

Abstract: Spacecraft observations of Saturn's rings show evidence of an active aggregation-disaggregation process triggered by periodic inuences from the nearby moons. This leads to clumping and break-up of the ring particles at time-scales of the order of a few hours. A mathematical model has been developed to explain these dynamics in the Saturn's F-ring and B-ring [3], the implications of which are in close agreement with the empirical results. In this paper, we conduct a rigorous analysis of the proposed forced dynamical system for a class of continuous, periodic and zero-mean forcing functions that model the ring perturbations caused by the moon ybys. In speci_c, we derive the existence of at least one periodic solution to the dynamic system with the period equal to the forcing period of the moon. Further, conditions for the uniqueness and stability of the solution and bounds for the amplitudes of the periodic solution are derived.

Bradley, E.T., J.E. Colwell, L.W. Esposito, 2013. Scattering properties of Saturn’s rings from Cassini UVIS spectra. Icarus. 225, 726-739.

http://www.sciencedirect.com/science/article/pii/S0019103513001681 Abstract: We use Cassini UVIS data to determine the scattering properties of Saturn’s ring particles in the

FUV. We have replaced the scattering function from the classical Chandrasekhar single scattering radiative

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transfer equation for reflectance with a ring wake model for the A and B rings derived from stellar occultations. The free parameters in this model are the ring particle Bond albedo, AB, and the ring particle asymmetry parameter, g, which equals the cosine of the most probable scattering angle of a photon from a ring particle. The spectrum of Saturn’s rings from 140 to 190 nm shows an absorption feature due to water ice shortward of 165 nm. We compare our model values for I/F to lit-side data at 155 nm and at 180 nm for regions in both the A and B rings. We used the unmodified Chandrasekhar model for the C ring and Cassini Division, and in all cases we determined AB and g in the FUV for the first time. Values of AB vary between 0.04 and 0.091 at 180 nm and between 0.012 and 0.019 at 155 nm. The variations across the ring of AB at 180 nm is consistent with a greater abundance of non-ice contaminant in the C ring and Cassini Division and a minimum in contaminant abundance in the outer B ring. There is little variation in AB at 155 nm across the rings, which suggests that the reflectance of the water ice and non-water ice material shortward of the 165 nm absorption edge are about the same. Values of g vary between _0.68 and _0.78 at 180 nm and between _0.63 and _0.77 at 155 nm showing that the ring particles are highly backscattering in the FUV. We find that the wavelength of the absorption feature varies with ring region and viewing geometry indicating a different photon mean path length, L, through the outer layer of the ring particle (Bradley, E.T., Colwell, J.E., Esposito, L.W., Cuzzi, J.N., Tollerud, H., Chambers, L. [2010]. Icarus 206 (2), 458–466). We compared I/F from 152 to 185 nm to a radiative transfer spectral model developed by Shkuratov et al. (Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G. [1999]. Icarus 137, 235–246) and modified by Poulet et al. (Poulet, F., Cuzzi, J.N., Cruikshank, D.P., Roush, T., Dalle Ore, C.M. [2002]. Icarus 160, 313–324). We find that L is positively correlated with phase angle, which we attribute to multiple scattering within the particle on length scales comparable to L. We extrapolate L to zero phase angle and find values of L at zero phase ranging from _2 to 3 lm. This provides a direct measure of the distance from the surface of a ring particle to the first scattering center. L at zero phase is roughly constant across the rings suggesting the outermost 1.25 lm of the ring particles have the same structural properties in all ring regions. We azimuthally binned and interpolated observations of the unlit side of the A ring taken during Saturn orbit insertion to a 100 km resolution radial profile. We see halos (enhanced brightness) surrounding the Janus 4:3 and Janus 5:4 density waves. We also computed I/F across the A ring using the SOI observational geometry along with AB and the power-law index, n, derived from the retrieval approach from lit side observations. I/F determined by this technique agrees with results from the lit side analysis for the A2 ring but diverge for the inner and outer A ring, which we attribute to multiple scattering effects.

