Sykes et al.: Interplanetary Dust Complex and Comets 677

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The Interplanetary Dust Complex and Comets

Mark V. SykesSteward Observatory

Eberhard GrünMax Planck Institut für Kernphysik

William T. ReachCalifornia Institute of Technology

Peter JenniskensSETI Institute

With the advent of spacebased in situ and remote sensing technologies, our knowledge ofthe structure and composition of the interplanetary dust cloud has changed significantly. Bothasteroidal and cometary sources of the cloud interior to Jupiter have been directly detected. Adistant contributing source, the Kuiper belt, has been also been detected beyond Saturn. Analysisof the morphology and composition of collected interplanetary dust particles (IDPs) is increas-ingly sophisticated. Meteor storms are more accurately predicted. Yet the fundamental ques-tion of whether asteroids or comets are the principal sources of interplanetary dust is still openand more complex. By understanding the origin and evolution of our own interplanetary dustcloud, and tracing its constituent particles to their roots, we are able to use these particles toprovide insights into the origin and evolution of their precursor bodies and look at dust pro-duction in the disks about other stars and compare what is going on in those solar systems tothe present and past of our own.

1. INTRODUCTION

Earth moves through a cloud of interplanetary dust anddebris, extending in size from submicrometer to kilometersand in distance from a few solar radii through the Kuiperbelt and beyond. That portion of the micrometer-sized par-ticle cloud in the vicinity of the Earth’s orbit gives rise tothe zodiacal light, seen most prominently after sunset in thespring and before dawn in autumn at northern latitudes(Fig. 1). In 1693, Giovanni Cassini ascribed this to sunlightscattering off dust particles orbiting the Sun. It was in thelate eighteenth and early nineteenth centuries that it wasrealized that material from space might be showering downon Earth. In 1794, Ernst Chladni, the father of acousticalscience, argued for the extraterrestrial origin of meteors,fireballs, and meteorites (Yeomans, 1991). The spectacularLeonid meteor shower in 1833, appearing to emanate froma single location in the sky, convinced many scientists ofthe day that these were indeed of extraterrestrial origin.Almost immediately, a connection was made with cometsby W. B. Clarke and Denison Olmsted. Then Hubert Newtoncorrectly determined the orbit of the Leonids and predictedtheir return in 1866. Work by Giovanni Schiaparelli andothers in the mid-1800s continued to press the connectionbetween meteor streams and comets. This was reinforcedwhen Comet Biela was seen to have broken up and Earth

Fig. 1. The zodiacal light from Mauna Kea, Hawai‘i. Courtesyof M. Ishiguro, ISAS.

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experienced a meteor shower in 1872 when Earth subse-quently passed near its orbit (Yeomans, 1991).

Comets are a logical source for the interplanetary dustcloud. They are the only solar system objects visually ob-served to emit dust, and meteor streams were also linkedto specific comets. In 1955, Fred Whipple applied his new“icy conglomerate” model of comet nuclei to determine a“quantitative relationship between comets and the zodiacallight.” Since dust evolves toward the Sun under Poynting-Robertson drag (e.g., Burns et al., 1979), it needs to be re-plenished if it is to be maintained. Whipple (1955) estimatedthat approximately 1 ton per second of meteoritic materialwas required to maintain the zodiacal cloud against suchloss. He concluded that comets could easily supply thatamount of material. In the same paper, however, he alsonoted that calculations by Piotrowski indicated that thecrushing of asteroids may also be an adequate source of zo-diacal material. “Whether the comets or the asteroids pre-dominate in zodiacal contribution must be decided on thebasis of other criteria . . . from meteoric and micrometeor-itical information as well as from the shape of the zodiacalcloud” (Whipple, 1955).

In the 1930s, “cosmic spherules” found in deep sea sedi-ments were found to be compositionally similar to meteor-ites. Analysis of these findings led Ernst Öpik to estimatethat the Earth was accumulating 8 × 109 kg of meteoriticmaterial per year (Öpik, 1951). With the advent of the spaceage, the question of the dust environment beyond Earth’satmosphere and its potential hazard to spacecraft and futureastronauts grew quickly in importance, later subsiding asbetter spacebased detectors replaced those that had beenproviding anomalously high densities due to sensitivity tomore than just dust (cf. Fechtig et al., 2001). Returned orbit-ing surfaces from Skylab, orbiting facilities such as the LongDuration Exposure Facility (LDEF), and a series of space-craft possessing dust detection systems, including Helios,Hiten, Pioneer 9, Galileo, Ulysses, and Cassini, amongothers, soon gave new information and constraints on theinterplanetary dust environment as did microcrater studieson returned lunar samples. These indicated that the inter-planetary dust cloud was complex, having a number of dif-ferent components of possibly different origins (e.g., Grünet al., 1985).

In 1983, the first large-scale survey of the zodiacal cloudat thermal infrared wavelengths by the Infrared Astronomi-cal Satellite (IRAS) (Hauser et al., 1984), revealed the over-all cloud shape and, more importantly, the first spatial struc-tures within the dust cloud (Low et al., 1984; Sykes et al.,1986; Sykes, 1988) directly relating to its asteroidal andcometary origins. This was followed by a thermal survey bythe Diffuse Infrared Background Experiment (DIRBE) onNASA’s Cosmic Background Explorer (COBE) satellite in1990 (Silverberg et al., 1993; Hauser et al., 1998).

Since the late 1960s, the increasing amount and diver-sity of dust observations sampling different components ofthe dust complex (Fig. 2) have spurred a series of regularinternational conferences with associated volumes summa-

rizing the state of knowledge at that time, the most recentof which is Grün et al. (2001). The reader is referred tothis book and its antecedents for the detailed backgroundthat provides some of the context for this chapter.

2. MORPHOLOGY OF THE CLOUD ANDITS RELATION TO COMETS

AND ASTEROIDS

At thermal infrared wavelengths, sky brightness viewedfrom Earth orbit is dominated by the thermal emission ofinterplanetary dust particles (IDPs) heated by sunlight.These particles are about 20–200 µm in size (Reach, 1988).The IRAS observed the sky in four bands between 12 and100 µm over a range of solar elongations between 60° and120° at a resolution of arcminutes, mapping the entire skyalmost three times over the course of the mission. DIRBEsimultaneously observed in 10 wavebands from 1.25 to240 µm, covering the portion of sky between 64° and 124°solar elongation with fully sampled images at 1° resolution.Using the DIRBE sky maps, the zodiacal light was charac-terized using three components (Kelsall et al., 1998). Thesmooth cloud is the dominant component, and its spatialdistribution was mathematically fitted with a parameterizedfunction as follows in terms of spherical coordinates (r,θ,z)in AU where r is the cloud center distance, θ is the azimuthalangle, and z is the vertical distance from the cloud mid-plane:

n = nor–1.34exp[–4.14g(ζ)0.942]

Fig. 2. Different methods of studying the interplanetary dustcomplex are sensitive to different size/mass ranges. Consideredtogether, a more complete picture of the production and evolutionof interplanetary dust can be constructed.

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where

ζ = z/r

and

g(ζ) = ζ2/0.378 for ζ < 0.189

and

g(ζ) = ζ – 0.0945 for ζ ≥ 0.189

The smooth cloud is azimuthally symmetric, with its mid-plane tilted by 2.03° from the ecliptic and its center offsetfrom the Sun by 0.013 AU. Both the tilt and offset are ex-plained by gravitational perturbations by the planets (Der-mott et al., 1986); while the radial variation and the depend-ence on z/r are to first order explained by Poynting-Robertsondrag, which yields an r–1 distribution and leaves orbital incli-nation unchanged (Wyatt and Whipple, 1950).

