The Interplanetary Dust Complex and Comets

Transcription

1 Sykes et al.: Interplanetary Dust Complex and Comets 677 The Interplanetary Dust Complex and Comets Mark V. Sykes Steward Observatory Eberhard Grün Max Planck Institut für Kernphysik William T. Reach California Institute of Technology Peter Jenniskens SETI Institute With the advent of spacebased in situ and remote sensing technologies, our knowledge of the structure and composition of the interplanetary dust cloud has changed significantly. Both asteroidal and cometary sources of the cloud interior to Jupiter have been directly detected. A distant contributing source, the Kuiper belt, has been also been detected beyond Saturn. Analysis of the morphology and composition of collected interplanetary dust particles (IDPs) is increasingly sophisticated. Meteor storms are more accurately predicted. Yet the fundamental question of whether asteroids or comets are the principal sources of interplanetary dust is still open and more complex. By understanding the origin and evolution of our own interplanetary dust cloud, and tracing its constituent particles to their roots, we are able to use these particles to provide insights into the origin and evolution of their precursor bodies and look at dust production in the disks about other stars and compare what is going on in those solar systems to the present and past of our own. 1. INTRODUCTION Earth moves through a cloud of interplanetary dust and debris, extending in size from submicrometer to kilometers and in distance from a few solar radii through the Kuiper belt and beyond. That portion of the micrometer-sized particle cloud in the vicinity of the Earth s orbit gives rise to the zodiacal light, seen most prominently after sunset in the spring and before dawn in autumn at northern latitudes (Fig. 1). In 1693, Giovanni Cassini ascribed this to sunlight scattering off dust particles orbiting the Sun. It was in the late eighteenth and early nineteenth centuries that it was realized that material from space might be showering down on Earth. In 1794, Ernst Chladni, the father of acoustical science, argued for the extraterrestrial origin of meteors, fireballs, and meteorites (Yeomans, 1991). The spectacular Leonid meteor shower in 1833, appearing to emanate from a single location in the sky, convinced many scientists of the day that these were indeed of extraterrestrial origin. Almost immediately, a connection was made with comets by W. B. Clarke and Denison Olmsted. Then Hubert Newton correctly determined the orbit of the Leonids and predicted their return in Work by Giovanni Schiaparelli and others in the mid-1800s continued to press the connection between meteor streams and comets. This was reinforced when Comet Biela was seen to have broken up and Earth Fig. 1. The zodiacal light from Mauna Kea, Hawai i. Courtesy of M. Ishiguro, ISAS. 677

2 678 Comets II Fig. 2. Different methods of studying the interplanetary dust complex are sensitive to different size/mass ranges. Considered together, a more complete picture of the production and evolution of interplanetary dust can be constructed. experienced a meteor shower in 1872 when Earth subsequently passed near its orbit (Yeomans, 1991). Comets are a logical source for the interplanetary dust cloud. They are the only solar system objects visually observed to emit dust, and meteor streams were also linked to specific comets. In 1955, Fred Whipple applied his new icy conglomerate model of comet nuclei to determine a quantitative relationship between comets and the zodiacal light. Since dust evolves toward the Sun under Poynting- Robertson drag (e.g., Burns et al., 1979), it needs to be replenished if it is to be maintained. Whipple (1955) estimated that approximately 1 ton per second of meteoritic material was required to maintain the zodiacal cloud against such loss. He concluded that comets could easily supply that amount of material. In the same paper, however, he also noted that calculations by Piotrowski indicated that the crushing of asteroids may also be an adequate source of zodiacal material. Whether the comets or the asteroids predominate in zodiacal contribution must be decided on the basis of other criteria... from meteoric and micrometeoritical information as well as from the shape of the zodiacal cloud (Whipple, 1955). In the 1930s, cosmic spherules found in deep sea sediments were found to be compositionally similar to meteorites. Analysis of these findings led Ernst Öpik to estimate that the Earth was accumulating kg of meteoritic material per year (Öpik, 1951). With the advent of the space age, the question of the dust environment beyond Earth s atmosphere and its potential hazard to spacecraft and future astronauts grew quickly in importance, later subsiding as better spacebased detectors replaced those that had been providing anomalously high densities due to sensitivity to more than just dust (cf. Fechtig et al., 2001). Returned orbiting surfaces from Skylab, orbiting facilities such as the Long Duration Exposure Facility (LDEF), and a series of spacecraft possessing dust detection systems, including Helios, Hiten, Pioneer 9, Galileo, Ulysses, and Cassini, among others, soon gave new information and constraints on the interplanetary dust environment as did microcrater studies on returned lunar samples. These indicated that the interplanetary dust cloud was complex, having a number of different components of possibly different origins (e.g., Grün et al., 1985). In 1983, the first large-scale survey of the zodiacal cloud at thermal infrared wavelengths by the Infrared Astronomical Satellite (IRAS) (Hauser et al., 1984), revealed the overall cloud shape and, more importantly, the first spatial structures within the dust cloud (Low et al., 1984; Sykes et al., 1986; Sykes, 1988) directly relating to its asteroidal and cometary origins. This was followed by a thermal survey by the Diffuse Infrared Background Experiment (DIRBE) on NASA s Cosmic Background Explorer (COBE) satellite in 1990 (Silverberg et al., 1993; Hauser et al., 1998). Since the late 1960s, the increasing amount and diversity of dust observations sampling different components of the dust complex (Fig. 2) have spurred a series of regular international conferences with associated volumes summarizing the state of knowledge at that time, the most recent of which is Grün et al. (2001). The reader is referred to this book and its antecedents for the detailed background that provides some of the context for this chapter. 2. MORPHOLOGY OF THE CLOUD AND ITS RELATION TO COMETS AND ASTEROIDS At thermal infrared wavelengths, sky brightness viewed from Earth orbit is dominated by the thermal emission of interplanetary dust particles (IDPs) heated by sunlight. These particles are about µm in size (Reach, 1988). The IRAS observed the sky in four bands between 12 and 100 µm over a range of solar elongations between 60 and 120 at a resolution of arcminutes, mapping the entire sky almost three times over the course of the mission. DIRBE simultaneously observed in 10 wavebands from 1.25 to 240 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 characterized using three components (Kelsall et al., 1998). The smooth cloud is the dominant component, and its spatial distribution was mathematically fitted with a parameterized function as follows in terms of spherical coordinates (r,θ,z) in AU where r is the cloud center distance, θ is the azimuthal angle, and z is the vertical distance from the cloud midplane: n = n o r 1.34 exp[ 4.14g(ζ) ]

