analyzer is mounted on the magnetometer boom ofthe spinning section of the Galileo spacecraft suchthat this rotation, together with multiple sensors,provides almost complete coverage of the entiresolid angle for charged partide velocity vectors at thespacecraft position. Electronic sectioning divides therotation into a number of azimuthal sectors that arepreselected by ground command. The E/Q range is0.8 V to 52 kV for positive ions and electrons. Theoperational mode at Venus is such that 32 E/Qpassbands are sampled over this E/Q range duringapproximately 1200 of spacecraft rotation. Thespacecraft rotation period is 19.1 s. Typical energyresolution AW/E is 0.11. By control ofthe spin phaseof the above sectors, 32 E/Q samples of intensitieswithin eight azimuthal sectors for each sensor wereacquired during four contiguous spacecraft rota-tions. The spacecraft spin axis was aligned with thedirection to the sun to within a few degrees duringVenus encounter. A sunshade that is used to preventoverheating of the instrument obscured the fields ofview P7 and E7, which were directed most nearly tothe solar direction. This sunshade prevents the de-tection of the solar wind ions and the bulk of themagnetosheath ion distributions.

3. Energy-time (E-t) spectrograms display the countsper accumulation period for each E/Q scan (abscis-sa) of the instrument. The accumulation period is0.2 s. For the instrument operation at Venus, 32samples at equal logarithmic intervals in E/Q weretaken. The responses are logarithmically coded ac-cording to the two color bars displayed in Fig. 1,one each for the ion and electron analyzers, respec-tively.

4. T. V. Johnson, Science 253, 1516 (1991).5. M. G. Kivelson et al., ibid., p. 1518.6. Values for the solar wind speeds and proton densi-

ties from the plasma instrument on Pioneer VenusOrbiter were supplied by J. D. Mihalov of theNational Aeronautics and Space AdministrationAmes Research Center. The electron densities de-rived from the Galileo plasma instrument have beencross-calibrated with the Langmuir-line frequencyobserved with the Galileo plasma wave instrumen-tation (D. A. Gurnett et al., ibid., p. 1522).

7. W. C. Feldman, in Collisionless Shocks in the Helio-sphere: Reviews ofCurrent Research, B. T. Tsurutaniand R. G. Stone, Eds. (Geophysical Monograph 35,American Geophysical Union, Washington, DC,1985), pp. 195-205.

8. The solar wind electron distribution exhibits a coolcore of electrons convecting with the solar wind ionsand a suprathermal velocity distribution that in-cludes a magnetic field-aligned "strahl" at higherenergies. See, for example, W. G. Pilipp et al. [J.Geophys. Res. 92, 1075 (1987)].

9. J. T. Gosling, M. F. Thomson, S. J. Bame, C. T.Russell, ibid. 94, 10011 (1989); R. J. Fitzenreiter, J.D. Scudder, A. J. Klimas, ibid. 95, 4155 (1990).

10. M. F. Thomsen, in Collisionless Shocks in the Helio-sphere: Reviews ofCurrent Research, B. T. Tsurutaniand R. G. Stone, Eds. (Geophysical Monograph 35,American Geophysical Union, Washington, DC,1985), pp. 253-270.

11. K. R. Moore, D. J. McComas, C. T. Russell, J. D.Mihalov, J. Geophys. Res. 94, 3743 (1989).

12. D. J. Williams et al., Science 253, 1525 (1991).13. On the day side of Venus the densities of atomic

oxygen and hydrogen are approximately equal,-100 atoms per cubic centimeter, at an altitude of3000 km [G. M. Keating et al., in The VenusInternational Reference Ionosphere, A. J. Kliore, V. I.Moroz, G. M. Keating, Eds., COSPAR Advances inSpace Research, vol. 5, no. 11 (Pergamon, Oxford,1985), pp. 117-171]. The hot hydrogen is notgravitationally bound and can be coarsely extrapo-lated to Galileo positions by assuming an inverse-square radial dependence beyond 3500 km. Theatomic oxygen is mostly gravitationally bound andthe densities are considerably less than those forhydrogen beyond 3500 km.

14. L. H. Brace, R. F. Theis, S. A. Curtis, L. W. Parker,J. Geophys. Res. 93,12735 (1988).

15. J. D. Mihalov and A. Barnes, Geophys. Res. Le-. 8,1277 (1981).

16. This particular coordinate system is chosen becauseofthe convenience ofcomputing the positions of theplasma samples in phase space. Specifically, the

27 SEPTEMBER 1991

V3-axis is directed parallel to B, and V2 is parallel toB x V,, where V, is the antisolar direction in aright-handed Cartesian coordinate system. The gy-ration speed is V = IV x BI/B.

17. Pickup ions in the vicinity of comet Halley havebeen reported to be scattered in velocity space aftertheir initial gyromotion in a ring [T. Mukai, W.Miyake, T. Terasawa, M. Kitayama, K. Hirao, Na-ture 321, 299 (1986)].

18. E. Mobius et al., ibid., 318, 426 (1985).19. To obtain an estimate of the anticipated densities of

pickup hydrogen ions, we use the atmospheric mod-el for atomic hydrogen (13) and assume a one-dimensional geometry with V, * B = 0 and a radialcolumn density of atomic hydrogen. Then the cor-responding ion densi7 is 1.5 x 101/V.TR, wheieV. = 4.5 x 1O7cm s- ,R is the radial distance fromVenus center to the spacecraft in units of Venusradii, and r is the lifetime ofatomic hydrogen, 1.2 x106 s [P. D. Feldman, in Comets, L. L. Wilkening,

Ed. (Univ. of Arizona Press, Tucson, 1982), pp.461-479].