The morphology of the main zodiacal cloud allows us,in principal, to assess the fraction of dust that derives fromcomets and asteroids. The Kelsall model above yields a half-width at half-maximum number density of about 14° fordust particle inclinations, while some models based on insitu particle detections suggest that the inclination distribu-tion may have a half-width as wide as 20° to 30° (Dikarevet al., 2002). For comparison, the half-width of the distri-bution of asteroid orbital inclinations (which is reasonablystructured) is between 12° and 16° (Minor Planet Center,2003), consistent with the Kelsall model, whereas short-period comets have inclinations generally less than 30°(Marsden, 1974) and half have inclinations less than about10° — not very different from asteroids. Evidently cloudscale-height does not clearly distinguish between asteroidsand comets as principal suppliers of dust.

The smooth cloud is supplemented by structures discov-ered by IRAS: asteroidal dust bands (Low et al., 1984) andcometary dust trails (Sykes et al., 1986) (Fig. 3). Trails rep-resent the principal means by which comets contribute dustto the zodiacal complex and are discussed in a later section.The dust bands stretch across the ecliptic plane in north-south pairs that straddle a midplane tilted by about 1° fromthe ecliptic. Soon after their discovery in the IRAS data,the bands were associated with the major Hirayama aster-oid families (Dermott et al., 1984) and explained as a natu-ral consequence of the general collisional comminution ofthe asteroid belt as a whole (since the families are regionsof asteroid concentration). An alternative to this “equilib-rium” theory was proposed by Sykes and Greenberg (1986)and Sykes (1990) in which the large-scale production of dustin the asteroid belt is stochastic (the “nonequilibrium” the-ory). In this case, observable dust bands would most likelybe associated with recent disruptions of small (~20 km) as-teroids. Detailed studies indicated that some bands seemedto be associated with the Themis and Koronis families(Sykes, 1990; Reach et al., 1997) and the Maria family

(Reach et al., 1997), but that other prominent bands suchas that initially associated with the Eos family (Dermott etal., 1984) were difficult to reconcile (Sykes, 1990; Reach,1992; Grogan et al., 2001).

Another means of understanding the relationship of thezodiacal cloud to asteroids and comets is by simulation ofits creation and the direct comparison with observations. Ina review, Dermott et al. (2001) concluded on the basis ofsuch models that dust in the asteroid bands observed byIRAS and COBE contributes about 30% of the total zodia-cal thermal emission. If only the Hirayama families (The-mis, Koronis, and Eos) contribute that much, then the restof the asteroid belt, according to the equilibrium model ofdust production, should contribute at least double that value,leaving at most only 10% to a cometary contribution. Thiswould be difficult to reconcile with a zodiacal cloud span-ning heliocentric ecliptic latitudes larger than the distribu-tion of asteroids (or comets for that matter) available tosupply it.

However, such a discrepancy (if it exists) may have beenmitigated by the identification of smaller asteroid familiesthat are dynamically younger and associated with two ofthe three major pairs of dust bands (Nesvorný et al., 2002,2003). This has given support to the nonequilibrium theoryof dust production in the asteroid belt. A consequence ofthis result is that the contribution of the asteroid belt as awhole to the zodiacal dust cloud is not expected to simplyscale with local asteroid density, increasing the complexity(and uncertainty) of the detailed relationship between aster-oids and the observed zodiacal cloud.

Whether asteroidal dust is produced continuously or sto-chastically, asteroids present a solid alternative to a purelycometary origin of the zodiacal cloud. Asteroid particles aredynamically distinct from cometary particles in that theyhave smaller initial orbital eccentricities. After being gen-erated in the asteroid belt, this dust evolves toward the Sun

Fig. 3. (a) The last scan of the ecliptic plane made by IRAS(76% complete). Ecliptic longitude increases from 0° (left) to 360°(right) with ecliptic latitudes between –30° and 30°. The diagonalstructure crossing the ecliptic plane near 90° and 270° longitudeis the galactic plane. The zodiacal cloud appears bright and wideat lower solar elongations, picking up the brighter thermal emis-sions of the warmer dust that lies closer to the Sun. At higher solarelongations, one looks through less dust near the Earth and seesa greater fraction of colder fainter dust. (b) High-pass filtering inecliptic latitude reveals structures in the zodiacal cloud associatedwith asteroid collisions and comets (Sykes, 1988).

(a)

(b)

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under Poynting-Robertson drag. As it passes the orbit ofEarth, some of it forms a circumsolar ring due to dust or-bits resonant with that of Earth (Dermott et al., 1994). Thering is just outside Earth’s orbit, at 1.03 AU, as the exte-rior resonances are more stable. The peak density in the ringoccurs in an enhancement that trails Earth, and amounts toabout 30% in excess of the density of the smooth cloud(Kelsall et al., 1998). The ring density is actually highlyuncertain, being derived from a line-of-sight integral, butthe estimate may be significantly improved using back-ground measurements by the Space Infrared Telescope Fa-cility (SIRTF) as it travels through the trailing enhancementduring its planned mission lifetime (Werner et al., 2001).

An intriguing aspect of the zodiacal cloud observed byIRAS and COBE/DIRBE is its azimuthal smoothness, about5% with the principal variation arising from dynamics-in-duced asymmetries in the dust ring near Earth’s orbit (Der-mott et al., 1994). This smoothness requires a source thatis similarly distributed over ecliptic longitude (i.e., withrandomized orbital nodes or a very efficient process forrandomizing the orbital nodes of the particles before theyspiral past Earth). The existence of the dust bands demon-strates that observed asteroid dust arises from a node-ran-domized population of parent bodies. Comets are muchfewer in number than asteroids, and material tracing theircurrent orbits would produce a relatively “lumpy” cloud.Whether a cometary component could be “smooth” dependson the outcome of the race between differential precessionof the ejected cometary particle orbits [104–107 yr depend-ing on ejection velocity (Sykes and Greenberg, 1986)], colli-sional lifetimes, and orbital decay timescales. If the comet-ary contribution to the zodiacal complex were due to singleor multiple generational collision fragments from largemillimeter–centimeter cometary particles [suggested as apossibility by Liou and Zook (1996)], the dispersion time-scales would tend toward tens of millions of years. Disrup-tion of such particles would be most likely by the smallestparticles capable of fragmenting them, resulting in low ejec-tion velocities on timescales (e.g., Dohnanyi, 1970) com-parable to or smaller than the dispersion timescales. Therapid orbital decay of small fragments (wherein lie the sur-face area) would result in the partial formation of a torus,extended toward the Sun (Sykes, 1990). Azimuthal smooth-ness of a purely cometary zodiacal cloud could be difficultto achieve. Better understanding of the detailed evolution ofthe nodes of cometary dust in addition to the other orbitalelements is a necessary step toward solving this problem.

The morphology of the zodiacal cloud changes with thesize of particles being considered. As particle sizes increasefrom tens of micrometers in size (to which IRAS and COBE/DIRBE were sensitive), their sensitivity to radiation forcesdecrease and their spatial distribution begins to convergeupon the distribution of yet-larger bodies from which theyultimately derive.

Information on objects in the millimeter to several-metersize range is obtained by meteor observations at Earth (sinceno such observations have yet been made at other planets).

The large end of this size range connects directly to thesize range accessible by near-Earth-object searches such asSpacewatch (Bottke et al., 2002a) and others. Ceplecha(1992) analyzes interplanetary bodies from millimeter-sizedmeteoroids to kilometer-sized boulders. The size distributionof smaller meteoroids is best documented in the lunar micro-crater record (Grün et al., 1985). Over this entire range theslope of the number density of these interplanetary bodiesvs. mass in a log-log plot is close to 0.83. This slope indi-cates that the population of these interplanetary bodies isin overall collisional equilibrium (Dohnanyi, 1970), i.e., thenumber of particles created in a mass interval by fragmen-tation of larger objects equals the number of particles in thatmass interval that are destroyed by collisions. However, inthe local mass distribution there are significant deviationsfrom this slope: One hump is at 10–9 kg and another is at104 kg. A hump in the mass distribution at mh signals thatthere is an excess of particles more massive than mh, prob-ably from an additional input in this mass range. One inputis in the 100-µm to centimeter size range, i.e., the range ofradar to visual meteors, and the other is in the 1–10-m sizerange, i.e., the range of very bright meteors, the fireballs.