3 Sykes et al.: Interplanetary Dust Complex and Comets 679 where (a) ζ = z/r and (b) g(ζ) = ζ 2 /0.378 for ζ < and g(ζ) = ζ for ζ The smooth cloud is azimuthally symmetric, with its midplane tilted by 2.03 from the ecliptic and its center offset from the Sun by AU. Both the tilt and offset are explained by gravitational perturbations by the planets (Dermott et al., 1986); while the radial variation and the dependence on z/r are to first order explained by Poynting-Robertson drag, which yields an r 1 distribution and leaves orbital inclination unchanged (Wyatt and Whipple, 1950). The morphology of the main zodiacal cloud allows us, in principal, to assess the fraction of dust that derives from comets and asteroids. The Kelsall model above yields a halfwidth at half-maximum number density of about 14 for dust particle inclinations, while some models based on in situ particle detections suggest that the inclination distribution may have a half-width as wide as 20 to 30 (Dikarev et al., 2002). For comparison, the half-width of the distribution of asteroid orbital inclinations (which is reasonably structured) is between 12 and 16 (Minor Planet Center, 2003), consistent with the Kelsall model, whereas shortperiod comets have inclinations generally less than 30 (Marsden, 1974) and half have inclinations less than about 10 not very different from asteroids. Evidently cloud scale-height does not clearly distinguish between asteroids and comets as principal suppliers of dust. The smooth cloud is supplemented by structures discovered by IRAS: asteroidal dust bands (Low et al., 1984) and cometary dust trails (Sykes et al., 1986) (Fig. 3). Trails represent the principal means by which comets contribute dust to the zodiacal complex and are discussed in a later section. The dust bands stretch across the ecliptic plane in northsouth pairs that straddle a midplane tilted by about 1 from the ecliptic. Soon after their discovery in the IRAS data, the bands were associated with the major Hirayama asteroid families (Dermott et al., 1984) and explained as a natural consequence of the general collisional comminution of the asteroid belt as a whole (since the families are regions of asteroid concentration). An alternative to this equilibrium theory was proposed by Sykes and Greenberg (1986) and Sykes (1990) in which the large-scale production of dust in the asteroid belt is stochastic (the nonequilibrium theory). In this case, observable dust bands would most likely be associated with recent disruptions of small (~20 km) asteroids. Detailed studies indicated that some bands seemed to be associated with the Themis and Koronis families (Sykes, 1990; Reach et al., 1997) and the Maria family 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 diagonal structure crossing the ecliptic plane near 90 and 270 longitude is the galactic plane. The zodiacal cloud appears bright and wide at lower solar elongations, picking up the brighter thermal emissions of the warmer dust that lies closer to the Sun. At higher solar elongations, one looks through less dust near the Earth and sees a greater fraction of colder fainter dust. (b) High-pass filtering in ecliptic latitude reveals structures in the zodiacal cloud associated with asteroid collisions and comets (Sykes, 1988). (Reach et al., 1997), but that other prominent bands such as that initially associated with the Eos family (Dermott et al., 1984) were difficult to reconcile (Sykes, 1990; Reach, 1992; Grogan et al., 2001). Another means of understanding the relationship of the zodiacal cloud to asteroids and comets is by simulation of its creation and the direct comparison with observations. In a review, Dermott et al. (2001) concluded on the basis of such models that dust in the asteroid bands observed by IRAS and COBE contributes about 30% of the total zodiacal thermal emission. If only the Hirayama families (Themis, Koronis, and Eos) contribute that much, then the rest of the asteroid belt, according to the equilibrium model of dust production, should contribute at least double that value, leaving at most only 10% to a cometary contribution. This would be difficult to reconcile with a zodiacal cloud spanning heliocentric ecliptic latitudes larger than the distribution of asteroids (or comets for that matter) available to supply it. However, such a discrepancy (if it exists) may have been mitigated by the identification of smaller asteroid families that are dynamically younger and associated with two of the three major pairs of dust bands (Nesvorný et al., 2002, 2003). This has given support to the nonequilibrium theory of dust production in the asteroid belt. A consequence of this result is that the contribution of the asteroid belt as a whole to the zodiacal dust cloud is not expected to simply scale with local asteroid density, increasing the complexity (and uncertainty) of the detailed relationship between asteroids and the observed zodiacal cloud. Whether asteroidal dust is produced continuously or stochastically, asteroids present a solid alternative to a purely cometary origin of the zodiacal cloud. Asteroid particles are dynamically distinct from cometary particles in that they have smaller initial orbital eccentricities. After being generated in the asteroid belt, this dust evolves toward the Sun

4 680 Comets II under Poynting-Robertson drag. As it passes the orbit of Earth, some of it forms a circumsolar ring due to dust orbits resonant with that of Earth (Dermott et al., 1994). The ring is just outside Earth s orbit, at 1.03 AU, as the exterior resonances are more stable. The peak density in the ring occurs in an enhancement that trails Earth, and amounts to about 30% in excess of the density of the smooth cloud (Kelsall et al., 1998). The ring density is actually highly uncertain, being derived from a line-of-sight integral, but the estimate may be significantly improved using background measurements by the Space Infrared Telescope Facility (SIRTF) as it travels through the trailing enhancement during its planned mission lifetime (Werner et al., 2001). An intriguing aspect of the zodiacal cloud observed by IRAS and COBE/DIRBE is its azimuthal smoothness, about 5% with the principal variation arising from dynamics-induced asymmetries in the dust ring near Earth s orbit (Dermott et al., 1994). This smoothness requires a source that is similarly distributed over ecliptic longitude (i.e., with randomized orbital nodes or a very efficient process for randomizing the orbital nodes of the particles before they spiral past Earth). The existence of the dust bands demonstrates that observed asteroid dust arises from a node-randomized population of parent bodies. Comets are much fewer in number than asteroids, and material tracing their current orbits would produce a relatively lumpy cloud. Whether a cometary component could be smooth depends on the outcome of the race between differential precession of the ejected cometary particle orbits [ yr depending on ejection velocity (Sykes and Greenberg, 1986)], collisional lifetimes, and orbital decay timescales. If the cometary contribution to the zodiacal complex were due to single or multiple generational collision fragments from large millimeter centimeter cometary particles [suggested as a possibility by Liou and Zook (1996)], the dispersion timescales would tend toward tens of millions of years. Disruption of such particles would be most likely by the smallest particles capable of fragmenting them, resulting in low ejection velocities on timescales (e.g., Dohnanyi, 1970) comparable to or smaller than the dispersion timescales. The rapid orbital decay of small fragments (wherein lie the surface area) would result in the partial formation of a torus, extended toward the Sun (Sykes, 1990). Azimuthal smoothness of a purely cometary zodiacal cloud could be difficult to achieve. Better understanding of the detailed evolution of the nodes of cometary dust in addition to the other orbital elements is a necessary step toward solving this problem. The morphology of the zodiacal cloud changes with the size of particles being considered. As particle sizes increase from tens of micrometers in size (to which IRAS and COBE/ DIRBE were sensitive), their sensitivity to radiation forces decrease and their spatial distribution begins to converge upon the distribution of yet-larger bodies from which they ultimately derive. Information on objects in the millimeter to several-meter size range is obtained by meteor observations at Earth (since no such observations have yet been made at other