20. We acknowledge discussions with M. G. Kivelson,D. A. Gurnett, D. J. Williams, J. D. Mihalov, L. H.Brace, A. I. Stewart, and E. Marsch. M. G. Kivelsonand D. A. Gurnett provided data from the magne-tometer and plasma wave instrument, respectively.Major contributions to the implementation of theplasma instrumentation at The University of Iowawere made by J. A. Lee, M. R. English, and G. L.Pickett. The authors express their appreciation to thefollowing personnel ofthe Jet Propulsion Laborato-ry for their contributions to our efforts: C. M.Yeates, J. R. Casani, W. G. Fawcett, H. W. Eyerly,M. S. Spehalski, W. J. O'Neil, R. F. Ebbett, T. V.Johnson, and S. J. Bolton. This research was sup-ported in part at The University of Iowa by the JetPropulsion Laboratory under contract 958778.

26 April 1991; accepted 9 July 1991

Images from Galileo of the Venus Cloud DeckMICHAEL J. S. BELTON, PETER J. GIERASCH, MICHAEL D. SMrIH,PAUL HELFENSTEIN, PAUL J. SCHINDER, JAMES B. POLLACK,KATHY A. RAGES, ANDREW P. INGERSOLL, KENNETH P. KLAASEN,JOSEPH VEVERKA, CLIFFORD D. ANGER, MICHAEL H. CARR,CLARK R. CHAPMAN, MERTON E. DAVIES, FRASER P. FANALE,RONALD GREELEY, RICHARD GREENBERG, JAMES W. HEAD III,DAVID MORRISON, GERHARD NEUKUM, CARL B. PILCHER

Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometersshow that the morphology and motions oflarge-scale features change with depth in thecloud deck. Poleward meridional velocities, seen in both spectral regions, are muchreduced in the NIR In the south polar region the markings in the two wavelengthbands are strongly anticorrelated. The images follow the changing state of the uppercloud layer downwind of the subsolar point, and the zonal flow field shows alongitudinal periodicity that may be coupled to the formation of large-scale planetarywaves. No optical lightning was detected.

T~HE SOLID STATE IMAGING (SSI)camera on Galileo returned 77 usefulimages from Venus documenting the

M. J. S. Belton, National Optical Astronomy Observa-tories, Tucson, AZ 85719.P. J. Gierasch, M. D. Smith, P. Helfenstein, P. J.Schinder, J. Veverka, Cornell University, Ithaca, NY14853.J. B. Pollack, K. A. Rages, D. Morrison, NASA AmesResearch Center, Moffet Field, CA 94035.A. P. Ingersoll, California Institute of Technology, Pas-adena, CA 91125.K. P. Klaasen, Jet Propulsion Laboratory, Pasadena, CA91109.C. D. Anger, ITRES Research Ltd., Calgary, Canada,T2E 7H7.M. H. Carr, United States Geological Survey, MenloPark, CA 94025.C. R. Chapman, Planetary Science Institute, ScienceApplications International Corporation, Tucson, AZ85f7P19.M. E. Davies, RAND Corporation, Santa Monica, CA90406.F. P. Fanale, Institute for Geophysics, Honolulu, HI96822.R. Greeley, Arizona State University, Tempe, AZ 85281.R. Greenberg, University of Arizona, Tucson, AZ85721.J. W. Head m, Brown University, Providence, RI02912.G. Neukum, Deutsche Luft und Raumfahrt, 8031 Ober-pfaffenhofen, Federal Republic of Germany.C. B. Pilcher, NASA Headquarters, Washington, DC20546.

dynamical state of the cloud tops during theweek after the encounter on 10 February1990 universal time (UT). The geometry ofthe encounter (1) and the ability of the SSIcamera (2) to image in the near-infrared(NIR) and the violet allowed an imagingsequence that probed to different depths inthe cloud layers and that could follow theevolution of small-scale dynamical phenom-ena in the atmosphere as it flowed throughthe subsolar region and downwind towardthe afternoon terminator. This region of theatmosphere has not undergone detailed ex-ploration by previous missions to Venus.Mariner 10, Pioneer Venus, Vega bal-

loon, and recent ground-based infrared ob-servations have provided a great deal ofinformation about the dynamical state ofthevenusian clouds (3-5). The pressure levelwhere the short-wavelength contrasts areformed is -50 mbar (6). This is at a levelsome 65 to 70 km above the surface, wherethe temperature is about 230 K. The cloudtops are part of a deep system of haze andcloud layers, consisting ofsulfuric acid drop-lets, that extends upward above the 45-km