Collisions govern the lifetimes of particles having these“excess” size ranges as well as larger particles up to the sizeof asteroids (whereas sublimation and breakup determinethe lifetime of comets in the inner solar system, <5 AU).Transport of these larger meteoroids through the solar sys-tem is facilitated by stochastic gravitational interactions withplanets, by the systematically inward (toward the Sun) driftcaused by the Pointing-Robertson effect, and by the Yar-kovsky effect (a thermal radiation force) acting in directionsthat depend on the orientation of the spin axis, spin rate,and thermal properties of the object (e.g., Bottke et al.,2002b). However, since the collisional lifetime is shorterthan the transport time, meteoroids only slowly diffuse awayfrom their place of origin while at the same time they areground down by collisions. Only meteoroids smaller than0.1 mm rapidly drift by the Pointing-Robertson effect to-ward the Sun where they sublimate. For fragments smallerthan about 1 µm in size in general, solar radiation pressurereduces solar gravity sufficiently to drive them out of thesolar system — they become beta-meteoroids — althoughparticles much smaller than 1 µm are less affected in thismanner (Burns et al., 1979).

Many clear orbital associations have been found betweenmeteor streams and comets from which they are presumedto originate. [Even some asteroids may be extinct comets.Several authors suggested that Apollo and Amor asteroidsare defunct comets. An argument for this thesis is that atleast the peculiar asteroid Phaethon is associated with theGeminid meteor stream (Halliday, 1988). By dynamicalstudies Gustafson (1989) showed that Phaeton’s cometaryactive phase lasted for several hundred orbits about 1000years ago.] On the other hand, it has been observed thatvisual meteor streams (millimeter and bigger sizes) are mostprominent in brightness and apparent point of origin on thesky compared with the sporadic meteor background. The

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contrast in these aspects between stream meteors and spo-radic meteors is reduced for smaller meteors and is onlyweakly recognizable in radar meteor observations [100-µmsize range (Galligan and Baggaley, 2001)]. No meteorstreams have yet been identified in the dust range (microme-ter size range). This observation indicates that the inputfrom comets into the interplanetary dust cloud near Earth’sorbit is most significant in the millimeter and larger sizerange, whereas smaller meteoroids are mostly collisionalfragments of the bigger asteroidal ones, ground down andtransported to Earth’s distance by radiation forces.

3. INFERENCES FROM THE PHYSICALPROPERTIES OF INTERPLANETARY DUST

3.1. Density: Meteors and Microcrater Studies

The notion that fragments of comets have low densitycomes from Whipple’s dirty snowball model where a cometnucleus consists of an intimate mixture of ice and dust.When the ice sublimates it leaves a filigree structure of dustwith much pore space from which the ice has been lost.Laboratory sublimation experiments of ice dust mixturesconfirm this picture (Grün et al., 1993a). A second line ofevidence comes from meteor observations that show thatthe terminal height (at which a meteoroid is sufficientlydecelerated so that it does not generate anymore light dur-ing its passage through the atmosphere), after scaling to thesame initial mass, inclination to horizon, and velocity dif-fer so much that the whole range of heights covers an airdensity ratio of 1 : 1000 (Ceplecha, 1994). This observationis interpreted as a consequence of a wide range of meteor-oid densities: Low-density meteoroids are decelerated athigher altitudes in the much more tenuous atmosphere thanhigh-density meteoroids. It was found that especially me-teor stream particles that have a clear genetic relation tocomets have very low material density, e.g., the Perseidsthat originate from Comet P/Swift-Tuttle. Ceplecha (1977)arrives at a classification of meteoroid orbits and densitiesfrom radar meteors (m ~ 10–8–10–6 kg) over photographicmeteors (10–6–1 kg) to fireballs (1–106 kg). “Asteroidal”meteoroids have high densities (~3 g/cm3) and orbits withmedium eccentricities (0.6) and low inclinations (10°).“Short-period comet”-like meteoroids have orbits withhigher eccentricities and their densities are lower (1–2 g/cm3). “Long-period comet”-like meteoroids have almostparabolic orbits, random inclinations, and very low densi-ties (0.2–0.6 g/cm3). From the study of fireballs from me-teoroids larger than 1 m, Ceplecha (1994) concludes thatthe majority of these bodies are of cometary origin and ofthe weakest known structure.

At the lower end of the size distribution, lunar micro-crater studies (Brownlee et al., 1973) suggested that about30% of all craters observed on lunar rocks were generatedby low-density meteoroids. This result was derived frommeasurements of the depth-to-diameter ratio of lunar micro-craters. Crater simulation experiments with hypervelocity

projectiles in the laboratory found that microcraters gener-ated by low-density projectiles had a significantly shallowerdepth than those generated in the same material by high-density projectiles [ρ > 3 g/cm3 (Vedder and Mandeville,1974; Nagel and Fechtig; 1980)]. Since cometary materialis generally associated with lower-density material thanasteroidal material, these authors conclude that at least 30%of interplanetary meteoroids originate from comets. Theeffects of the irregular shape of IDPs (e.g., Brownlee, 1985),however, are not known. Analysis of impacts on NASA’sLDEF and ESA’s Eureca satellite indicate a mean densityof IDP impactors between 2.0 and 2.4 g/cm3 (McDonnelland Gardner, 1998), somewhere in between canonical com-etary and asteroidal values.

3.2. Interplanetary Dust Particles

Interplanetary dust particles collected in Earth’s upperatmosphere provide clues to their asteroidal or cometaryorigin. Some of these particles have the kind of open struc-tures that are associated with dust expected from comets(Fig. 4). Compositions of IDPs range from chondritic (most)to iron-sulfide-nickel and mafic silicates. Some particles aremelted as a consequence of atmospheric entry. Measure-ments of the density of about 100 of these stratosphericIDPs (having diameters of 5–15 µm) found that unmeltedchondritic particles have densities between 0.5 and 6.0 g/cm3, about half of which are below 2 g/cm3, but with noobserved bimodality as one might hope with two distinctsource populations (Love et al., 1993).

Atmospheric entry heating was proposed as a means ofdistinguishing between IDPs arising from comets and aster-oids, given the greater average orbital eccentricity (henceaverage impact velocity) of the former (Flynn, 1989). Entryvelocities of IDPs were measured by Joswiak et al. (2000)

Fig. 4. A 10-µm-diameter particle collected in Earth’s strato-sphere. It is carbonaceous and very porous, suggesting that it maybe of cometary origin, although an asteroidal origin is not excludedas a possibility. Courtesy of NASA.

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applying a model of atmospheric heating and helium release(Love and Brownlee, 1994). High- and low-velocity groupswere distinguished, with the high-velocity (cometary) grouphaving an average density of 1.1 g/cm3 and exhibiting “fluffy,porous, aggregate textures” and the low-velocity (asteroidal)group having average density of 2.5 g/cm3 and tending “to-ward smoother, compact forms” (Joswiak et al., 2000).However, while the means were distinct, there was signifi-cant overlap between the two groups in their ranges of prop-erties, making it difficult to assign a particle to one group oranother on the basis of density and morphology unless itresided at the extremities associated with those groups. Thisdifficulty is in part due to the potential pumping up of a par-ticle’s orbital eccentricity (or that of its collisional precursor)by planetary perturbations. The properties of dust from aspecific known comet will be obtained by the Stardust mis-sion when it collects dust from the environment of P/Wild 2and returns it to Earth (Brownlee et al., 2000). This will bea great aid to further distinguishing cometary from asteroi-dal particles among collected IDPs.