planets). The large end of this size range connects directly to the size range accessible by near-earth-object searches such as Spacewatch (Bottke et al., 2002a) and others. Ceplecha (1992) analyzes interplanetary bodies from millimeter-sized meteoroids to kilometer-sized boulders. The size distribution of smaller meteoroids is best documented in the lunar microcrater record (Grün et al., 1985). Over this entire range the slope of the number density of these interplanetary bodies vs. mass in a log-log plot is close to This slope indicates that the population of these interplanetary bodies is in overall collisional equilibrium (Dohnanyi, 1970), i.e., the number of particles created in a mass interval by fragmentation of larger objects equals the number of particles in that mass interval that are destroyed by collisions. However, in the local mass distribution there are significant deviations from this slope: One hump is at 10 9 kg and another is at 10 4 kg. A hump in the mass distribution at m h signals that there is an excess of particles more massive than m h, probably from an additional input in this mass range. One input is in the 100-µm to centimeter size range, i.e., the range of radar to visual meteors, and the other is in the 1 10-m size range, 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 size of asteroids (whereas sublimation and breakup determine the lifetime of comets in the inner solar system, <5 AU). Transport of these larger meteoroids through the solar system is facilitated by stochastic gravitational interactions with planets, by the systematically inward (toward the Sun) drift caused by the Pointing-Robertson effect, and by the Yarkovsky effect (a thermal radiation force) acting in directions that 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 shorter than the transport time, meteoroids only slowly diffuse away from their place of origin while at the same time they are ground down by collisions. Only meteoroids smaller than 0.1 mm rapidly drift by the Pointing-Robertson effect toward the Sun where they sublimate. For fragments smaller than about 1 µm in size in general, solar radiation pressure reduces solar gravity sufficiently to drive them out of the solar system they become beta-meteoroids although particles much smaller than 1 µm are less affected in this manner (Burns et al., 1979). Many clear orbital associations have been found between meteor streams and comets from which they are presumed to originate. [Even some asteroids may be extinct comets. Several authors suggested that Apollo and Amor asteroids are defunct comets. An argument for this thesis is that at least the peculiar asteroid Phaethon is associated with the Geminid meteor stream (Halliday, 1988). By dynamical studies Gustafson (1989) showed that Phaeton s cometary active phase lasted for several hundred orbits about 1000 years ago.] On the other hand, it has been observed that visual meteor streams (millimeter and bigger sizes) are most prominent in brightness and apparent point of origin on the sky compared with the sporadic meteor background. The

5 Sykes et al.: Interplanetary Dust Complex and Comets 681 contrast in these aspects between stream meteors and sporadic meteors is reduced for smaller meteors and is only weakly recognizable in radar meteor observations [100-µm size range (Galligan and Baggaley, 2001)]. No meteor streams have yet been identified in the dust range (micrometer size range). This observation indicates that the input from comets into the interplanetary dust cloud near Earth s orbit is most significant in the millimeter and larger size range, whereas smaller meteoroids are mostly collisional fragments of the bigger asteroidal ones, ground down and transported to Earth s distance by radiation forces. 3. INFERENCES FROM THE PHYSICAL PROPERTIES OF INTERPLANETARY DUST 3.1. Density: Meteors and Microcrater Studies The notion that fragments of comets have low density comes from Whipple s dirty snowball model where a comet nucleus consists of an intimate mixture of ice and dust. When the ice sublimates it leaves a filigree structure of dust with much pore space from which the ice has been lost. Laboratory sublimation experiments of ice dust mixtures confirm this picture (Grün et al., 1993a). A second line of evidence comes from meteor observations that show that the terminal height (at which a meteoroid is sufficiently decelerated so that it does not generate anymore light during its passage through the atmosphere), after scaling to the same initial mass, inclination to horizon, and velocity differ so much that the whole range of heights covers an air density ratio of 1 : 1000 (Ceplecha, 1994). This observation is interpreted as a consequence of a wide range of meteoroid densities: Low-density meteoroids are decelerated at higher altitudes in the much more tenuous atmosphere than high-density meteoroids. It was found that especially meteor stream particles that have a clear genetic relation to comets have very low material density, e.g., the Perseids that originate from Comet P/Swift-Tuttle. Ceplecha (1977) arrives at a classification of meteoroid orbits and densities from radar meteors (m ~ kg) over photographic meteors ( kg) to fireballs ( kg). Asteroidal meteoroids have high densities (~3 g/cm 3 ) and orbits with medium eccentricities (0.6) and low inclinations (10 ). Short-period comet -like meteoroids have orbits with higher eccentricities and their densities are lower (1 2 g/ cm 3 ). Long-period comet -like meteoroids have almost parabolic orbits, random inclinations, and very low densities ( g/cm 3 ). From the study of fireballs from meteoroids larger than 1 m, Ceplecha (1994) concludes that the majority of these bodies are of cometary origin and of the weakest known structure. At the lower end of the size distribution, lunar microcrater studies (Brownlee et al., 1973) suggested that about 30% of all craters observed on lunar rocks were generated by low-density meteoroids. This result was derived from measurements of the depth-to-diameter ratio of lunar microcraters. Crater simulation experiments with hypervelocity projectiles in the laboratory found that microcraters generated by low-density projectiles had a significantly shallower depth than those generated in the same material by highdensity projectiles [ρ > 3 g/cm 3 (Vedder and Mandeville, 1974; Nagel and Fechtig; 1980)]. Since cometary material is generally associated with lower-density material than asteroidal material, these authors conclude that at least 30% of interplanetary meteoroids originate from comets. The effects of the irregular shape of IDPs (e.g., Brownlee, 1985), however, are not known. Analysis of impacts on NASA s LDEF and ESA s Eureca satellite indicate a mean density of IDP impactors between 2.0 and 2.4 g/cm 3 (McDonnell and Gardner, 1998), somewhere in between canonical cometary and asteroidal values Interplanetary Dust Particles Interplanetary dust particles collected in Earth s upper atmosphere provide clues to their asteroidal or cometary origin. Some of these particles have the kind of open structures 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 are melted as a consequence of atmospheric entry. Measurements of the density of about 100 of these stratospheric IDPs (having diameters of 5 15 µm) found that unmelted chondritic particles have densities between 0.5 and 6.0 g/ cm 3, about half of which are below 2 g/cm 3, but with no observed bimodality as one might hope with two distinct source populations (Love et al., 1993). Atmospheric entry heating was proposed as a means of distinguishing between IDPs arising from comets and asteroids, given the greater average orbital eccentricity (hence average impact velocity) of the former (Flynn, 1989). Entry velocities of IDPs were measured by Joswiak et al. (2000) Fig. 4. A 10-µm-diameter particle collected in Earth s stratosphere. It is carbonaceous and very porous, suggesting that it may be of cometary origin, although an asteroidal origin is not excluded as a possibility. Courtesy of NASA.