Esposito, L.W.E, Colaprete, A, English, J., Haberle, R.M., Kahre, M.A. 2014. Clouds and aerosols on the terrestrial planets. In Comparative climatology of terrestrial planets. S. Mackwell, Ed. University of AZ Press, Tucson, AZ. Pp. 329-353.http://www.barnesandnoble.com/w/comparative-climatology-of-terrestrial-planets-steve-mackwell/1115100100?ean=9780816530595

Abstract Clouds and aerosols are common on the terrestrial planets, highly variable on Earth and Mars, and completely covering Venus. Clouds form by condensation and photochemical processes. Nucleation of cloud droplets by certain aerosols provides an indirect linkage. Earth clouds cover over half of the planet, are composed of mainly liquid water or ice, and are a significant component of Earth’s surface and top of atmosphere energy balance. On Venus, H2SO4 is the dominant cloud constituent, produced by chemical cycles operating on SO2, likely produced from geologic activity. Martian water ice clouds generally have smaller particles than on Earth, although they form by the same processes. Mars clouds affect the deposition of radiation, drive photochemical reactions, and couple to the dust cycle. In the past, Mars clouds may have produced a significant greenhouse effect at times of high obliquity and early in its history. Mars atmospheric dust has both a seasonal cycle and great dust storms. Dust significantly influences the thermal and dynamical structure of the martian atmosphere. Mars CO2 clouds provide both latent heat and radiative effects on the atmosphere, possibly more important on the early, wet, and warmer Mars climate.

Esposito, L.W.E, Colaprete, A, English, J., Haberle, R.M., Kahre, M.A. 2014. Clouds and aerosols on the terrestrial planets. In Comparative climatology of terrestrial planets. S. Mackwell, Ed. University of AZ Press, Tucson, AZ. Pp. 329-353.http://www.barnesandnoble.com/w/comparative-climatology-of-terrestrial-planets-steve-mackwell/1115100100?ean=9780816530595

Abstract: Clouds and aerosols are common on the terrestrial planets, highly variable on Earth and Mars, and completely covering Venus. Clouds form by condensation and photochemical processes. Nucleation of cloud droplets by certain aerosols provides an indirect linkage. Earth clouds cover over half of the planet, are composed of mainly liquid water or ice, and are a significant component of earth’s surface and top of atmosphere energy balance. On Venus, H2SO4 is the dominant cloud constituent, produced by chemical cycles operating on SO2, likely produced from geologic activity. Martian water ice clouds generally have

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smaller particles than on Earth, although they form by the same processes. Mars clouds affect the deposition of radiation, drive photochemical reactions, and couple to the dust cycle. In the past, Mars clouds may have produced a significant greenhouse effect at times of high obliquity and early in its history. Mars atmospheric dust has both a seasonal cycle and great dust storms. Dust significantly influences the thermal and dynamical structure of the Martian atmosphere. Mars CO2 clouds provide both latent heat and radiative effects on the atmosphere, possibly more important on the early, wet and warmer Mars climate.

Esposito, L.W. 2014. Planetary Rings. A post-equinox view. Cambridge, UK: Cambridge University Press. http://www.amazon.com/Planetary-Rings-Post-Equinox-Cambridge-Science/dp/1107028825/ref=sr_1_1?s=books&ie=UTF8&qid=1393341539&sr=1-1&keywords=planetary+rings+a+post+equinox+view

Description: Fully updated and expanded, this new edition presents a cutting-edge summary of planetary rings, including results from Cassini's Saturn System, Equinox and Solstice missions, and the New Horizons flyby of Jupiter. The book introduces basic physical processes and simple mathematical approaches in an accessible manner, including N-body and stochastic models of ring dynamics. Further revised chapters present highlighted topics including Saturn's F ring, Uranus' rings and moons, Neptune's partial rings, dusty rings, and Jupiter's ring-moon system after Galileo and New Horizons. Cassini results are fully integrated throughout, including new images in color, and a new Afterword links ring images in the Cassini 'Hall of Fame' gallery to the relevant explanation in the text. An online cache of images and videos from NASA's collection makes it easy to locate relevant and beautiful illustrative materials. This is a key resource for students, researchers and professionals in planetary science, astronomy and space-mission research.