3.3. Helios and Other Missions

Other evidence for distinguishing cometary and asteroi-dal particles on the basis of their orbits and physical prop-erties came from the Helios dust experiment. The dust in-strument on the Helios spacecraft consisted of two sensorsthat were mounted differently in the spacecraft. The eclipticsensor was sensitive to impacts arriving from both north andsouth sides of the ecliptic plane. Since this sensor viewedthe Sun once per spin revolution (the spacecraft spin axiswas perpendicular to the ecliptic plane) it was covered byan aluminum-coated 0.3-µm-thick plastic film in order toprevent heat and solar UV radiation entering into the sen-sor. This film caused a penetration cut-off for meteoroidsthat depended on the mass, density, and velocity of impact-ing dust particles (Pailer and Grün, 1980). The south sensorhad an open aperture that was shielded from solar radiationby the spacecraft rim and hence recorded only dust impactsarriving from south of the ecliptic plane. This sensor wassensitive to somewhat smaller and/or lower-density meteor-oids. Both sensors had overlapping fields of view.

Helios measurements covered the range from 0.3 to1 AU heliocentric distance. The measured dust flux displaysa steady increase toward the Sun by about a factor of 10.There are significant differences between the measurementsby both sensors. The ecliptic sensor detected most impactsin a band centered about the apex direction (i.e., 90° off theSun in the direction of spacecraft motion), while the southsensor observed particles from all around during a spinrevolution with a predominance of small particles from thesolar direction (Grün et al., 1980). Modeling of the Heliosresults show that these “apex” particles have low eccentrici-ties (eave ≤ 0.6) and small semimajor axes (averaging about0.6 AU). Since apex particles did penetrate the front filmof the ecliptic sensor, their density cannot be below 1 g/cm3

(Pailer and Grün, 1980), at least not for the smallest par-

ticles detected. On the other hand, impacts outside the bandwere mostly observed by the south sensor and must havehigher eccentricities. Modeling shows that these “eccentric”particles have eccentricities, eave ~ 0.7, and semimajor axes,aave ~ 0.9. Grün et al. (1980) conclude that at least half theeccentric particles should have densities below 1 g/cm3, sug-gesting a cometary origin.

Four planned comet flybys (and an unintended one) fromwhich in situ dust data became available were performed todate. Four spacecraft took dust measurements within 104 kmof the nuclei of different comets. In 1985 the InternationalCometary Explorer (ICE) mission flew through the coma ofComet Giacobini-Zinner, and the plasma wave instrumentrecorded dust impacts in the tailward region of the coma(Gurnett et al., 1986). One year later, a five-spacecraft ar-mada flew by Comet Halley, of which three spacecraft car-ried a range of dust instruments from simple impact count-ers to sophisticated dust-mass analyzers. The two RussianVega spacecraft crossed the sunward side of the coma andrecorded dust impacts from the outer boundary at a distanceof 2 × 105 km down to about 8000 km from the nucleus(Mazets et al., 1987; Simpson et al., 1987; Vaisberg et al.,1987). ESA’s Giotto spacecraft flew closest (600 km) to thenucleus, but most measurements ended on approach at a dis-tance of about 3000 km when a millimeter-sized pebble hitthe spacecraft with a speed of almost 70 km/s, causing somedamage onboard and interrupting telecommunication. Sometime later ground control over the spacecraft was regainedand several instruments continued their measurements. Sixyears later, in 1992, the Giotto spacecraft was redirected tofly through the coma of Comet Grigg-Skjellerup at a dis-tance of only 200 km from the nucleus (McDonnell et al.,1993), although it ended up passing at a distance of ~100 km(McBride et al., 1997). This was possible because the dustproduction of this comet was very low and only three parti-cles between approximately 1 and 100 µm and a fourth par-ticle ~10 mg were recorded to hit the approximately 2-m2

big bumper shield (a fairly flat distribution over several or-ders of magnitude, consistent with the mass distributionseen at Halley). The latest flyby of Comet Borrelly by theDeep Space 1 spacecraft occurred in 2001. The plasma waveinstrument onboard recorded several dust impacts within5 minutes of closest approach at 2000 km distance from thenucleus (Tsurutani et al., 2003). Almost 30 years earlier,in 1974, the dust instrument onboard the HEOS-2 satelliterecorded an enhanced impact rate of micrometer-sized parti-cles by a factor of 3 over what the instrument had observedin previous years (Hoffmann et al., 1976; Grün et al., 1976).During this period the instrument was pointed in the direc-tion where Comet Kohoutek (1993 XII) was about one yearearlier and had displayed strong dust emission that led to itsdetection at about 4 AU from the Sun. At the time of the re-corded dust impacts the comet was already 3 AU from theSun past its perihelion. Therefore, the dust recordings con-stitute measurements in the very distant tail of this comet.

Besides the spatial extent of dust in cometary comae andtails, the dust production rate and size distribution of comet-

Sykes et al.: Interplanetary Dust Complex and Comets 683

ary dust was derived from the in situ measurements. Mostdata, of course, came from the comprehensive measure-ments at Comet Halley. It was found that the dust size distri-bution extends over a much wider range than was expectedfrom astronomical observations, mostly in the optical wave-length range. The size distribution extends to both muchsmaller particles in the submicrometer and even nanometersize range, and to much bigger particles in the millimetersize range (McDonnell et al., 1987). It was also found thatsome particles fragment shortly after their release from thenucleus (Simpson et al., 1987; Vaisberg et al., 1987), whichindicates that their initial structure is very fluffy and containsmaterials that sublimates at distances as small as 1 AU fromthe Sun. As a consequence of this extended mass distributionthe dust production is significantly bigger than that whichhas been derived from astronomical observations alone. Inaddition, the dust-to-gas mass ratio of Comet Halley ex-ceeds a value of 1 (McDonnell et al., 1991) compared to avalue of 0.1 from earlier estimates.

The dust mass spectrometer onboard the Giotto and Vegaspaceprobes provided elemental and isotopic data for smallnewly ejected cometary particles (Kissel et al., 1986). Amajor discovery was that of “CHON” particles, which con-sisted of material high in content of the elements H, C, N,and O, showing that the dust was rich in organics (Kisseland Krüger, 1987). Magnesium isotope ratios showed onlya slight variation around the nominal solar value, whereasthe isotopic ratio of 12C/13C showed large variations fromgrain to grain, but on average it was also solar like (Jess-berger and Kissel, 1991). The average elemental composi-tion was found to be solar like, but significantly enriched involatile elements H, C, N, and O compared to C1 chondrites(Jessberger et al., 1987).

4. HOW COMETS SUPPLY THE CLOUD

4.1. General

When a comet is discovered, it is identified by virtue ofits fuzzy appearance, with perhaps a tail, arising from theloss of gas and dust. This dust represents the smallest-sizedparticle emissions from a comet [generally tens of microme-ters and smaller, although significant coma surface area isargued to reside in very large particles (see Fulle, 2004)].These are entrained in the gas outflow and accelerated tospeeds up to ~1 km/s for the smallest particles. After de-coupling from the gas, they are generally lost to the solarradiation field. The sensitivity of a particle to solar radiationpressure is described by the parameter, β, the ratio of radia-tion force to gravitational force felt by the particle (Burnset al., 1979). Most of the particles observed in a comet’stail at visible wavelengths are micrometer-sized and haveβ > 1. These particles are not gravitationally bound to theSun and escape the solar system, not contributing to theinterplanetary dust complex. Particles having lower β canalso escape from the solar system when they are releasedfrom an orbiting object like a comet, because of the con-

tribution of the comet’s motion. For emission at perihelion,the value of β for escape is a function of orbital eccentricityof the parent body

βp ≥ (1 – e)/2

Thus, for a parabolic comet, all emitted particles would belost. The solar system loses particles tens of micrometersand smaller from long-period comets, while retaining par-ticles on the order of several micrometers from short-pe-riod Jupiter-family comets (Fig. 5). Almost all dust particlestens of micrometers and smaller are released from theselatter comets into Jupiter-crossing orbits — even cometswhose orbits are completely interior to that of Jupiter’s(Fig. 6). Subsequent perturbations on the orbits of theseparticles by Jupiter results in their loss while the distribu-tion of many bear little resemblance to the elements of theirparent comets (e.g., Gustafson et al., 1987). Making theassumption that such scattered particles have randomizednodes (required to match the azimuthal symmetry of thecloud), Liou et al. (1995) was able to model a contributionto the cloud by single-sized particles from Encke, takinginto account radiation pressure, Poynting-Robertson andcorpuscular drag, and perturbations by Jupiter, which whencombined with a model contribution from asteroid dustmade good matches to selected scans of the zodiacal cloudby IRAS.