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level (2 bar; 370 K). There are three domi-nant layers (7) with additional thin, highlystratified haze layers found at great heightsabove the main cloud deck (8, 9). Featuresin the ultraviolet show variable zonal mo-tions of -100 m s` over the planet'sequator. This "super"-rotation is in the samedirection as that of the solid planet but isabout 50 times faster; it is also found to bevariable on time scales of years. At highlatitudes there are large-scale jets with am-plitudes that also vary; these jets sometimesare absent, causing the cloud tops to followa rigid-body rotation profile. Poleward me-ridional motions (up to -10 m sol) haveconsistently been measured, and there isevidence for a solar-locked pattern in themarkings that indicates strong horizontaldivergence in the flow related to the subsolarregion. Finally, there is evidence for thepresence of equatorial and mid-latitudeplanetary-scale waves that propagate in thecloud layers relative to the flow. Thesewaves, through some mechanism not yetunderstood, appear to give rise to the darkmarkings at the largest scale.Even with this knowledge, researchers

have a poor understanding of the processesthat maintain the vigorous circulation of theatmosphere (10). In particular, there is noobservational evidence for an equatorwardmomentum transport, which would be animportant factor in explaining the observedspin of the atmosphere. Very little is knownabout the dynamical transports at small spa-tial scales and short time scales, and thepossibility that transports might be orga-nized to provide equatorward transport ofzonal momentum is ofconsiderable interest.The capabilities of the Galileo camera per-

mitted us to address a major part of ourexperiment to this question. It has beenestablished that small-scale motions existand can be dynamically complex. The Vegaballoons in 1985 discovered vertical mo-tions of -1 m s-l, varying with time scalesof a fraction of an hour in the vicinity of themiddle cloud layer. In addition, recent the-oretical work (11) has shown that trappedinternal waves are possible near the clouddeck.The Galileo camera (2) utilizes a virtual

phase charge-coupled silicon detector withlinear photometric response from 350 to1000 rm. Broad-band spectral resolution isobtained with an eight-position filter wheel,but only the clear (633 nm), violet (418nm), and NIR (986 nm) filters were used atVenus. The principal limitation on the ex-periment was the volume of data that couldbe accommodated on the tape recorder be-cause the main communications antennawas not available for use. The imaging se-quence (1, 12) was therefore limited to 81frames.We have used Irvine's (13) ground-based

photometry, with phase-angle dependence,to calibrate the brightnesses in the images.We found very small residuals between ob-served and calculated brightnesses and ob-tained good removal of terminator and limbgradients with the empirical function:

I=B-F 0 - exp(- [o/a)Irp 1 - exp(- p/b)

(1)I is the reflected intensity and F0 is the solarflux, both integrated over the instrumentpassband, go) is the cosine of the solarincidence angle (measured from vertical),and p. is the cosine of the emission angle. Inthe NIR, B = 0.85, k = 1.14, a = 0.118,and b = 0.0019. In the violet, B = 0.59, k =0.90, a = 0.0547, and b = 0.0039. The fit

Fig. 1. Time sequence of violet and NIR image pairs.Violet frames are to the left and NIR to the right. Northis at the top. The brightnesses have been divided by themean photometric function specified in Eq. 1 to empha-size the morphology of the markings. The bright limb inthe NIR images is an artifact of this process. Otherartifacts are small, faint circular features, which are

residual blemishes due to incomplete compensation ofthe shadows of small dust particles (12), and the faintcurved lines in the terminator and subsolar regions,which are digitization contours due to the heavy stretch.Time increases from top to bottom. Within each pair theimages are separated by 30 s; each row of images isseparated by approximately 2.01 hours. The retrogrademotion of the markings is easily seen at both wave-

lengths. The north-south brightness boundary stretchingacross the equator in the NIR frames correlates withmotions in the violet frames. The frame numbers are as

follows: 18494400, 18494445, 18506300, 18506345,18518400, 18518445, 18530200, and 18530245. Theresolution in the last two frames is 17 km per pixel, andthe phase angle is 46°.

1532

Fig. 2. Correlation image formed from the lastpair displayed in Fig. 1. A correlation box about10% of the planetary radius on a side was em-ployed. The background shade represents no cor-relation. The darkest and lightest regions on theimage represent correlation coefficients of about-0.7 and +0.5.

was made at a phase angle of 4T. We usedsuch models as a tool to remove the back-ground run ofbrightness from the images sothat the morphology of the smaller scalemarkings is easier to discern. Most of thedifference between the data and model pre-dictions is due to real contrast variationsover the disk. These are about 25% in theviolet and 3% in the NIR.

Figure 1 displays a series of NIR andviolet image pairs. The violet frames showsloping striations that form a global spiraltoward the pole, familiar from previous im-aging experiments; on global scales the Ve-nus cloud deck has a strikingly symmetricaland laminar-like dynamical appearance. Themottled area at low latitudes in the after-noon quadrant is also familiar and has beenattributed to convection (4). The generalpattern is consistent with formation ofclouds at low latitudes with no particularalignment or elongation, followed by shear-ing and stretching as the clouds drift pole-ward (11). The bright collar at high latitudesis thought to be part of a polar circulationand has also been observed previously.The patterns in the NIR image have never

been observed before. There is a suggestionof a spiral pattern at mid-latitudes. Thecontrast in the polar regions seems to bereversed, with the bright violet collar re-placed by a dark one. In addition, there is astrong discontinuity in brightness runningnorth-south across the equator, just down-wind ofthe subsolar region. This feature canbe seen to migrate with the zonal flowthrough the subsolar region, apparently instep with a related feature seen in the violet(probably associated with the structures atthe "root" of the horizontal Y feature oftenseen in images of the Venus ultravioletmarkings, but only weakly evident in theGalileo images). The tilt of the mid-latitudespirals is smaller than that of the violet