Stevens, M., Evans, S., Lumpe, J. Westlake, J.H., Ajello, J.M., Bradley, T., Esposito, L.W. 2015. Molecular Nitrogen and Methane Density Retrievals from Cassini UVIS Dayglow Observations of Titan's Upper Atmosphere. Icarus, 247, 301-312.

http://www.sciencedirect.com/science/article/pii/S0019103514005478 Abstract. We retrieve number densities of molecular nitrogen (N2) and methane (CH4) from Titan’s upper

atmosphere using the UV dayglow. We use Cassini Ultraviolet Imaging Spectrograph (UVIS) limb observations from 800 to 1300 km of the N I 1493 Å and N II 1085 Å multiplets, both produced directly from photofragmentation of N2. UVIS N2 and CH4 densities are in agreement with measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) from the same flyby if INMS densities are scaled up by a factor of 3.0 as reported in previous studies. Analysis of three Cassini flybys of Titan shows that (1) the CH4 homopause on Titan is between 900 and 1100 km, (2) upper atmospheric temperatures vary by less than 10 K over 6 h at the same geographic location and (3) from 1100 to 1700 local solar time temperatures also vary by less than 10 K. The capability of retrieving the global-scale composition from these data complements existing techniques and significantly advances the study of upper atmospheric variability at Titan and for any other atmosphere with a detectable UV dayglow.

Shemansky, D.E., Y. L. Yung, X. Liu, J. Yoshii, C. J. Hansen, A. R. Hendrix, and L.W. Esposito. 2014. A new understanding of the Europa atmosphere and limits on geophysical activity, ApJ 797:84. doi: 10.1088/0004-637X/797/2/84.

http://iopscience.iop.org/0004-637X/797/2/84/Abstract: Deep extreme ultraviolet spectrograph exposures of the plasma sheet at the orbit of Europa,

obtained in 2001 using the Cassini Ultraviolet Imaging Spectrograph experiment, have been analyzed to determine the state of the gas. The results are in basic agreement with earlier results, in particular with Voyager encounter measurements of electron density and temperature. Mass loading rates and lack of detectable neutrals in the plasma sheet, however, are in conflict with earlier determinations of atmospheric composition and density at Europa. A substantial fraction of the plasma species at the Europa orbit are long-lived sulfur ions originating at Io, with ~25% derived from Europa. During the outward radial diffusion process to the Europa orbit, heat deposition forces a significant rise in plasma electron temperature and latitudinal size accompanied with conversion to higher order ions, a clear indication that mass loading from Europa is very low. Analysis of far ultraviolet spectra from exposures on Europa leads to the conclusion that earlier reported atmospheric measurements have been misinterpreted. The results in the present work are also in conflict with a report that energetic neutral particles imaged by the Cassini ion and neutral camera experiment originate at the Europa orbit. An interpretation of persistent energetic proton pitch angle distributions near the Europa orbit as an effect of a significant population of neutral gas is also in conflict with the results of the present work. The general conclusion drawn here is that Europa is geophysically far less active than inferred in previous research, with mass loading of the plasma sheet ≤4.5 × 1025 atoms s–1 two orders of magnitude below earlier published calculations. Temporal variability in the region joining the

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Io and Europa orbits, based on the accumulated evidence, is forced by the response of the system to geophysical activity at Io. No evidence for the direct injection of H2O into the Europa atmosphere or from Europa into the magnetosphere system, as has been observed at Enceladus in the Saturn system, is obtained in the present investigation.