Cometary particles may undergo considerable orbitalevolution with time, increasing the difficulty of distinguish-ing them from asteroidal particles. Liou and Zook (1996)

Fig. 5. Maximum β (minimum radius, assuming a density of 1 g/cm3) of particles from known comets on escape trajectories fromthe solar system, assuming perihelion emission and zero ejectionvelocity. Aphelion distances of source comets are dashed lines.For circular orbits, βp = 0.5. Particles are assumed to have zeroalbedo.

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determined that some cometary particles (from Tempel 2–like comets) could be injected into mean-motion resonanceswith Jupiter and trapped for thousands of years, after whichtheir orbital eccentricities would be quite small. They wouldapproach Earth with the low velocities expected for aster-oid particles. This would help explain the overlap in mor-phologies and compositions among collected “cometary”and “asteroidal” IDPs, identified by their model atmosphericentry speeds.

4.2. Dust Trails

4.2.1. Discovery and observations. In 1983, the firstsurvey of the entire sky at thermal infrared wavelengths wasconducted by IRAS (Neugebauer et al., 1984). Part of theongoing analysis during this mission was the IRAS FastMover Program (Davies et al., 1984; Stewart et al., 1984;Green et al., 1985) in which fast-moving solar system ob-jects were sought. Six comets and a couple of Apollo as-teroids (including Phaethon) were discovered. However, acuriously extended tail associated with P/Tempel 2 was de-tected over the course of a number of IRAS scans. This wasmanifested by about 50 faint, relatively collinear sources at25 µm. It was found to extend 10° on the sky with a width of4', and no similar feature was found by the program associ-ated with any other comet observed by IRAS (Davies et al.,1984; Steward et al., 1984). Dynamical analysis showed this

“anomalous tail” to be the result of low-velocity emissionsof large particles, some of which may have occurred at least1500 days prior to the observations (Eaton et al., 1984).

An examination of IRAS image products, in which in-dividual IRAS scans were merged into images, revealed thecontinuous emission of the reported Tempel 2 tail extendingover 48° of sky (Fig. 7). Similar features were found asso-ciated with other comets (Sykes et al., 1986). Clearly, a newcometary phenomenon had been discovered by IRAS andwas referred to as “trails.” An examination of all the IRASdata yielded trails associated with eight short-period comets(Table 1) and about an equal number of “orphans” not as-sociated with any known comet (Sykes and Walker, 1992).As with Eaton et al. (1984), all trails were found to be con-sistent with low-velocity emissions and emissions fromyears to more than a century before the time of observation.Thus, trails offered a continuous record of the emission ofcomets over that period of time. At the comet orbits, trailshave widths of several 104–105 km.

After IRAS, trails were no longer observed in the infra-red until the launch of the Infrared Space Observatory (ISO)more than a decade later. The ISO observed segments ofthe Kopff (Davies et al., 1997) and Encke trails (Reach etal., 2000). Unlike IRAS, ISO was a pointed and not a sur-vey instrument, so its ability to study trails was limited.However, the Kopff trail showed changes due to emissionssince IRAS. The Encke trail was observed from a particu-larly favorable angle of 35° above its orbital plane and in-cluded the comet allowing the emergence of the trail fromthe comet coma to be studied. Dynamical modeling of theEncke trail showed that the mass lost in trail particles (me-teoroids with radii of at least several millimeters) is muchlarger than the mass lost in gaseous or small-particle form,and the comet can only survive these large-particle lossesfor ~10,000 years (Reach et al., 2000).

At the time of its observation by IRAS, the Tempel 2 trailposition was sent to ground observers who were unable todetect it at visual wavelengths (Davies et al., 1984; Stewartet al., 1984). Several years later, a trail associated with P/Faye was accidentally detected in the visible by the Space-

Fig. 6. Perihelion and aphelion distances of parent comets (opencircles) and the aphelia of emitted particles of different β (filledcircles). Jupiter’s perihelion and aphelion distances are indicatedby solid lines.

Fig. 7. The Tempel 2 dust trail is seen to extend over 30° in thiscomposite image constructed from IRAS scans. The comet coma isseen at the left end of the trail. In the upper right corner is part ofthe central asteroid dust band. Background cloud-like structuresare interstellar cirrus.

Sykes et al.: Interplanetary Dust Complex and Comets 685

watch Survey in the course of searching for near-Earthobjects. One evening, a band 1'–2' in width lay across oneof their half-degree scans. The next night, they traced theband back to its cometary source. The trail extended 10°(Rabinowitz and Scotti, 1991). More recently, a programto observe trails from the ground at visual wavelengths hasbeen successful (Ishiguro et al., 2002), and other observersare beginning to report similar detections of dust trails (e.g.,Lowry et al., 2003). These observations are the harbingerof a new era of dust trail studies that will allow more ex-tensive mapping and characterization of large particle massloss from comets than has been previously been available.

4.2.2. Particle properties. Syndyne analysis of theTempel 2 trail (comparing it with the predicted locations ofcontinuously emitted particles with zero emission velocity)suggested particle diameters of about 1 mm (β ~ 0.001),assuming density of 1 g/cm3 (Eaton et al., 1984; Sykes et al.,1990). Trail particles are dark. This is evidenced by the earlyfailure to detect the Tempel 2 dust trail from the ground(Davies et al., 1984; Stewart et al., 1984) and supported byinitial estimates of the albedo of those particles using IRASmeasurements (Sykes, 1987). An extremely low albedo (onthe order of a percent) for Kopff trail particles has beenestimated on the basis of its groundbased detection (Ishiguroet al., 2002).

Trail particles have color temperatures that tend to be inexcess of blackbody values (Sykes, 1987; Walker et al., 1989;Sykes et al., 1990; Sykes and Walker, 1992), suggestingeither low-emissivity materials, a small particle component,or sustaining temperature gradients. The low emissivitiesrequired do not match that of any known nonmetallic ma-terials (Sykes and Walker, 1992). Particles small enough tohave the observed low β values would need to be smallerthan tens of nanometers (Burns et al., 1979), which wouldnot radiate efficiently at infrared wavelengths. Trail particlesappear to be uniformly large, dark, rapidly rotating particlesthat have thermal conductivity low enough to allow them

to sustain a latitudinal temperature gradient (Fig. 8). Lowthermal conductivity can be achieved with porosity, whichtranslates to low mass density.

4.2.3. What trails tell us about comets and their contri-bution to the interplanetary dust cloud. Dust trails revealthe principal mechanism by which short-period comets losemass: via the low-velocity emission of large particles. Esti-mates of refractory (dust) mass loss rates (Table 1), combinedwith gas mass loss from visible groundbased observations,reveal an average cometary dust to gas mass ratio of about3 (Sykes and Walker, 1992), which is significantly higherthan the canonical 0.1–1. This translates to roughly equalvolumes of “rock” and “ice” in a comet nucleus (assuminga rock density of 3 g/cm3 and an ice density of 1 g/cm3), andis consistent with the dust to gas limit for Comet Halley’s,

TABLE 1. Cometary dust trail information from Sykes and Walker (1992).