6 682 Comets II applying a model of atmospheric heating and helium release (Love and Brownlee, 1994). High- and low-velocity groups were distinguished, with the high-velocity (cometary) group having an average density of 1.1 g/cm 3 and exhibiting fluffy, porous, aggregate textures and the low-velocity (asteroidal) group having average density of 2.5 g/cm 3 and tending toward smoother, compact forms (Joswiak et al., 2000). However, while the means were distinct, there was significant overlap between the two groups in their ranges of properties, making it difficult to assign a particle to one group or another on the basis of density and morphology unless it resided at the extremities associated with those groups. This difficulty is in part due to the potential pumping up of a particle s orbital eccentricity (or that of its collisional precursor) by planetary perturbations. The properties of dust from a specific known comet will be obtained by the Stardust mission when it collects dust from the environment of P/Wild 2 and returns it to Earth (Brownlee et al., 2000). This will be a great aid to further distinguishing cometary from asteroidal particles among collected IDPs Helios and Other Missions Other evidence for distinguishing cometary and asteroidal particles on the basis of their orbits and physical properties came from the Helios dust experiment. The dust instrument on the Helios spacecraft consisted of two sensors that were mounted differently in the spacecraft. The ecliptic sensor was sensitive to impacts arriving from both north and south sides of the ecliptic plane. Since this sensor viewed the Sun once per spin revolution (the spacecraft spin axis was perpendicular to the ecliptic plane) it was covered by an aluminum-coated 0.3-µm-thick plastic film in order to prevent heat and solar UV radiation entering into the sensor. This film caused a penetration cut-off for meteoroids that depended on the mass, density, and velocity of impacting dust particles (Pailer and Grün, 1980). The south sensor had an open aperture that was shielded from solar radiation by the spacecraft rim and hence recorded only dust impacts arriving from south of the ecliptic plane. This sensor was sensitive to somewhat smaller and/or lower-density meteoroids. Both sensors had overlapping fields of view. Helios measurements covered the range from 0.3 to 1 AU heliocentric distance. The measured dust flux displays a steady increase toward the Sun by about a factor of 10. There are significant differences between the measurements by both sensors. The ecliptic sensor detected most impacts in a band centered about the apex direction (i.e., 90 off the Sun in the direction of spacecraft motion), while the south sensor observed particles from all around during a spin revolution with a predominance of small particles from the solar direction (Grün et al., 1980). Modeling of the Helios results show that these apex particles have low eccentricities (e ave 0.6) and small semimajor axes (averaging about 0.6 AU). Since apex particles did penetrate the front film of the ecliptic sensor, their density cannot be below 1 g/cm 3 (Pailer and Grün, 1980), at least not for the smallest particles detected. On the other hand, impacts outside the band were mostly observed by the south sensor and must have higher eccentricities. Modeling shows that these eccentric particles have eccentricities, e ave ~ 0.7, and semimajor axes, a ave ~ 0.9. Grün et al. (1980) conclude that at least half the eccentric particles should have densities below 1 g/cm 3, suggesting a cometary origin. Four planned comet flybys (and an unintended one) from which in situ dust data became available were performed to date. Four spacecraft took dust measurements within 10 4 km of the nuclei of different comets. In 1985 the International Cometary Explorer (ICE) mission flew through the coma of Comet Giacobini-Zinner, and the plasma wave instrument recorded dust impacts in the tailward region of the coma (Gurnett et al., 1986). One year later, a five-spacecraft armada flew by Comet Halley, of which three spacecraft carried a range of dust instruments from simple impact counters to sophisticated dust-mass analyzers. The two Russian Vega spacecraft crossed the sunward side of the coma and recorded dust impacts from the outer boundary at a distance of 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 the nucleus, but most measurements ended on approach at a distance of about 3000 km when a millimeter-sized pebble hit the spacecraft with a speed of almost 70 km/s, causing some damage onboard and interrupting telecommunication. Some time later ground control over the spacecraft was regained and several instruments continued their measurements. Six years later, in 1992, the Giotto spacecraft was redirected to fly through the coma of Comet Grigg-Skjellerup at a distance 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 dust production of this comet was very low and only three particles between approximately 1 and 100 µm and a fourth particle ~10 mg were recorded to hit the approximately 2-m 2 big bumper shield (a fairly flat distribution over several orders of magnitude, consistent with the mass distribution seen at Halley). The latest flyby of Comet Borrelly by the Deep Space 1 spacecraft occurred in The plasma wave instrument onboard recorded several dust impacts within 5 minutes of closest approach at 2000 km distance from the nucleus (Tsurutani et al., 2003). Almost 30 years earlier, in 1974, the dust instrument onboard the HEOS-2 satellite recorded an enhanced impact rate of micrometer-sized particles by a factor of 3 over what the instrument had observed in previous years (Hoffmann et al., 1976; Grün et al., 1976). During this period the instrument was pointed in the direction where Comet Kohoutek (1993 XII) was about one year earlier and had displayed strong dust emission that led to its detection at about 4 AU from the Sun. At the time of the recorded dust impacts the comet was already 3 AU from the Sun past its perihelion. Therefore, the dust recordings constitute measurements in the very distant tail of this comet. Besides the spatial extent of dust in cometary comae and tails, the dust production rate and size distribution of comet-

7 Sykes et al.: Interplanetary Dust Complex and Comets 683 ary dust was derived from the in situ measurements. Most data, of course, came from the comprehensive measurements at Comet Halley. It was found that the dust size distribution extends over a much wider range than was expected from astronomical observations, mostly in the optical wavelength range. The size distribution extends to both much smaller particles in the submicrometer and even nanometer size range, and to much bigger particles in the millimeter size range (McDonnell et al., 1987). It was also found that some particles fragment shortly after their release from the nucleus (Simpson et al., 1987; Vaisberg et al., 1987), which indicates that their initial structure is very fluffy and contains materials that sublimates at distances as small as 1 AU from the Sun. As a consequence of this extended mass distribution the dust production is significantly bigger than that which has been derived from astronomical observations alone. In addition, the dust-to-gas mass ratio of Comet Halley exceeds a value of 1 (McDonnell et al., 1991) compared to a value of 0.1 from earlier estimates. The dust mass spectrometer onboard the Giotto and Vega spaceprobes provided elemental and isotopic data for small newly ejected cometary particles (Kissel et al., 1986). A major discovery was