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Fig. 3. A sequence of violet images at intervals of1 day. The image numbers are 18494400,18633900, 18773300, and 18912800. The clouddeck rotates from east to west, or from right toleft, in these frames. The time sequence is fromleft to right and then top to bottom.

striations, although closer inspection of theviolet images shows a smaller tilt for large-scale features than that for small ones. As inthe violet, clouds tend to be patchy at lowlatitudes and linear or streaky at mid- andhigh latitudes. This result is consistent withthe same hypothesis described above forformation of the general pattern in the vio-let. It suggests that the same mechanism is atwork at the deeper level where the NIRcontrasts are formed.We have checked this hypothesis by

studying a short movie sequence of violetfeature track frames. The movie allows theeye to discern relationships that are some-

times hard to recognize on the individualpictures. We were able to identify severalsmall-scale features migrating polewardfrom low latitudes and melding into thestriated spiral pattern. At higher latitudes,elongated features squirt along the lanesformed by the large-scale striation pattern.The movie sequence also shows that

blotchy areas occur in patches astride theequator and that two distinct patches are

separated in longitude by a few thousandkilometers. Similar patches of bright-rimmed cellular features are also seen inimages taken much later in the sequence.

These features may be triggered by a large-scale wave. The small blotches themselvesare a few hundred kilometers in diameterand have lifetimes that can exceed 1 day(because we were able to track them for thistime). They are thus much larger and slowerthan convective elements should be accord-ing to simple theory, which would predictdimensions on the order of a scale height(about 5 kin) and time scales of less than an

hour. In Earth's atmosphere, fields of con-

vection cells are often seen with similarlylarge aspect ratios; however, whether theyare manifestations ofthe same phenomena isnot clear. A radiative instability (14) is an

alternative cause of these features, as could

Fig. 4. (A) Cylindricalprojection of a set ofimages taken over a

7-day period. A 4.4-day rotation periodwas adopted to definea longitude systemthat approximatelymoves with the equa-torial clouds. Theglobal dark and lightpattern displays the"y1 and shows thatthere is a wavenum-ber-1 albedo variation.(B) The zonal windspeed, averaged over

local times between 1and 2 p.m. and lati-tudes 00 to 15SN, isdisplayed, also exhibit-ing a wavenumber-1variation.

27 SEPTEMBER 1991

-120

-90[

i -60

o 30

0

60 -30 0 30 60Latitude (degrees)

Fig. 5. Velocities ofNIR and violet (VI) featuresas a function of latitude; (A) eastward, (B) north-ward. The vertical bars indicate the estimatederror, based on the sample standard deviationswithin each 15° latitude averaging bin.

be convective overshoot into an overlyingstable region (15). In any of these cases smallvertical shear would most likely be needed to

permit unaligned features to develop. Theorigin of the mesoscale blotches and cellularmarkings remains a puzzle.

In our examination of the images we havenot found any visible evidence for changes indoud morphology on time scales less than an

hour or two. Even at time separations of 2hours we see only gradual evolution in theshape of features with lifetimes on the orderof a day. Thus, we have not yet, for example,seen any evidence for small-scale internalgravity wave activity.We have investigated whether there are

cloud features in common between the NIRand violet images by forming a cross-corre-

lation image. Using the last pair of images ofFig. 1, we evaluated the cross-correlationwithin a sliding box 30 pixels square (theplanetary diameter is about 600 pixels) andformed the image displayed in Fig. 2. Thearea outside the disk is filled with a shade ofgray indicating zero correlation. The darkestregions in the image represent a correlationof about -0.7. We have experimented withrandom patterns and believe that the anti-correlation indicated by these dark regions ismeaningful. In some large areas on theimages it is quite clear by inspection that an

anticorrelation exists, such as near the polarcollar. In low latitudes the sense of correla-tion is mixed, except in the region upwindof where the NIR picture shows a north-south brightness boundary and the correla-tion is positive.The inverse relationship between bright-

nesses in the NIR and violet at high latitudescan be explained by changes in cloud density

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Latitude (degree)Fig. 6. Zonal mean velocities compared to earlierepochs; (A) eastward, (B) northward. The Gali-leo data is displayed as a solid line. Profiles from1974 are from Limaye and Suomi (19). Profilesfrom 1979 and 1982 were measured from PioneerVenus images by Limaye et at. (20). Profiles from1980 and 1983 are our measurements from Pio-neer Venus images.

in the altitude region between 60 and 65km. We have used Crisp's (16) descriptionof the cloud particles and optical propertiesto evaluate radiative fluxes in the two-streamapproximation, and we examined the sensi-tivity of reflected fluxes to the particle num-ber density at different heights. An increaseof the doud density between 60 and 65 kmhas the effect of increasing the NIR reflec-tivity but decreasing the violet one, becausecloud particles are more absorbing in theviolet. Other violet contrasts, which may or

may not be correlated with the NIR bright-ness, can arise from cloud changes at otherlevels in the atmosphere.Four violet images taken at 24-hour time

intervals are displayed in Fig. 3. Because thecloud top rotation period is -4 days, theseframes show the morphology of the entirecloud deck (as well as it can be determinedfrom monitoring one side of the planet).Frames covering the entire 7-day period ofGalileo imaging are linked together in cylin-drical projection in Fig. 4A. At this epoch,the large-scale dark and light patterns thatsometimes form the famous YT' feature donot seem to be strong but are evident.Patches of bright-rimmed cellular features(noted above) can also be seen in this pic-ture. Figure 4B shows a correlation ofzonalvelocity measurements with this pattern,which is discussed more fillly below.The National Aeronautics and Space Ad-

ministration (NASA) Jet Propulsion Labo-ratory (JPL) computer software programfor tracking cloud patches, AMOS (17), hasbeen used to measure displacements of fea-