Name θ W ∆vp Age LM D/G

Churyomov-Gerasimenko* 1 50 2 3–11 13.1 4.6Encke 93 680 40 19–21 14.9 3.5Gunn 6 111 3 27–74 13.8 3.6Kopff 17 47 3 47–158 13.5 1.2Pons-Winnecke 3 40 3 6–21 13.2 2.4Schwassmann-Wachmann 1 10 769 5 114–148 14.5 1.6Tempel 1† 7 68 4 14–38 13.4 3.5Tempel 2 65 31 2 140–665 13.7 2.9

*Rosetta target.†Deep Impact target.

Includes the observed angular extent, θ (deg.); the width, W (103 km); normal velocity assuming peri-helion emission, ∆vp (m/s); an estimate of the age (in years) of the oldest emissions observed; anestimate of the corresponding comet mass loss rates, LM (log g/century); and an estimate of the dustto gas mass ratio of the comet, D/G.

Fig. 8. Color temperatures for individual 12, 25, and 60-µmscans of the Tempel 2 dust trail were calculated and scaled to1 AU. The top dashed line corresponds to the color temperatureof a sphere on which each point is in instantaneous radiative equi-librium with solar insolation. The bottom dashed line correspondsto the color temperature of a blackbody. The central solid linecorresponds to the color temperature of randomly oriented, rapidlyrotating spheres where each local latitude is in radiative equilib-rium with the average diurnal solar insolation. Geometric albedois assumed to be zero.

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based on observations by Giotto (McDonnell et al., 1991).Assuming all short-period comets have trails similar to

those identified in the IRAS observations, with a corre-sponding average mass loss rate of 8.4 × 108 kg/yr (Sykesand Walker, 1992), the amount of material contributed tothe zodiacal dust complex (assuming 150 comets) would be1.3 × 1011 kg/yr, a significant fraction of the ~2.9 × 1011 kg/yr lost within 1 AU that needs to be replenished if the cloudis in steady state (Grün et al., 1985). A recent optical/ther-mal imaging survey of comets also concludes that cometdust is a major supplier of the IDP cloud (Lisse, 2002).

4.3. Meteor Streams

The largest-sized particles supplied to the interplanetarydust cloud by comets are observed as meteor showers. Theybegin as dust trails, with individual returns by the parentcomet to perihelion causing separate trails in a pattern re-flecting planetary perturbations of the comet orbit itself.Perturbations on individual grains in the trail cause a cyclicmotion of the node of the trail near Earth’s orbit. Meteoroutbursts (including meteor storms) are seen when Earthpasses through a dust trail (e.g., Kresák, 1993). Differencesin perturbations acting on different trail fragments result inthese fragments superposing and smearing to the point thatthey populate a filament. Dispersal of comet trail/meteorstream material into the background zodiacal cloud can beinhibited by orbital resonances, which can maintain trailcohesion for long periods of time [Asher et al. (1999), whodetermined that the Leonid outburst of 1998 was dustejected in 1366]. Close encounters cause the grains to bedispersed into a broader meteor stream responsible for anannual shower, which can be identified with that comet’sorbit for thousands of years before dissipating and becom-ing indistinguishable from the background population ofparticles in that size range.

Success at predicting storms has been recently achievedwith the work of Jenniskens (1994, 1997), who forecast thereturn of the 1994 α-Monocerids based on the dust-trailhypothesis, and Kondrat’eva and Reznikov (1985), who pre-dicted the return of the 1998 Draconid storm (Fujiwara et al.,2001) and the 2001/2002 Leonid storms. The latter consid-ered the ejection of a single particle at perihelion in the direc-tion of motion of the parent comet and calculated the subse-quent gravitational perturbations on an orbit with enough lagto allow for a timely collision with Earth. McNaught andAsher (1999) and Lyytinen (1999) applied a refined modelto the Leonids, identifying the spatial distributions of dusttrails from its parent, P/Tempel Tuttle, which allowed mete-ors to be identified with emissions from specific epochs ofperihelion passage (e.g., Fig. 9). This allows for future stud-ies of the effects of age on larger cometary dust particles.

A principal means of analyzing particle number densitieswithin a meteor stream is via its zenith hourly rate (ZHR),the rate of visible meteors seen by a standard observer underideal conditions (radiant in the zenith and the star limitingmagnitude = 6.5) (Jenniskens, 1995). Precise measurementsof the ZHR from aircraft during the 1999 Leonid event

showed the dust density in Earth’s path and along the orbitto exhibit a sharp core, but with “wings” well described bya Lorentzian, while the perpendicular dispersion in a sun-ward direction is wider and exponential (Jenniskens et al.,2000). Possible explanations include emission at large he-liocentric distances combined with a higher degree of frag-mentation of dust particles within the coma near perihelion(Jenniskens, 2001). A mass loss rate of ~2.6 × 1010 kg/re-turn was measured.

4.4. Kuiper Belt Dust

Another direct source of interplanetary dust in both theinner solar system and beyond Jupiter is the source regionof the short-period comets themselves — the Kuiper belt.Flynn (1994) suggested that this might constitute a signifi-cant contribution to the interplanetary dust collected inEarth’s stratosphere. Liou et al. (1996) found that 20% ofthe grains generated in the Kuiper belt would evolve all theway to the Sun (the remainder being scattered out of thesolar system by the giant planets), and that particles between9 and 50 µm diameter would be depleted due to mutualcollisions and collisions with interstellar dust. They furtherfound that particles surviving into the inner solar systemwould have low-eccentricity, low-inclination orbits, mak-ing them dynamically indistinguishable from evolving as-teroidal particles (offering a possible explanation for theIDPs having “cometary” physical properties, but heatingcharacteristics of asteroidal sources, above). More recently,Moro-Martín and Malhotra (2003) modeled the dynami-cal evolution of dust particles from the Kuiper belt, takinginto consideration the combined effects of radiation pres-sure, Poynting-Robertson drag, solar wind drag, and thegravitational forces of the planets (excluding Mercury andPluto) and concluded that near Earth, these grains wouldhave high eccentricities and inclinations, similar to cometarygrains and not asteroidal grains, contradicting Liou et al.(1996). They further concluded that between 11% and 21%of particles with 0.01 ≤ β ≤ 0.4 would drift from the Kuiper

Fig. 9. Activity curve of the 2001 Leonid meteor storms. Closedsymbols are from the Leonid Multi-Instrument Aircraft Campaign;open circles are data gathered by the International Meteor Orga-nization (Jenniskens, 2002).

Sykes et al.: Interplanetary Dust Complex and Comets 687

belt to interior to Jupiter [in rough agreement with Liou etal. (1996)], and that [assuming the dust production rates ofLandgraf et al. (2002)] the contribution to the IDPs cap-tured in Earth’s atmosphere may be as low as 1–2%.

Because of their large heliocentric distance (beyond30 AU), Kuiper belt objects (KBOs) are not active. Dustproduction in the Kuiper belt depends upon collisional ac-tivity, constrained by their size distribution and detailedorbital-element distribution. This production has been esti-mated by Stern (1996) to be between 9.5 × 108 to 3.2 ×1011 g/s (far more than needed to replenish the loss of dustestimated to be lost each year within 1 AU!). In addition,they determine that recent impacts would produce betweenzero and several hundred short trail-like structures, havingannual parallaxes of up to 2.6°. However, a parallactic sur-vey making use of two-week to six-month baselines pro-vided by separate maps of the sky by IRAS (Sykes et al.,1994) produced no evidence of parallax in any extendedstructures at 60 and 100 µm. Yamamoto and Mukai (1998)estimated a production rate of dust grains between 3.7 × 105

and 3.1 × 107 g/s with radii <10 µm as a consequence ofimpacts of interstellar dust on KBOs.