that of CHON particles, which consisted of material high in content of the elements H, C, N, and O, showing that the dust was rich in organics (Kissel and Krüger, 1987). Magnesium isotope ratios showed only a slight variation around the nominal solar value, whereas the isotopic ratio of 12 C/ 13 C showed large variations from grain to grain, but on average it was also solar like (Jessberger and Kissel, 1991). The average elemental composition was found to be solar like, but significantly enriched in volatile elements H, C, N, and O compared to C1 chondrites (Jessberger et al., 1987). When a comet is discovered, it is identified by virtue of its fuzzy appearance, with perhaps a tail, arising from the loss of gas and dust. This dust represents the smallest-sized particle emissions from a comet [generally tens of micrometers and smaller, although significant coma surface area is argued to reside in very large particles (see Fulle, 2004)]. These are entrained in the gas outflow and accelerated to speeds up to ~1 km/s for the smallest particles. After decoupling from the gas, they are generally lost to the solar radiation field. The sensitivity of a particle to solar radiation pressure is described by the parameter, β, the ratio of radiation force to gravitational force felt by the particle (Burns et al., 1979). Most of the particles observed in a comet s tail at visible wavelengths are micrometer-sized and have β > 1. These particles are not gravitationally bound to the Sun and escape the solar system, not contributing to the interplanetary dust complex. Particles having lower β can also escape from the solar system when they are released from an orbiting object like a comet, because of the contribution of the comet s motion. For emission at perihelion, the value of β for escape is a function of orbital eccentricity of the parent body β p (1 e)/2 Thus, for a parabolic comet, all emitted particles would be lost. The solar system loses particles tens of micrometers and smaller from long-period comets, while retaining particles on the order of several micrometers from short-period Jupiter-family comets (Fig. 5). Almost all dust particles tens of micrometers and smaller are released from these latter comets into Jupiter-crossing orbits even comets whose orbits are completely interior to that of Jupiter s (Fig. 6). Subsequent perturbations on the orbits of these particles by Jupiter results in their loss while the distribution of many bear little resemblance to the elements of their parent comets (e.g., Gustafson et al., 1987). Making the assumption that such scattered particles have randomized nodes (required to match the azimuthal symmetry of the cloud), Liou et al. (1995) was able to model a contribution to the cloud by single-sized particles from Encke, taking into account radiation pressure, Poynting-Robertson and corpuscular drag, and perturbations by Jupiter, which when combined with a model contribution from asteroid dust made good matches to selected scans of the zodiacal cloud by IRAS. Cometary particles may undergo considerable orbital evolution with time, increasing the difficulty of distinguishing them from asteroidal particles. Liou and Zook (1996) 4. HOW COMETS SUPPLY THE CLOUD 4.1. General Fig. 5. Maximum β (minimum radius, assuming a density of 1 g/ cm 3 ) of particles from known comets on escape trajectories from the solar system, assuming perihelion emission and zero ejection velocity. Aphelion distances of source comets are dashed lines. For circular orbits, β p = 0.5. Particles are assumed to have zero albedo.

8 684 Comets II Fig. 6. Perihelion and aphelion distances of parent comets (open circles) and the aphelia of emitted particles of different β (filled circles). Jupiter s perihelion and aphelion distances are indicated by solid lines. determined that some cometary particles (from Tempel 2 like comets) could be injected into mean-motion resonances with Jupiter and trapped for thousands of years, after which their orbital eccentricities would be quite small. They would approach Earth with the low velocities expected for asteroid particles. This would help explain the overlap in morphologies and compositions among collected cometary and asteroidal IDPs, identified by their model atmospheric entry speeds Dust Trails Discovery and observations. In 1983, the first survey of the entire sky at thermal infrared wavelengths was conducted by IRAS (Neugebauer et al., 1984). Part of the ongoing analysis during this mission was the IRAS Fast Mover Program (Davies et al., 1984; Stewart et al., 1984; Green et al., 1985) in which fast-moving solar system objects were sought. Six comets and a couple of Apollo asteroids (including Phaethon) were discovered. However, a curiously extended tail associated with P/Tempel 2 was detected over the course of a number of IRAS scans. This was manifested by about 50 faint, relatively collinear sources at 25 µm. It was found to extend 10 on the sky with a width of 4', and no similar feature was found by the program associated 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 emissions of large particles, some of which may have occurred at least 1500 days prior to the observations (Eaton et al., 1984). An examination of IRAS image products, in which individual IRAS scans were merged into images, revealed the continuous emission of the reported Tempel 2 tail extending over 48 of sky (Fig. 7). Similar features were found associated with other comets (Sykes et al., 1986). Clearly, a new cometary phenomenon had been discovered by IRAS and was referred to as trails. An examination of all the IRAS data yielded trails associated with eight short-period comets (Table 1) and about an equal number of orphans not associated with any known comet (Sykes and Walker, 1992). As with Eaton et al. (1984), all trails were found to be consistent with low-velocity emissions and emissions from years to more than a century before the time of observation. Thus, trails offered a continuous record of the emission of comets over that period of time. At the comet orbits, trails have widths of several km. After IRAS, trails were no longer observed in the infrared until the launch of the Infrared Space Observatory (ISO) more than a decade later. The ISO observed segments of the Kopff (Davies et al., 1997) and Encke trails (Reach et al., 2000). Unlike IRAS, ISO was a pointed and not a survey instrument, so its ability to study trails was limited. However, the Kopff trail showed changes due to emissions since IRAS. The Encke trail was observed from a particularly favorable angle of 35 above its orbital plane and included the comet allowing the emergence of the trail from the comet coma to be studied. Dynamical modeling of the Encke trail showed that the mass lost in trail particles (meteoroids with radii of at least several millimeters) is much larger than the mass lost in gaseous or small-particle form, and the comet can only survive these large-particle losses for ~10,000 years (Reach et al., 2000). At the time of its observation by IRAS, the Tempel 2 trail position was sent to ground observers who were unable to detect it at visual wavelengths (Davies et al., 1984; Stewart et al., 1984). Several years later, a trail associated with P/ Faye was accidentally detected in the visible by the Space- Fig. 7. The Tempel 2 dust trail is seen to extend over 30 in this composite image constructed from IRAS scans. The comet coma is seen at the left end of the trail. In the upper right corner is part of the central asteroid dust band. Background cloud-like structures are interstellar cirrus.