1534

tures and to infer velocities. These velocitiesare true fluid velocities only if clouds act asinert tracers, which is not known to bestrictly true on Venus. Belton et al. (4) haveargued that because small- and moderate-scale features (up to a few hundred kilome-ters) seem to move together on Venus andatmospheric waves are in general dispersive,these features are unlikely to be waves. Fur-thermore, cloud tracking gives results thatare in accord with in situ probe determina-tions (18). However, very large scale albedofeatures are found to move with differentvelocities from small tracers and have beenidentified with planetary-scale waves propa-gating in the flow (4, 5). In the case of theGalileo data, the results are still preliminary.We have measured 1529 tracers in violet and126 in the NIR. The NIR features are largerthan the violet ones and much more diffuse.There are a large number of violet imagesyet to be measured, and improved tech-niques are being developed to increase theaccuracy with the NIR images.

Zonal mean velocities of violet featuresare displayed in Fig. 5. Averages wereformed by binning the data in 24-hourintervals and then averaging these bins withequal weight. Thus, each sector of the rotat-ing cloud deck is given roughly equalweight, in spite of the fact that many moretracers were located in the images of higherresolution. The average in latitude wasformed with sliding bins 150 wide. Theviolet tracers show somewhat larger zonalwinds (u = 101 -+ 1 m s-1 at the equator,where u is the eastward velocity) than those inmost previous epochs (Fig. 6) (19, 20) andexhibit weak mid-latitude jets. The 1979 Pi-oneer Venus epoch stands out as showing thevelocity profile most similar to solid-bodyrotation. Zonal mean meridional velocitiesare poleward, as in other epochs, and proba-bly represent a poleward branch of a plane-tary Hadley circulation at doud level.The NIR tracers (Fig. 5) show smaller

zonal velocities (u = 78 + 3 m s-1 at theequator), as would be expected if their ori-gin were seated deeper in the atmosphere.Doppler tracking of Pioneer Venus atmo-spheric descent probes has consistentlyshown vertical shear of this kind near thecloud region (18), whereas the experiencewith the Venera probes is mixed (10). Wecan make a very rough estimate of thepossible vertical separation of the violet andNIR markings using the average shear esti-mated on the basis of the sum of the probeexperience, that is, -1.5 m s'` km-l. Withthis as a guide, the violet and NIR markingscould be separated by as much as 15 kin,which would place the NIR markings some-where toward the bottom of the middlecloud layer. However, we must stress that

Fig. 7. High-resolution mosaic of images18218000 and 18218045. North is to the top.The left side is afternoon and the right side ismorning. The background photometric functionhas been removed. Note the two distinct patchesof activity on the equator, separated by a low-contrast zone.

this estimate cannot be taken as much morethan an educated guess. Perhaps the mostinteresting results on NIR motions are theextremely small meridional velocities, whichindicate a very weak Hadley circulation atthe level where these features originate. Ifwe assume random measurement error, anestimate of the uncertainty can be madefrom the standard deviation and the numberof samples. At 250 latitude this procedureyields u = 70 ± 3 m s' and v = 0.1 ± 2 ms , where v is the northward velocity.An independent estimate of the meridional

drift velocity of NIR features can be made ifone adopts the assumption described abovethat mid-latitude streaks are caused by shearingofblobs formed near the equator. The amountofshear, and therefore the tilt ofstreaks, is thenrelated to the shear stngth and the length oftime that it takes for a blob to drift to mid-latitudes. Under these conditions, blobs stretchout to conform approximately to streamlines(11), as observedin the frame rotatingwith theequatorial velocity. The slope of strmlin isdX/dO = -v cos O/(u - uq cos 0), where X ando are east longitude and latitude and uq iseastward velocity at the equator. The imagesshow that for large-scale streaky features dX/dOis about 0.1 and that the zonal velocity is nearlyindependent oflatitude. Solving for the merid-ional velocity at 300 givesv 1.2ms -

Figure 7 displays a two-frame mosaicfrom a series of high-resolution images,again with the background Fhotometricfunction removed, sequenced to follow thedevelopment of small-scale dynamical fea-tures downstream ofthe subsolar point. Theimages are rich in detail; hundreds of tiepoints can be located in each set ofpairs, andwe have made preliminary measurements ofmotions. Velocity vectors from one set ofpairs are displayed in Fig. 8. A transitionfrom organized, laminar-like behavior in themorning sector to disorganized behavior inthe afternoon is dearly evident. Cloud deckinstability forced by solar heating seems alikely cause, although at the levels in the

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LongitudeFig. 8. Map of high-resolution velocity vectors.The mean rotation profile has been subtracted.Note the more turbulent appearance of the after-noon sector. These measurements were madefrom images 18229900, 18229945, 18240700,and 18240745. The arrows show the displace-ment that would occur in a 12-hour time period;the actual time lapse between the image pairs is 2hours.