That dust from the Kuiper belt is being generated andtransported to smaller heliocentric distances, however, issupported by Landgraf et al. (2002) in their analysis of datafrom the dust experiments onboard Pioneer 10 and 11.Humes (1980) had reported an essentially constant spatialdensity between 1 and 18 AU. Landgraf et al. (2002) con-sidered three potential sources for the impacts recordedbeyond Jupiter: P/Halley-type comets, P/Schwassmann-Wachmann 1-type comets, and dust from the Kuiper belt.They found that the amount of dust detected beyond Saturncould only be explained by dust originating in the Kuiperbelt and evolving toward the Sun. Under their model, about5 × 107 g/s would need to be generated in the Kuiper belt(between 0.01 and 6 mm in size), more than an order of mag-nitude below the minimum estimate of Stern (1996). Therequired contributions from P/Halley-type comets were 3 ×105 g/s and that from P/Schwassmann-Wachmann 1 cometswere 8 × 104 g/s. It is interesting to note that dust produc-tion from P/Schwassmann-Wachmann 1 itself was estimatedto be ~105 g/s (Sykes and Walker, 1992, Table 1) in particles~1 mm in size.

5. EFFECT OF INTERSTELLARDUST PARTICLES

Interstellar dust particles are thought to play a role inthe production of interplanetary dust (e.g., Yamamoto andMukai, 1998) and its comminution (e.g., Liou et al., 1996).As the solar system moves through the galaxy, dust grainsthat pass through the planetary system have been detectedby the dust detector onboard the Ulysses spacecraft (Grünet al., 1993b). It came as a big surprise that after Ulyssesflew by Jupiter, the dust detector recorded impacts of in-terstellar dust (ISD) that arrived from a direction that wasopposite to the expected flow direction of interplanetarydust grains. It was found that on average the impact veloci-

ties of these particles exceeded the local solar system es-cape velocity (Grün et al., 1994).

The motion of ISD through the solar system was foundto be parallel to the flow of neutral interstellar hydrogenand helium gas with a speed of 26 km/s both for gas anddust. This proves that local interstellar dust and gas arenearly at rest with respect to each other. The interstellar dustflow was continuously monitored by Ulysses and persistedat a constant level at all latitudes above the ecliptic planeeven over the poles of the Sun, whereas interplanetary dustwas strongly depleted away from the ecliptic plane. Start-ing in mid-1996 the flux of ISD began slowly to decreaseand, in the year 2000, was about a factor of 3 lower [this isrelated to the reversal of the magnetic field in the courseof the solar cycle (Landgraf, 2000].

Measurements in the ecliptic plane by Galileo confirmedthat outside about 3 AU the interstellar dust flux exceeds theflux of micrometer-sized interplanetary grains. Interstellargrains observed by Ulysses and Galileo range from 10–18 kgto more than 10–13 kg. If compared with the ISD mass dis-tribution derived by astronomers, the mass distribution ob-served by spacecraft overlaps only with the biggest massesobserved by remote sensing. More recently, even bigger(10–10 kg) interstellar meteoroids have been reliably identi-fied by their hyperbolic speed (>100 km/s) at 1 AU (Bag-galy, 2000). The flow direction of these big particles variesover a much wider angular range than that of small grainsobserved by Ulysses and Galileo.

The deficiency of measured small grain masses is notsolely caused by the detection threshold of the in situ in-strumentation, but it indicates a depletion of small interstel-lar grains in the heliosphere. Model calculations by Frischet al. (1999) of the filtering of electrically charged grainsin the heliospheric bow shock region and in the heliosphereitself show that 0.1-µm-sized and smaller particles arestrongly impeded from entering the planetary system by theinteraction with the solar wind magnetic field.

6. PRODUCTION OF DUST IN OTHERPLANETARY SYSTEMS

The interplanetary dust cloud offers a “blueprint” or“laboratory” for understanding dust in other planetary sys-tems. The scattered starlight and thermal emission from dustaround other stars — exozodiacal light — is, for most stars,the only indicator we can observe from Earth of the colli-sional processes and small bodies around those stars. Further-more, these small bodies also provide evidence for planetsaround other stars. An inner hole and significant warp werediscovered in the β Pictoris disk (Lagage and Pantin, 1994;Heap et al., 2000), and blobs were discovered around Vega(Wilner et al., 2002) and ε Eridani (Greaves et al., 1998;Quillen and Thorndike, 2002) and Fomalhaut (Holland etal., 1998; Wyatt and Dent, 2002). These structures have allbeen explained by perturbations by planets around the stars,with the direct analogy to Earth’s circumsolar ring (dis-cussed above) providing key supporting evidence (Kuchnerand Holman, 2003). If viewed from afar, our own solar sys-

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tem might be recognizable as having at least two planets asa consequence of structure in dust evolving from the Kuiperbelt (Liou and Zook, 1999). Dust bands and trails in theinterplanetary cloud have been tied to asteroid families andindividual short-period comets; comparable structures havenot yet been observed around other stars, so the direct con-nection between parent bodies and the dust cloud can onlybe studied in detail in the solar system.

Exozodiacal light studies also provide valuable clues tounderstanding aspects of interplanetary dust that cannot bereadily discerned from our vantage point inside the systemand in its midplane. Among nearby main-sequence stars,some 15% have far-infrared emission, in excess of the pho-tosphere, that is believed to be due to circumstellar mate-rial (Backman et al., 1997; Habing et al., 2001). Most of thestars have “cold” infrared excess, detected at wavelengthsof 60 µm and greater. This is partly due to the rapid declinein photospheric emission at longer wavelengths, making far-infrared excess more prominent than mid-infrared excess.The cold excesses, with color temperatures ~80 K, are lo-cated relatively far from the central star and roughly corre-spond to the Kuiper belt in the solar system. In at least fourcases, dubbed the “fabulous four” by observers, the disksare resolved: β Pictoris, Vega, ε Eridani, and Fomalhaut allhave disks extended at least 100 AU from their central stars.In all cases, the Poynting-Robertson loss time is shorter thanthe stellar lifetime, indicating that the disks must be replen-ished by collisions among a reservoir of larger bodies. Thissuggests it is likely that our own Kuiper belt may be colli-sionally active and could be a source of dust, although thatdust between 9 and 50 µm in diameter may only reside inthe outer solar system because its transport to the inner solarsystem is hindered by collisions with interstellar dust alongthe way (Liou et al., 1996).

Exozodiacal light due to “warm” dust in the “terrestrialplanet” zone around other stars is more rare than the colderdust. This warmer dust is difficult to discern photometri-cally, because the disk emission is generally fainter than thephotosphere at wavelengths less than 30 µm (as opposed tothe cold excesses, which are often larger than the photo-sphere). A recent photometric survey found warm disksaround 5 out of 81 stars, and none of them had color tem-peratures higher than 120 K (Laureijs et al., 2002). Further-more, none of the stars older than 400 m.y. had warm exo-zodiacal light. In contrast, the zodiacal light as viewed fromEarth has a color temperature around 262 K (Reach et al.,1996) and the dust density has been shown to increase as apower-law all the way in to less than 0.15 AU form the Sun(Leinert et al., 1981). One problem with searching for thewarmer dust, which would be the analog of dust producedby asteroid collisions and short-period comets, is angularresolution. Mid-infrared spectra of the β Pictoris disk re-veal a bright silicate emission feature from the warm dust,with a shape that is different from that of the silicate featurefound in the zodiacal light (Reach et al., 2003). A Keck ob-servation with high angular resolution showed that the sili-cate feature arises only very close to the star (Weinbergeret al., 2003).

The very different vantage points for viewing the zodia-cal and exozodiacal light allow for very different insightsinto the collisional processes and evolution of small solidbodies. From inside the solar system, it is possible to use thebrightness as a function of look direction to obtain the scat-tering phase function (Hong, 1985; Kelsall et al., 1998); thephase function is needed to invert brightness distributions.From the zodiacal light, it is possible to measure the densityof interplanetary dust to within tens of solar radii, which isnot possible around other stars because of glare from thephotosphere. From the exozodiacal light, it is possible tomeasure the distribution of material to hundreds of AU,which is not possible from Earth-based observatories be-cause of the bright foreground from dust in the inner solarsystem.