9 Sykes et al.: Interplanetary Dust Complex and Comets 685 TABLE 1. Cometary dust trail information from Sykes and Walker (1992). Name θ W v p Age LM D/G Churyomov-Gerasimenko* Encke Gunn Kopff Pons-Winnecke Schwassmann-Wachmann Tempel Tempel *Rosetta target. Deep Impact target. Includes the observed angular extent, θ (deg.); the width, W (10 3 km); normal velocity assuming perihelion emission, v p (m/s); an estimate of the age (in years) of the oldest emissions observed; an estimate of the corresponding comet mass loss rates, LM (log g/century); and an estimate of the dust to gas mass ratio of the comet, D/G. watch Survey in the course of searching for near-earth objects. One evening, a band 1' 2' in width lay across one of their half-degree scans. The next night, they traced the band back to its cometary source. The trail extended 10 (Rabinowitz and Scotti, 1991). More recently, a program to observe trails from the ground at visual wavelengths has been successful (Ishiguro et al., 2002), and other observers are beginning to report similar detections of dust trails (e.g., Lowry et al., 2003). These observations are the harbinger of a new era of dust trail studies that will allow more extensive mapping and characterization of large particle mass loss from comets than has been previously been available Particle properties. Syndyne analysis of the Tempel 2 trail (comparing it with the predicted locations of continuously emitted particles with zero emission velocity) suggested particle diameters of about 1 mm (β ~ 0.001), assuming density of 1 g/cm 3 (Eaton et al., 1984; Sykes et al., 1990). Trail particles are dark. This is evidenced by the early failure to detect the Tempel 2 dust trail from the ground (Davies et al., 1984; Stewart et al., 1984) and supported by initial estimates of the albedo of those particles using IRAS measurements (Sykes, 1987). An extremely low albedo (on the order of a percent) for Kopff trail particles has been estimated on the basis of its groundbased detection (Ishiguro et al., 2002). Trail particles have color temperatures that tend to be in excess of blackbody values (Sykes, 1987; Walker et al., 1989; Sykes et al., 1990; Sykes and Walker, 1992), suggesting either low-emissivity materials, a small particle component, or sustaining temperature gradients. The low emissivities required do not match that of any known nonmetallic materials (Sykes and Walker, 1992). Particles small enough to have the observed low β values would need to be smaller than tens of nanometers (Burns et al., 1979), which would not radiate efficiently at infrared wavelengths. Trail particles appear to be uniformly large, dark, rapidly rotating particles that have thermal conductivity low enough to allow them to sustain a latitudinal temperature gradient (Fig. 8). Low thermal conductivity can be achieved with porosity, which translates to low mass density What trails tell us about comets and their contribution to the interplanetary dust cloud. Dust trails reveal the principal mechanism by which short-period comets lose mass: via the low-velocity emission of large particles. Estimates of refractory (dust) mass loss rates (Table 1), combined with gas mass loss from visible groundbased observations, reveal an average cometary dust to gas mass ratio of about 3 (Sykes and Walker, 1992), which is significantly higher than the canonical This translates to roughly equal volumes of rock and ice in a comet nucleus (assuming a rock density of 3 g/cm 3 and an ice density of 1 g/cm 3 ), and is consistent with the dust to gas limit for Comet Halley s, Fig. 8. Color temperatures for individual 12, 25, and 60-µm scans of the Tempel 2 dust trail were calculated and scaled to 1 AU. The top dashed line corresponds to the color temperature of a sphere on which each point is in instantaneous radiative equilibrium with solar insolation. The bottom dashed line corresponds to the color temperature of a blackbody. The central solid line corresponds to the color temperature of randomly oriented, rapidly rotating spheres where each local latitude is in radiative equilibrium with the average diurnal solar insolation. Geometric albedo is assumed to be zero.

10 686 Comets II 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 corresponding average mass loss rate of kg/yr (Sykes and Walker, 1992), the amount of material contributed to the zodiacal dust complex (assuming 150 comets) would be kg/yr, a significant fraction of the ~ kg/ yr lost within 1 AU that needs to be replenished if the cloud is in steady state (Grün et al., 1985). A recent optical/thermal imaging survey of comets also concludes that comet dust is a major supplier of the IDP cloud (Lisse, 2002) Meteor Streams The largest-sized particles supplied to the interplanetary dust cloud by comets are observed as meteor showers. They begin as dust trails, with individual returns by the parent comet to perihelion causing separate trails in a pattern reflecting planetary perturbations of the comet orbit itself. Perturbations on individual grains in the trail cause a cyclic motion of the node of the trail near Earth s orbit. Meteor outbursts (including meteor storms) are seen when Earth passes through a dust trail (e.g., Kresák, 1993). Differences in perturbations acting on different trail fragments result in these fragments superposing and smearing to the point that they populate a filament. Dispersal of comet trail/meteor stream material into the background zodiacal cloud can be inhibited by orbital resonances, which can maintain trail cohesion for long periods of time [Asher et al. (1999), who determined that the Leonid outburst of 1998 was dust ejected in 1366]. Close encounters cause the grains to be dispersed into a broader meteor stream responsible for an annual shower, which can be identified with that comet s orbit for thousands of years before dissipating and becoming indistinguishable from the background population of particles in that size range. Success at predicting storms has been recently achieved with the work of Jenniskens (1994, 1997), who forecast the return of the 1994 α-monocerids based on the dust-trail hypothesis, and Kondrat eva and Reznikov (1985), who predicted the return of the 1998 Draconid storm (Fujiwara et al., 2001) and the 2001/2002 Leonid storms. The latter considered the ejection of a single particle at perihelion in the direction of motion of the parent comet and calculated the subsequent gravitational perturbations on an orbit with enough lag to allow for a timely collision with Earth. McNaught and Asher (1999) and Lyytinen (1999) applied a refined model to the Leonids, identifying the spatial distributions of dust trails from its parent, P/Tempel Tuttle, which allowed meteors to be identified with emissions from specific epochs of perihelion passage (e.g., Fig. 9). This allows for future studies of the effects of age on larger cometary dust particles. A principal means of analyzing particle number densities within a meteor stream is via its zenith hourly rate (ZHR), the rate of visible meteors seen by a standard observer under ideal conditions (radiant in the zenith and the star limiting magnitude = 6.5) (Jenniskens, 1995). Precise measurements of the ZHR from aircraft during the 1999 Leonid event Fig. 9. Activity curve of the 2001 Leonid meteor storms. Closed symbols are from the Leonid Multi-Instrument Aircraft Campaign; open circles are data gathered by the International Meteor Organization (Jenniskens, 2002). showed the dust density in Earth s path and along the orbit to exhibit a sharp core, but with wings well described by a Lorentzian, while the perpendicular dispersion in a sunward direction is wider and exponential (Jenniskens et al., 2000). Possible explanations include emission at large heliocentric distances combined with a higher degree of fragmentation of dust particles within the coma near perihelion (Jenniskens, 2001). A mass loss rate of ~ kg/return was measured Kuiper Belt Dust Another direct source of interplanetary dust in both the inner solar system and beyond Jupiter is the source region of the short-period comets themselves the Kuiper belt. Flynn (1994) suggested that this might constitute a significant contribution to the interplanetary dust collected in Earth s stratosphere. Liou et al. (1996) found that 20% of the grains generated in the Kuiper belt would evolve all the way to the Sun (the remainder being scattered out of the solar system by the giant planets), and that particles between 9 and 50 µm diameter would be depleted due to mutual collisions and collisions with interstellar dust. They further found that particles surviving into the inner solar system would have low-eccentricity, low-inclination orbits, making them dynamically indistinguishable from evolving asteroidal particles (offering a possible explanation for the IDPs having cometary physical properties, but heating characteristics of asteroidal sources, above). More recently, Moro-Martín and Malhotra (2003) modeled the dynamical evolution of dust particles from the Kuiper belt, taking into consideration the combined effects of radiation pressure, Poynting-Robertson drag, solar wind drag, and the gravitational forces of the planets (excluding Mercury and Pluto) and concluded that near Earth, these grains would have high eccentricities and inclinations, similar to cometary grains 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