Esposito, L.W. 2013. Saturn’s Rings. Discoveries in Modern Science. Ed. James Trefil. Vol. 3. Farmington Hills, MI: Macmillan Reference USA. p987-991.

http://www.amazon.com/Discoveries-Modern-Science-Exploration-Technology/dp/002866244X Abstract: Planetary rings, those enduring symbols of space, exist around all the giant planets of the solar

system. Perhaps, once, every planet has had a ring. Earth once possessed a ring of debris that coalesced to form its own Moon. It is likely that planets around other stars (called extrasolar planets, or exoplanets) will also have rings. In Earth’s solar system, each ring system is unique: this can be explained as the results of the random events that create them from small nearby satellites. Saturn’s rings are the biggest and brightest. The youthful appearance of these rings is a result of recent events that create them, like the shattering of a small moon. However, that moon itself may be formed from constituents of former rings: thus, nature recycles the basic raw material to renew rings. Saturn’s rings are astronomers’ best local laboratory for studying phenomena in flattened disks: many processes occurring now in rings resemble those in a protoplanetary disk where planets are forming, particularly interactions between the disk and embedded objects

Parkinson, C. D., Yung, Y.L., Esposito, L.W.E., Gao, P., Bougher, S.W., and Hirtzig, M.. 2015. Photochemical Control of the Distribution of Venusian Water and Comparison to Venus Express SOIR Observations. PSS, 113–114, p 226–236

Online at: http://www.sciencedirect.com/science/article/pii/S0032063315000501Abstract: We use the JPL/Caltech 1-D photochemical model to solve continuity diffusion equation for

atmospheric constituent abundances and total number density as a function of radial distance from the planet Venus. Photochemistry of the Venus atmosphere from 58 to 112 km is modeled using an updated and expanded chemical scheme (Zhang et al., 2010 and Zhang et al., 2012), guided by the results of recent observations and we mainly follow these references in our choice of boundary conditions for 40 species. We model water between 10 and 35 ppm at our 58 km lower boundary using an SO2 mixing ratio of 25 ppm as our nominal reference value. We then vary the SO2 mixing ratio at the lower boundary between 5 and 75 ppm holding water mixing ratio of 18 ppm at the lower boundary and finding that it can control the water distribution at higher altitudes. SO2 and H2O can regulate each other via formation of H2SO4. In regions of high mixing ratios of SO2 there exists a “runaway effect” such that SO2 gets oxidized to SO3, which quickly soaks up H2O causing a major depletion of water between 70 and 100 km. Eddy diffusion sensitivity studies performed characterizing variability due to mixing that show less of an effect than varying the lower boundary mixing ratio value. However, calculations using our nominal eddy diffusion profile multiplied and divided by a factor of four can give an order of magnitude maximum difference in the SO2 mixing ratio and a factor of a few difference in the H2O mixing ratio when compared with the respective nominal mixing ratio for these two species. In addition to explaining some of the observed variability in SO2 and H2O on Venus, our work also sheds light on the observations of dark and bright contrasts at the Venus cloud tops observed in an ultraviolet spectrum. Our calculations produce results in agreement with the SOIR Venus Express results of 1 ppm at 70–90 km (Bertaux et al., 2007) by using an SO2 mixing ratio of 25 ppm SO2 and 18 ppm water as our nominal reference values. Timescales for a chemical bifurcation causing a collapse of water concentrations above the cloud tops (>64 km) are relatively short and on the order of a less than a few months, decreasing with altitude to less than a few days.

Nicholson, P. and Esposito, L.W. 2016. Introduction: Special issue on planetary rings. Icarus, 279, 1.