7. THE FUTURE

7.1. Dust Detectors on Spacecraft

The first dust detectors flown in space were simple mi-crophones that responded to dust impacts, but also to a widerange of interferences that in interplanetary space occurredmore frequently than dust impacts. Once this was appreci-ated more sophisticated multicoincidence detectors weredeveloped that permitted the detection of dust impacts at arate as low as one impact per month. Impact ionizationprovided the means of at least two independent coincidentmeasurements of a dust impact: the plasma cloud gener-ated by an hypervelocity impact onto a solid target is sepa-rated by an electric field so that positive ions and negativeions together with electrons are recorded separately. Time-of-flight analysis of the ions even provides mass analysisof the generated ions. Early detectors of this type had asensitive area of ≤0.01 m2, which had the consequence thatonly very few dust impacts were recorded in interplanetaryspace. Therefore, the more recent dust detectors on theGalileo, Ulysses, and Cassini missions had sensitive areasthat were 10 times larger. Several recent missions carry dustmass analyzers in which the impact-generated ions are ana-lyzed in a mass spectrometer (e.g., Kissel, 1986). The Cas-sini cosmic dust analyzer (Srama et al., 1996, 2004) is themost sophisticated dust instrument to date. It combines a0.1-m2 impact ionization detector with a time-of-flight massspectrometer and charge sensing entrance stage for coarsevelocity and direction determination.

It is hoped that all future missions, particularly to theouter solar system, will include dust experiments. Dustbeyond Saturn will be studied for the first time since Pio-neer 10 and 11 with the student dust counter planned forthe New Horizons mission to Pluto.

7.2. DUNE Observatory

From knowledge of the dust particles’ birthplace and theparticles’ bulk properties, we can learn about the remoteenvironment out of which the particles were formed. Thisapproach could be carried out by means of a dust telescope

Sykes et al.: Interplanetary Dust Complex and Comets 689

on a dust observatory in space. A dust telescope is a combi-nation of a dust trajectory sensor together with an analyzerfor the chemical composition of dust particles.

Potential targets of a dust telescope are interstellar dust,interplanetary dust (e.g., meteor stream dust, cometary, orasteroidal dust or dust from the Moon), and even spacedebris (e.g., fine grains from solid rocket burns).

The first goal of a dust telescope is to distinguish by theirtrajectories dust particles from different sources: interstellargrains from the different types of interplanetary dust grains.Interstellar dust flows narrowly collimated through the so-lar system. This flow can be easily distinguished from theflow of interplanetary particles. Young cometary particleshave highly eccentric orbits, whereas asteroidal particleshave low eccentricity orbits. These different orbits are sepa-rated by measurement of the flight direction and speed. Dustin meteor streams occurs only during specific periods andis directly related to the parent comet.

A state-of-the-art dust telescope would consist of anarray of parallel mounted dust analyzers (Grün et al., 2000)and consists of several instruments sharing a common im-pact plane of about 1 m2 in size. Potential components are ahigh-resolution impact mass spectrometer, a dust analyzerfor the determination of physical and chemical dust proper-ties, and large-area impact detectors with trajectory analysis.Dust particles’ trajectories are determined by the measure-ment of the electric signals that are induced when a chargedgrain flies through an appropriately configured electrodesystem. After the successful identification of dust chargesof >10–15 Coulombs in space by the Cassini cosmic dustanalyzer, trajectory analyzers that are in development havetenfold increased sensitivity of charge detection giving ustrajectories for submicrometer-sized dust grains.

Modern dust chemical analyzers have sufficient massresolution to resolve ions with atomic mass numbers above100. However, since their impact area is only 0.01 m2, theycan analyze statistically meaningful numbers of grains onlyin the dust-rich environments of comets or ringed planets.Therefore, a dust telescope should include several of theexisting mass analyzers or a large area chemical dust ana-lyzer of mass resolution >100 with at least 10 times greatersensitive area, in order to provide statistically significantmeasurements of interplanetary and interstellar dust grainsin space.

7.3. Thermal Infrared Observations

Since IRAS and COBE there have been no surveys ofthe sky at thermal wavelengths. ISO allowed some studiesof cometary trails (Davies et al., 1997; Reach et al., 2000),and detailed SIRTF observations of a large number of short-period comets may allow us to greatly improve upon theestimates of cometary dust production contributing to thezodiacal cloud (as well as an understanding of their emis-sion history, retained in the trails). Only by surveys, how-ever, are we able to observe the cloud as a whole and modelits evolution and supply by entire populations of objects.But we have been limited by trying to understand the inter-

planetary dust cloud from observations made within it, nearits plane of symmetry. This makes distinguishing radial com-ponents difficult, because they are all coincident along ourline of sight and the stochastic nature of dust productionwithin the cloud complicates the interpretation of structures,volume distributions, and excesses (or deficits) comparedto models. It would be extremely useful to have thermal ob-servations of the cloud from a vantage well away from theecliptic plane, to look over the “top” of the zodiacal cloudinterior to Jupiter and study outer solar system dust and dustin the Kuiper belt directly. Peaking over the half-width ofthe Kelsall model of the zodiacal cloud at 1 AU, and assum-ing the extent of this inner cloud runs with the main asteroidbelt, an orbital inclination of about 45° would be requiredto observe dust in the Kuiper belt. This would parallacticallyshift the position of the inner cloud by more than 10°, allow-ing for the contribution of distant cold dust to be more easilydistinguished. In addition to detecting or placing meaningfullimits on dust being generated in the Kuiper belt, studies ofthe inner zodiacal cloud by such a system would greatly im-prove our understanding of the morphology of the cloud andthe relative contributions of asteroid collisions and cometemissions.

7.4. Earth as a Detector

The valuable information provided by meteor streamobservations on detailed structures of cometary emissionsand the physical properties of those particles argue for theincreasing application of modern technology to their study.First, continued efforts to predict these events must be madeto allow their observation. Long-term video and radar moni-toring for the identification of new meteor streams shouldbe conducted. Campaigns should focus on predicted me-teor outbursts with high-speed imagers and photometers,and head-echo of meteors using high-power radar, particu-larly focusing on small meteoroid masses, should be con-ducted to assess their fragmentation properties. Informationon meteoroid composition could be greatly improved by theuse of cooled CCD cameras for slitless optical spectroscopy,and the development of instruments that would focus onindividual emission lines/bands and the indirect detectionof organics. Together these should not only provide infor-mation on properties, but also allow us to assess differencesamong comets that might relate to different formation con-ditions and locations as well as ages.

8. FINAL NOTE

The ability to associate IDPs collected at Earth’s orbitwith specific sources is of great value in that it provides“sample return” information on these sources that mightotherwise be impractical to obtain as well as provide greatinsight into the origin and evolution of those sources andthe solar system in general. The increase in our very diversemeans of studying interplanetary dust, including atmos-pheric collection, in situ studies by spacecraft, remote ob-servations at visual and thermal wavelengths, and collisional

690 Comets II

and dynamical modeling have substantially increased thecomplexity (and interest) of the problem. The interplanetarydust complex is not in a steady-state condition. Evidencetoday bolsters the significant episodic infusion of dust byasteroid collisions, a greater potential cometary source dueto the discovery of their large particle emissions, and thecontribution at some level of dust from the Kuiper belt.Studies of these sources and interplanetary dust are increas-ingly interrelated. We need to study both sources and dustin order to have a more complete understanding of all.

Acknowledgments. This chapter benefited from the detailedreviews of D. Brownlee and N. McBride, to whom the authorsexpress their appreciation.

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