11 Sykes et al.: Interplanetary Dust Complex and Comets 687 belt to interior to Jupiter [in rough agreement with Liou et al. (1996)], and that [assuming the dust production rates of Landgraf et al. (2002)] the contribution to the IDPs captured in Earth s atmosphere may be as low as 1 2%. Because of their large heliocentric distance (beyond 30 AU), Kuiper belt objects (KBOs) are not active. Dust production in the Kuiper belt depends upon collisional activity, constrained by their size distribution and detailed orbital-element distribution. This production has been estimated by Stern (1996) to be between to g/s (far more than needed to replenish the loss of dust estimated to be lost each year within 1 AU!). In addition, they determine that recent impacts would produce between zero and several hundred short trail-like structures, having annual parallaxes of up to 2.6. However, a parallactic survey making use of two-week to six-month baselines provided by separate maps of the sky by IRAS (Sykes et al., 1994) produced no evidence of parallax in any extended structures at 60 and 100 µm. Yamamoto and Mukai (1998) estimated a production rate of dust grains between and g/s with radii <10 µm as a consequence of impacts of interstellar dust on KBOs. That dust from the Kuiper belt is being generated and transported to smaller heliocentric distances, however, is supported by Landgraf et al. (2002) in their analysis of data from the dust experiments onboard Pioneer 10 and 11. Humes (1980) had reported an essentially constant spatial density between 1 and 18 AU. Landgraf et al. (2002) considered three potential sources for the impacts recorded beyond 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 Saturn could only be explained by dust originating in the Kuiper belt and evolving toward the Sun. Under their model, about g/s would need to be generated in the Kuiper belt (between 0.01 and 6 mm in size), more than an order of magnitude below the minimum estimate of Stern (1996). The required contributions from P/Halley-type comets were g/s and that from P/Schwassmann-Wachmann 1 comets were g/s. It is interesting to note that dust production from P/Schwassmann-Wachmann 1 itself was estimated to be ~10 5 g/s (Sykes and Walker, 1992, Table 1) in particles ~1 mm in size. 5. EFFECT OF INTERSTELLAR DUST PARTICLES Interstellar dust particles are thought to play a role in the production of interplanetary dust (e.g., Yamamoto and Mukai, 1998) and its comminution (e.g., Liou et al., 1996). As the solar system moves through the galaxy, dust grains that pass through the planetary system have been detected by the dust detector onboard the Ulysses spacecraft (Grün et al., 1993b). It came as a big surprise that after Ulysses flew by Jupiter, the dust detector recorded impacts of interstellar dust (ISD) that arrived from a direction that was opposite to the expected flow direction of interplanetary dust grains. It was found that on average the impact velocities of these particles exceeded the local solar system escape velocity (Grün et al., 1994). The motion of ISD through the solar system was found to be parallel to the flow of neutral interstellar hydrogen and helium gas with a speed of 26 km/s both for gas and dust. This proves that local interstellar dust and gas are nearly at rest with respect to each other. The interstellar dust flow was continuously monitored by Ulysses and persisted at a constant level at all latitudes above the ecliptic plane even over the poles of the Sun, whereas interplanetary dust was strongly depleted away from the ecliptic plane. Starting in mid-1996 the flux of ISD began slowly to decrease and, in the year 2000, was about a factor of 3 lower [this is related to the reversal of the magnetic field in the course of the solar cycle (Landgraf, 2000]. Measurements in the ecliptic plane by Galileo confirmed that outside about 3 AU the interstellar dust flux exceeds the flux of micrometer-sized interplanetary grains. Interstellar grains observed by Ulysses and Galileo range from kg to more than kg. If compared with the ISD mass distribution derived by astronomers, the mass distribution observed by spacecraft overlaps only with the biggest masses observed by remote sensing. More recently, even bigger (10 10 kg) interstellar meteoroids have been reliably identified by their hyperbolic speed (>100 km/s) at 1 AU (Baggaly, 2000). The flow direction of these big particles varies over a much wider angular range than that of small grains observed by Ulysses and Galileo. The deficiency of measured small grain masses is not solely caused by the detection threshold of the in situ instrumentation, but it indicates a depletion of small interstellar grains in the heliosphere. Model calculations by Frisch et al. (1999) of the filtering of electrically charged grains in the heliospheric bow shock region and in the heliosphere itself show that 0.1-µm-sized and smaller particles are strongly impeded from entering the planetary system by the interaction with the solar wind magnetic field. 6. PRODUCTION OF DUST IN OTHER PLANETARY SYSTEMS The interplanetary dust cloud offers a blueprint or laboratory for understanding dust in other planetary systems. The scattered starlight and thermal emission from dust around other stars exozodiacal light is, for most stars, the only indicator we can observe from Earth of the collisional processes and small bodies around those stars. Furthermore, these small bodies also provide evidence for planets around other stars. An inner hole and significant warp were discovered 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 et al., 1998; Wyatt and Dent, 2002). These structures have all been explained by perturbations by planets around the stars, with the direct analogy to Earth s circumsolar ring (discussed above) providing key supporting evidence (Kuchner and Holman, 2003). If viewed from afar, our own solar sys-

12 688 Comets II tem might be recognizable as having at least two planets as a consequence of structure in dust evolving from the Kuiper belt (Liou and Zook, 1999). Dust bands and trails in the interplanetary cloud have been tied to asteroid families and individual short-period comets; comparable structures have not yet been observed around other stars, so the direct connection between parent bodies and the dust cloud can only be studied in detail in the solar system. Exozodiacal light studies also provide valuable clues to understanding aspects of interplanetary dust that cannot be readily discerned from our vantage point inside the system and in its midplane. Among nearby main-sequence stars, some 15% have far-infrared emission, in excess of the photosphere, that is believed to be due to circumstellar material (Backman et al., 1997; Habing et al., 2001). Most of the stars have cold infrared excess, detected at wavelengths of 60 µm and greater. This is partly due to the rapid decline in photospheric emission at longer wavelengths, making farinfrared excess more prominent than mid-infrared excess. The cold excesses, with color temperatures ~80 K, are located relatively far from the central star and roughly correspond to the Kuiper belt in the solar system. In at least four cases, dubbed the fabulous four by observers, the disks are resolved: β Pictoris, Vega, ε Eridani, and Fomalhaut all have disks extended at least 100 AU from their central stars. In all cases, the Poynting-Robertson loss time is shorter than the stellar lifetime, indicating that the disks must be replenished by collisions among a reservoir of larger bodies. This suggests it is likely that our own Kuiper belt may be collisionally active and could be a source of dust, although that dust between 9 and 50 µm in diameter may only reside in the outer solar system because its transport to the inner solar system is hindered by collisions with interstellar dust along the way (Liou et al., 1996). Exozodiacal light due to warm dust in the terrestrial planet zone around other stars is more rare than the colder dust. This warmer dust is difficult to discern