Rehnberg, M., Esposito, L., Brown, Z., Albers, A., Sremcevic, M. A. 2016. A traveling feature in Saturn's rings. Icarus, 279, 100-108.http://www.sciencedirect.com/science/article/pii/S0019103516302901

Abstract: The co-orbital satellites of Saturn, Janus and Epimetheus, swap radial positions every 4.0 years. Since Cassini has been in orbit about Saturn, this has occurred on 21 January in 2006, 2010, and 2014. We describe the effects of this radial migration in the Lindblad resonance locations of Janus within the rings. When the swap occurs such that Janus moves towards Saturn and Epimetheus away, nonlinear interference between now-relocated density waves launches a solitary wave that travels through the rings with a velocity approximately twice that of the local spiral density wave group velocity in the A ring and commensurate with the spiral density wave group velocity in the B ring.

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Becker, T. M., J. E. Colwell, L. W. Esposito, and A. D. Bratcher 2016. Characterizing the Particle Size Distribution of Saturn’s A Ring with Cassini UVIS Occultation Data. Icarus (Accepted Nov. 2015). Icarus, 279, 20-35.http://www.sciencedirect.com/science/article/pii/S0019103515005035 Abstract: Stellar occultation data from Cassini’s Ultraviolet Imaging Spectrograph (UVIS) have revealed diffraction spikes near sharp edges in Saturn’s rings. The UVIS High Speed Photometer (HSP) observes these spikes as signals at ring edges that surpass measurements of the unocculted stellar signal. In Saturn’s A ring, diffracted light can augment the direct stellar signal by up to 6% and can be detected tens of kilometers radially from the edge. The radial profile of the diffraction signal is dependent on the size distribution of the particle population near the ring edge. These diffraction signals are clearly observed at sharp edges throughout Saturn’s ring system. In this paper we focus on the clearest detections at the outer edge of the A ring and at the edges of the Encke Gap. We present a forward model in which we reconstruct the spacecraft’s observations for each stellar occultation by ring edges. The model produces a synthetic diffraction signal for a given truncated power-law particle size distribution, which we compare with the observed signal. We find an overall steepening of the power-law size distribution and a decrease in the minimum particle size at the outer edge of the A ring when compared with the Encke Gap edges. This suggests that interparticle collisions caused by satellite perturbations in the region result in more shedding of regolith or fragmentation of particles in the outermost parts of the A ring. We rule out any significant population of sub-millimeter-sized particles in Saturn’s A ring, placing a lower limitation of 1-mm on the minimum particle size in the ring.

Esposito, L.W. and de Stefano, M. 2017. Space age studies of planetary rings. A chapter in Planetary Ring Systems, Cambridge Press, London, (Eds. Tiscareno and Murray). (Submitted May 2016).

Hansen, C. J., L. W. Esposito, K.-M. Aye, J. E. Colwell, A. R. Hendrix, G. Portyankina, D. Shemansky. 2017. Investigation of Diurnal Variability of Water Vapor in Enceladus’ Plume by the Cassini Ultraviolet Imaging Spectrograph. GRL. Published online Jan 2017.

Online at: http://onlinelibrary.wiley.com/doi/10.1002/2016GL071853/abstractAbstract: An occultation of ε Orionis by Enceladus’ plume was observed with Enceladus at an orbital

longitude near apoapsis in order to investigate whether water vapor flow is modulated diurnally, similar to ice particles. The occultation showed that the bulk water vapor emanating from Enceladus changes little with orbital position. The amount of gas in at least one supersonic jet increased significantly, implying that the increase in the number of particles lofted at apoapsis could be due to more gas coming from the supersonic jets and not the overall gas flux from the tiger stripe fissures that cross Enceladus’ south polar region.

Royer, E.M., Ajello J.M., Holsclaw, G.M., Bradley, E.T., Esposito, L.W. and West, R.A. 2017. Cassini UVIS observations of Titan ultraviolet airglow intensity dependence with solar zenith angle. GRL. 44, 1,88–96. DOI:10.1002/2016GL071756.