cloud deck associated with the violet mark-ings, the thermal structure of the atmo-sphere is not susceptible to buoyant insta-bility. There is no evidence in the Galileoimages for the circumequatorial belt phe-nomena seen by Mariner 10 (3-5) or theformation of "bowlike" waves upstream ofthe subsolar region. There is, however, evi-dence for a "detached" remnant of a bowlikewave structure that can be seen propagatingdownstream in the image sequence in Fig. 1.When the velocity field in solar-fixed co-

ordinates is examined as a function of time,an interesting behavior appears. It is dis-played in juxtaposition with the cylindricalprojection in Fig. 4B. A periodic oscillationof the zonal velocity with an amplitude ofabout 10 m sl is strongly suggested, al-though the data do not extend over a longenough time interval to verify the periodic-ity. Del Genio and Rossow (5) discovered asimilar zonal wind oscillation in PioneerVenus data. In the Galileo observations, theoscillation exhibits a particularly large am-plitude. Del Genio and Rossow attribute theoscillation to a Kelvin wave propagatingslightly faster than the cloud top rotation.

If venusian lightning flashes have powercharacteristics and frequency of occurrencesimilar to that of terrestrial lightning (21)and spectral characteristics similar to thosesuggested by Borucki et al. (22), then it isonly marginally possible that they could bedetected by the SSI camera. Nevertheless, inview of the considerable interest in Venuslightning, ten frames were devoted to asearch. No indications for the presence oflightning flashes were found in these pic-tures. By chance, four images included thebright star K-Geminorum (brightness V =3.57 magnitude; G8) at a distance of 1.3mrad from the dark limb, and we used thisobject as a guide to make a rough estimate of

27 SEPTEMBER 1991

the energy in a flash that we would have justfailed to detect. We found that any Venusflashes would have had to exceed a totaloptical energy per flash of -4 x 109 J tohave been detected. This is roughly threeorders of magnitude brighter than a typicalterrestrial flash.Three pairs of images of the limb of

Venus were obtained through the violet andNIR filters between 1.5 and 3.6 hours afterclosest approach to define the vertical struc-ture of the cloud layer above 85 km (-1mbar; 180 K). Two pairs ofimages capturedthe limb near 48° and 57N with resolutionsof 0.5 (violet) and 0.9 km per pixel (NIR),and one pair included the limb near 20Swhen the resolution had fallen to 1 and 1.8km per pixel, respectively (these resolutionestimates take into account smear during theexposures). Figure 9 illustrates the derivedrun of extinction coefficient with heightabove the surface at the three latitudes,derived with an inversion program devel-oped to analyze limb images in the Voyagerprogram (23). Allowance was made for scat-tering from both the direct solar beam andthe light reflected from the lower lying,optically thick cloud deck with a modeldeveloped by Hapke and co-workers (9).The zero point for the height scale was madewith the assumption that the half intensitypoints along the limb are at a radius of6136km in both filters (12) and that the surface isat a radius of 6051 km. The profiles shouldbe most accurate in the altitude range from83 to 96 km. At lower altitudes, the slantpath optical depth exceeds unity, and theinversion rapidly loses accuracy; at higheraltitudes, the data become too noisy. Theupturn in the extinction coefficients above96 km is an artifact ofthe inversion programthat results from a combination of low sig-nal-to-noise and uncertainties in the level ofthe zero-point brightness value. Additionaluncertainties are introduced by the presenceof weak ghost images induced by the frontaperture cover (12). We have simulated theeffects of these ghost images and find thatthey will not introduce any localized artifactsat the altitudes in the above range. The ghostscan, however, cause substantial changes in thederived extinction coefficients and aerosolscale heights. We estimate that the latter maybe increased by 10 and 20% (violet and NIR,respectively) over their true values between87 and 96 km by this effect. We have notincluded the influence of far-field scatteredlight in this preliminary analysis, but thisinfluence is not expected to be significant.The extinction profiles at common lati-

tudes are effectively the same at both wave-lengths and share similar shapes and magni-tudes (Fig. 9). This implies that particlescattering, rather than molecular scattering,

100 95 90 85 100 95 90 85 100 95 90 85Altitude (km)

Fig. 9. Run ofvolume extinction coefficient withheight in the cloud tops at three different lati-tudes. Continuous lines correspond to imagestaken through the violet (418 nm) filter; dashedlines correspond to the NIR (986 nm) filter. Thealtitude scale refers to the surface, assumed to beat a radius of 6051 km.

is chiefly responsible for the observed lightat relatively high altitudes in the atmosphereand that the mean size of the scatterers is atleast a few tenths of a micrometer, in accordwith results from Pioneer Venus at loweraltitudes (6). The more subdued amplitudeofthe prominent local maximum near 90 kmat 57N in the violet filter may indicate thatRayleigh scattering is beginning to show itspresence at 418 nm (8).The low latitude profiles show a smooth

decline in extinction with increasing altitudewith no evidence for a discrete layer. How-ever, the extinction coefficient displays asignificantly sharper decline with increasingaltitude from about 84 to 87 km (scaleheight = 1.7 km) than from 87 to 94 km(scale height = 4.5 km, approximately thesame as the local gas scale height). Thehigher latitude profiles differ from the abovein that they show evidence for a discretelayer near 90 km and a larger scale height(-3.1 km) in the lower altitude region.Generally, the behavior of limb hazes seenby Galileo is similar to that seen in Mariner10 images (8, 9). However, the local maxi-ma on the Mariner 10 images occurred atlower altitudes (79 to 85 km) and were mostmarked at low latitudes, and the discretelayers had larger amplitudes than the onesseen at higher latitudes in the Galileo imagesobtained 15 years later.The above comparisons imply that signif-

icant changes in the upper levels ofthe cloudoccur on time scales of years, in accord withchanges in gas abundance and particle prop-erties derived from Pioneer Venus data atsimilar altitudes (8, 24).