Online at: http://onlinelibrary.wiley.com/doi/10.1002/2016GL071756/full Abstract: The Cassini Ultraviolet Imaging Spectrometer (UVIS) observed the airglow (dayglow and

nightglow) of Titan over a range of solar zenith angles (SZA) from 14 to 150° on five separate observations obtained between 2008 and 2012. The modeling of the solar cycle normalized UVIS observations indicates that a Chapman layer function provides a satisfactory fit to the intensity of the EUV and FUV airglow molecular emissions of the N2 Lyman-Birge-Hopfield band system (LBH ), the Carroll-Yoshino band system ( ), and of several atomic multiplets of nitrogen (NI, II) as a function of SZA. This result shows that the strongest contribution to the Titan dayglow occurs by processes (photoelectrons and photodissociation) involving the solar EUV flux rather than magnetospheric particle precipitation that dominates emission excitation in the nightglow.

IN PRESSEsposito, L.W. and de Stefano, M. 2017. Space age studies of planetary rings. A chapter in

Planetary Ring Systems, Cambridge Press, London, (Eds. Tiscareno and Murray). (Submitted May 2016, In press).

SUBMITTED

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Becker, T. M., J. E. Colwell, L. W. Esposito, N. O. Attree, and C. D. Murray. 2017. Cassini UVIS Solar Occultations by Saturn’s F Ring and the Detection of Collision-Produced Micron-Sized Dust. Icarus. (Submitted November 2016)

Colwell, J. E., L. W. Esposito, and J. H. Cooney 2017. Particle Sizes in Saturn’s Rings from UVIS Stellar Occultations 1. Variations with Ring Region. Icarus. (Submitted November 2016)

Abstract: The Cassini spacecraft’s Ultraviolet Imaging Spectrograph (UVIS) includes a high speed photometer (HSP) that has observed more than 100 stellar occultations by Saturn’s rings. In the absence of intervening ring material, the time series of measurements by the HSP is described by Poisson statistics in which the variance equals the mean. The finite sizes of the ring particles occulting the star lead to a variance that is larger than the mean due to random variations in the number of individual particles in each measurement area. This effect was first exploited by Showalter and Nicholson (1990) with the stellar occultation observed by Voyager 2. At a given optical depth, a larger excess variance corresponds to larger particles or clumps that results in greater variation of the signal from measurement to measurement. Here we present analysis of the excess variance in occultations observed by Cassini UVIS. We observe differences in the derived correlation length scale in different ring regions. The C ring plateaus show a distinctly smaller length scale than the background C ring, while the background C ring itself shows a positive correlation between length scale and optical depth. The innermost 700 km of the B ring has a distribution of excess variance with optical depth that is consistent with the C ring ramp and C ring but not with the remainder of the B1 region. The Cassini Division, though similar to the C ring in spectral and structural properties, has a different particle size and behavior with optical depth. The Cassini Division ramp is intermediate between the Cassini Division and the A ring, while the C ring ramp smoothly transitions in particle size from the C ring to the B ring. The A ring is dominated by self-gravity wakes which present geometry-dependent clump sizes that peak in the central A ring. The spectral “halo” regions around the strongest density waves in the A ring correspond to decreases in the particle size. There is also a pronounced dip in particle size at the Mimas 5:3 bending wave corresponding to an increase in optical depth there, suggesting that at these waves small particles are liberated from clumps or self-gravity wakes leading to a reduction in apparent particle size and an increase in optical depth.

Kempf, S., Schmidt, J., Brockwell, T., Thomas Cravens, T., Esposito, L.W., Fiege, K., Giese, B., Hedman, M.M., Helfenstein, P., Hurford, T.A., Jaumann, R., Jones, G., Kanani, S., Kieffer, S.W., Lewis, W., Magee, B.A., Matson, D.L., Nimmo, F., Pappalardo, R., Perry, M.E., Postberg, F., Saur, J., Spencer, J., Sotin, C., Spahn, F., Teolis, B.D., Tobie, G., Waite, H., Young, D.T., Zolotov, M. 2016. Enceladus as an active body. SSR. (Submitted 3/3/14).

Abstract: Data from the Cassini-Huygens mission to Saturn revealed that the midsized icy satellite Enceladus is geologically active: A region around the south pole of Enceladus is anomalously warm and a mixture of gases, dominantly water vapor, is found to flow out there from a system of cracks in the ice crust. Micron sized ice particles, entrained in the gas, are ejected to space. It became clear, that Enceladus is the dominant source for magnetospheric plasma in the Saturn system, as well as for neutrals and dust, which forms the E ring of Saturn. What is so particular about Enceladus? Why is the activity localized at the south pole? How is the heat ultimately generated? How old is the phenomenon and how did it evolve? A complete, consistent answer to these questions is not known to date. Nevertheless, much progress has been made in analyzing and understanding various aspects of the activity, which gives important constraints and directions for future research. It is the purpose of this paper to review the current understanding of Enceladus' geodynamics, geochemistry, and the interaction with the saturnian system.

Jerousek, R. G., J. E. Colwell, P. D. Nicholson, M. M. Hedman, L. W. Esposito. 2016. The Smallest Particles in Saturn’s Rings from Self-Gravity Wake Observations. Icarus. 279, 36-50. http://www.sciencedirect.com/science/article/pii/S0019103516301221

Abstract: We present a forward modeling approach for determining, in part, the ring particle spatial distribution in the vicinity of sharp ring or ringlet edges. Synthetic edge occultation profiles are computed based on a two-parameter particle spatial distribution model. One parameter, h, characterizes the vertical extent of the ring and the other, δ, characterizes the radial scale over which the ring optical depth transitions from the background ring value to zero. We compare our synthetic occultation profiles to high resolution stellar occultation light curves observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) for occultations by the Titan ringlet and Huygens ringlet edges.

More than 100 stellar occultations of the Huygens Ringlet and Titan Ringlet edges were studied, comprising 343 independent occultation cuts of the edges of these two ringlets. In 237 of these profiles the measured light-curve was fit well with our two parameter edge model. Of the remaining edge occultations, 69 contained structure that could only be fit with extremely large values of the ring-plane vertical thickness (h>1

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km) or by adopting a different model for the radial profile of the ring optical depth. An additional 37 could not be fit by our two-parameter model.

Certain occultations at low ring-plane incidence angles as well as occultations nearly tangent to the ring edge allow the direct measurement of the radial scale over which the particle packing varies at the edge of the ringlet. In 24 occultations with these particular viewing geometries, we find a wide variation in the radial scale of the edge. We are able to constrain the vertical extent of the rings at the edge to less than ~300 meters in the 70% of the occultations with appropriate viewing geometry, however tighter constraints could not be placed on h due to the 1 weaker sensitivity of the occultation profile to vertical thickness compared to its sensitivity to δ.

Many occultations of a single edge could not be fit to a single value of δ, indicating large temporal or azimuthal variability, although the azimuthal variation in δ with respect to the longitudes of various moons in the system did not show any discernible pattern.

Madhusudhanan, P., L.W. Esposito, P.J. Torres. 2016. A combined dynamical transport model for producing haloes at resonances in Saturn’s rings. Icarus. (Submitted January 2015).

Rehnberg, M.E., Brown, Z.L., Esposito, L.W., Albers, N. 2017. Direct detection of gaps in Saturn’s A ring. Icarus. (Submitted fall 2016).

Abstract: Indirect observations spanning decades have indicated that Saturn's A ring is populated with a plethora of self-gravity wakes, small wavelike structures that arise from the gravitational attraction between ring particles. We present the direct detection of the gaps that represent the minima between the denser wakes. Through a statistical test, we analyze a series of seven high-resolution stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph to identify nearly half a million discrete regions with an optical depth less than a quarter of the surrounding ring. These gaps correlate strongly with previous observations of the A-ring brightness asymmetry.

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