REFERENCES AND NOTES

1. T. V. Johnson et al., Science 253, 1516 (1991).2. K. P. Klaasen, M. C. Clary, J. R. Janesick, Opt. Eng.

23, 334 (1984); M. J. S. Belton et al., Space Sd.Rev., in press.

3. B. C. Murrayetal., Science 183, 1307 (1974); L. D.Travis et al., ibid. 203, 781 (1979); W. B. Rossow,A. D. Del Genio, S. S. Limaye, L. D. Travis, P. H.Stone, J. Geophys. Res. 85, 8107 (1980); L. W.Esposito, R. G. Knollenberg, M. Ya. Marov, 0. B.Toon, R. P. Turco, in Venus, D. M. Hunten, L.

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Colin, T. M. Donahue, V. L. Moroz, Eds. (Univ. ofArizona Press, Tucson, 1983), pp. 484-564; D.Allen and J. W. Crawford, Nature 307, 222 (1984);V. M. Linkin et at., Science 231, 1417 (1986); S. S.Limaye, Icarus 73, 212 (1988); D. Crisp et al.,Science 246, 506 (1989); W. B. Rossow, A. D. DelGenio, T. Eichler, J. Atmos. Sci. 47, 2053 (1990).

4. M. J. S. Belton et al., J. Atmos. Sci. 33, 1383(1976); M. J. S. Belton, G. R. Smith, G. Schubert,A. D. Del Genio, ibid., p. 1394.

5. A. D. Del Genio and W. B. Rossow, ibid. 47, 293(1990).

6. K. Kawabata et al., J. Geophys. Res. 85, 8129(1980).

7. R. G. Knollenberg and D. M. Hunten, ibid., p.8039.

8. B. O'Leary, J. Atmos. Sci. 32, 1091 (1975).9. D. Wallach and B. Hapke, Icarus 63, 354 (1985).

10. G. Schubert, in Venus, D. M. Hunten, L. Colin, T.M. Donahue, V. I. Moroz, Eds. (Univ. of ArizonaPress, Tucson, 1983), pp. 681-765.

11. P. J. Schinder, P. J. Gierasch, S. S. Leroy, M. D.Smith, J. Atmos. Sci. 47, 2037 (1990).

12. M. J. S. Belton et al., Adv. Space Res., in press.13. W. M. Irvine, J. Atmos. Sci. 25, 610 (1968).14. P. J. Gierasch, A. P. Ingersoll, R. T. Williams, Icars

19,473 (1973); S. J. Ghan, J. Atmos. Sci. 46,2529(1989).

15. G. Schubert, personal communication.16. D. Crisp, Icamus 67, 484 (1986); ibid. 77, 391 (1989).17. G. M. Yagi, J. J. Lorre, P. L. Jepsen, inPnroceedis ofthe

Co,*rc on Atspheric Entvrnm ofAeropace Sys-tems and Applied M Aoy (American MeteoogicalSociety, Boston, 1978), pp. 110-117.

18. See, for example, C. C. Councelman, S. A. Goure-vitch, R. W. King, G. B. Loriot, and E. S. Ginzberg[J. Geophys. Res. 85, 8026 (1980)].

19. S. S. Limaye and V. E. Suomi, J. Atmos. Sci. 38,1220 (1981).

20. S. S. Limaye, C. J. Grund, S. P. Burre, Icars 51,416 (1982); S. S. Limaye, C. Grassotti, M. J.Kuetemeyer, ibid. 73, 191 (1988).

21. M. A. Williams, E. P. Krider, D. M. Hunten, Rev.Geophys. Space Phys. 21, 892 (1983); Z. Levin, W.J. Borucki, 0. B. Toon, Icars 56, 80 (1983).

22. W. J. Borucki et al., Geophys. Res. Lett. 10, 961(1983).

23. J. B. Pollacketal., J. Geo . Res. 92,15037 (1987).24. We gratfully acknowedge an interal review of the

manuscript by G. Schubert and the assiste of H.Breneman, B. Carcich, C. Cunningham, E. Dejong, S.Howell, B. Pakowsk, T. Thompson, A. Toigo, andL. Wainio in the planning and execmtion of the SSIexperiment and the subsequent reduction of the dam

21 June 1991; accepted 29 July 1991

Middle Infrared Thermal Maps of Venus at theTime of the Galileo EncounterGLENN S. ORTON, JOHN CALDWELL, A. JAMEs FRIEDSON,TERRY Z. MARTIN

Images ofthe disk ofVenus, taken at wavelengths between 8 and 22 micrometers, wereobtained a few days after the Galileo spacecraft's closest approach on 8 February 1990;these images show variations in the thickness of the main H2S04 cloud deck and theoverlying temperature structure. Several features are qualitatively similar to those ofearlier observations, such as a hot region at the south pole, surrounded by a cold"collar," and brightening toward the lower latitudes, where low-contrast bandingappears. The collar does have a northern counterpart that is warmer, however.Equatorial limb darkening is quantitatively similar to that of previous observations;fairly constant at wavelengths up to 20 micrometers, where limb darkening increasessubstantially. In contrast to what was found in previous observations, polar andequatorial limb darkening are nearly the same at most wavelengths. A longitudinalvariation is observable that is consistent with a wavenumber-2 behavior and a

brightness maximum near local midnight.

A S PART OF A PROGRAM TO SUPPORT

Galileo Venus science analysis andcompare observations of the atmo-

sphere of Venus with previous observationsof the main H2SO4 doud deck and overly-ing temperature structure, we imaged Venuson the mornings of 10, 11, and 12 February1990 in the middle infrared. Poor weatherprevented us from observing on 8 February,the day of the spacecrafts closest approachto Venus, or the following morning. Wemade the observations at the National Aero-nautics and Space Administration (NASA)Infrared Telescope Facility at the summit of

G. S. Orton, A. J. Friedson, T. Z. Martin, Jet PropulsionLaboratory, California Institute of Technology, 4800Oak Grove Drive, MS 169-237, Pasadena, CA 91109.J. Caldwell, Physics Department, York University, 4700Keele Street, North York, Ontario, Canada M3J 1P3.

1536

Manna Kea at wavelengths of 8.57, 11.52,13.00, 18.00, 20.24, and 21.88 gm. Ihewavelengths were chosen fbr comparison withearlier ground-based images (1, 2) and spatiallyresolved spacecraft data (3-5); the three longestwavelengths also overlap the spectral range ofthe Galileo Photopolarimeter-Radiometer(PPR) radiometric channels A, B. and C. The60 pixel by 60 pixel images were produced byraster scanning in a regular grid pattern with1arc sec spacing in both directions; the en-

trance aperture was just under 2 arc sec in

diameter. The diskofVenus, 23 to 25 days pastinferior conjunction, subtended 49 arc sec. andthe evening terminator was located about 420east (Fig. 1) of the central meridian. We usedthe results of well-calibrated spacecaft experi-ments (4, 5) to normalize our absolute intensi-ties.

The images from wavelengths of8.57 and11.52 Am (Fig. 1) show the greatest inten-sity contrast across the disk, a result of thestrong dependence of their brightness ontemperature. The east-west orientation ofthe scan rows is evident as narrow streaks,particularly near the limb of Venus, arisingfrom short-term seeing and transparencyvariations. Evident in most of these imagesis a hot feature at the south pole. Hotfeatures have been observed at the polessince the earliest thermal maps (6). Thisfeature is consistent with Pioneer VenusOrbiter Infrared Radiometer (OIR) obser-vations (7) of a bright, elongated feature atthe north pole, rotating retrograde with aperiod of some 3 days. A much cooler"collar" surrounds the hot pole. Closer tothe equator, the planet is brighter and con-tains lower contrast bands. None of thesefeatures is axisymmetric or stationary inearth- or solar-fixed coordinates.The cold circumpolar collar and the

brighter bands can also be seen in mosaicscreated from the images at each wavelength(Fig. 2). The direction of scans is evident asthe nearly horizontal narrow bands slopingslightly upward toward higher longitudes.The best match of features contained inseparate images implies an offset of roughly90° in longitude per earth day, consistentwith the 4-day retrograde period associatedwith ultraviolet features (8). The hot polarfeature is confined to latitudes poleward ofabout 70°, and the collar is confined tolatitudes between 500 and 700, also consis-tent with the Pioneer Venus OIR results (4,7). Except at 8.57 gm, the mosaics alsoshow a cool region poleward of 450N. Thepresence of a northern counterpart of thesouth pole collar is consistent with GalileoPPR maps and Near Infrared MappingSpectrometer (NIMS) images of Venusfrom 5-,um thermal emission from the samecloud deck (9), but this collar is warmer thanits southern counterpart. Images ofthe ther-mal emission of the night side of Venus atshorter wavelengths (9, 10) show a dark areanear the pole that could correspond to thecollar. Figure 2 shows a relatively brightstreak between 30° and 500S and a fainterstreak between 200 and 40WS. There is nomorphology in the Venus night side near-infrared data (9, 10) that immediately corre-sponds with these features. If the streaks arecaused by a variation of cloud optical thick-ness, the cloud layer in question may be onlyone of several different components of acloud system discernible at the shorter wave-lengths.The general morphology of brightness

temperature features on Venus appears to besimilar for all wavelengths greater than 8.57pum, even at 13.00 and 18.00 pLm, where the

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Images from Galileo of the Venus Cloud Deck

GREENBERG, JAMES W. HEAD III, DAVID MORRISON, GERHARD NEUKUM and CARL B. PILCHERMICHAEL H. CARR, CLARK R. CHAPMAN, MERTON E. DAVIES, FRASER P. FANALE, RONALD GREELEY, RICHARDPOLLACK, KATHY A. RAGES, ANDREW P. INGERSOLL, KENNETH P. KLAASEN, JOSEPH VEVERKA, CLIFFORD D. ANGER, MICHAEL J. S. BELTON, PETER J. GIERASCH, MICHAEL D. SMITH, PAUL HELFENSTEIN, PAUL J. SCHINDER, JAMES B.

DOI: 10.1126/science.253.5027.1531 (5027), 1531-1536.253Science 

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