photometrically, because the disk emission is generally fainter than the photosphere at wavelengths less than 30 µm (as opposed to the cold excesses, which are often larger than the photosphere). A recent photometric survey found warm disks around 5 out of 81 stars, and none of them had color temperatures higher than 120 K (Laureijs et al., 2002). Furthermore, none of the stars older than 400 m.y. had warm exozodiacal light. In contrast, the zodiacal light as viewed from Earth has a color temperature around 262 K (Reach et al., 1996) and the dust density has been shown to increase as a power-law all the way in to less than 0.15 AU form the Sun (Leinert et al., 1981). One problem with searching for the warmer dust, which would be the analog of dust produced by asteroid collisions and short-period comets, is angular resolution. Mid-infrared spectra of the β Pictoris disk reveal a bright silicate emission feature from the warm dust, with a shape that is different from that of the silicate feature found in the zodiacal light (Reach et al., 2003). A Keck observation with high angular resolution showed that the silicate feature arises only very close to the star (Weinberger et al., 2003). The very different vantage points for viewing the zodiacal and exozodiacal light allow for very different insights into the collisional processes and evolution of small solid bodies. From inside the solar system, it is possible to use the brightness as a function of look direction to obtain the scattering phase function (Hong, 1985; Kelsall et al., 1998); the phase function is needed to invert brightness distributions. From the zodiacal light, it is possible to measure the density of interplanetary dust to within tens of solar radii, which is not possible around other stars because of glare from the photosphere. From the exozodiacal light, it is possible to measure the distribution of material to hundreds of AU, which is not possible from Earth-based observatories because of the bright foreground from dust in the inner solar system. 7. THE FUTURE 7.1. Dust Detectors on Spacecraft The first dust detectors flown in space were simple microphones that responded to dust impacts, but also to a wide range of interferences that in interplanetary space occurred more frequently than dust impacts. Once this was appreciated more sophisticated multicoincidence detectors were developed that permitted the detection of dust impacts at a rate as low as one impact per month. Impact ionization provided the means of at least two independent coincident measurements of a dust impact: the plasma cloud generated by an hypervelocity impact onto a solid target is separated by an electric field so that positive ions and negative ions together with electrons are recorded separately. Timeof-flight analysis of the ions even provides mass analysis of the generated ions. Early detectors of this type had a sensitive area of 0.01 m 2, which had the consequence that only very few dust impacts were recorded in interplanetary space. Therefore, the more recent dust detectors on the Galileo, Ulysses, and Cassini missions had sensitive areas that were 10 times larger. Several recent missions carry dust mass analyzers in which the impact-generated ions are analyzed in a mass spectrometer (e.g., Kissel, 1986). The Cassini cosmic dust analyzer (Srama et al., 1996, 2004) is the most sophisticated dust instrument to date. It combines a 0.1-m 2 impact ionization detector with a time-of-flight mass spectrometer and charge sensing entrance stage for coarse velocity and direction determination. It is hoped that all future missions, particularly to the outer solar system, will include dust experiments. Dust beyond Saturn will be studied for the first time since Pioneer 10 and 11 with the student dust counter planned for the New Horizons mission to Pluto DUNE Observatory From knowledge of the dust particles birthplace and the particles bulk properties, we can learn about the remote environment out of which the particles were formed. This approach could be carried out by means of a dust telescope

13 Sykes et al.: Interplanetary Dust Complex and Comets 689 on a dust observatory in space. A dust telescope is a combination of a dust trajectory sensor together with an analyzer for the chemical composition of dust particles. Potential targets of a dust telescope are interstellar dust, interplanetary dust (e.g., meteor stream dust, cometary, or asteroidal dust or dust from the Moon), and even space debris (e.g., fine grains from solid rocket burns). The first goal of a dust telescope is to distinguish by their trajectories dust particles from different sources: interstellar grains from the different types of interplanetary dust grains. Interstellar dust flows narrowly collimated through the solar system. This flow can be easily distinguished from the flow of interplanetary particles. Young cometary particles have highly eccentric orbits, whereas asteroidal particles have low eccentricity orbits. These different orbits are separated by measurement of the flight direction and speed. Dust in meteor streams occurs only during specific periods and is directly related to the parent comet. A state-of-the-art dust telescope would consist of an array of parallel mounted dust analyzers (Grün et al., 2000) and consists of several instruments sharing a common impact plane of about 1 m 2 in size. Potential components are a high-resolution impact mass spectrometer, a dust analyzer for the determination of physical and chemical dust properties, and large-area impact detectors with trajectory analysis. Dust particles trajectories are determined by the measurement of the electric signals that are induced when a charged grain flies through an appropriately configured electrode system. After the successful identification of dust charges of >10 15 Coulombs in space by the Cassini cosmic dust analyzer, trajectory analyzers that are in development have tenfold increased sensitivity of charge detection giving us trajectories for submicrometer-sized dust grains. Modern dust chemical analyzers have sufficient mass resolution to resolve ions with atomic mass numbers above 100. However, since their impact area is only 0.01 m 2, they can analyze statistically meaningful numbers of grains only in the dust-rich environments of comets or ringed planets. Therefore, a dust telescope should include several of the existing mass analyzers or a large area chemical dust analyzer of mass resolution >100 with at least 10 times greater sensitive area, in order to provide statistically significant measurements of interplanetary and interstellar dust grains in space Thermal Infrared Observations Since IRAS and COBE there have been no surveys of the sky at thermal wavelengths. ISO allowed some studies of cometary trails (Davies et al., 1997; Reach et al., 2000), and detailed SIRTF observations of a large number of shortperiod comets may allow us to greatly improve upon the estimates of cometary dust production contributing to the zodiacal cloud (as well as an understanding of their emission history, retained in the trails). Only by surveys, however, are we able to observe the cloud as a whole and model its evolution and supply by entire populations of objects. But we have been limited by trying to understand the interplanetary dust cloud from observations made within it, near its plane of symmetry. This makes distinguishing radial components difficult, because they are all coincident along our line of sight and the stochastic nature of dust production within the cloud complicates the interpretation of structures, volume distributions, and excesses (or deficits) compared to models. It would be extremely useful to have thermal observations of the cloud from a vantage well away from the ecliptic plane, to look over the top of the zodiacal cloud interior to Jupiter and study outer solar system dust and dust in the Kuiper belt directly. Peaking over the half-width of the Kelsall model of the zodiacal cloud at 1 AU, and assuming the extent of this inner cloud runs with the main asteroid belt, an orbital inclination of about 45 would be required to observe dust in the Kuiper belt. This would parallactically shift the position of the inner cloud by more than 10, allowing for the contribution of distant cold dust to be more easily distinguished. In addition to detecting or placing meaningful limits on dust being generated in the Kuiper belt, studies of the inner zodiacal cloud by such a system would greatly improve our understanding of the morphology of the cloud and the relative contributions of asteroid collisions and comet emissions Earth as a Detector The valuable information provided by meteor stream observations on detailed structures of cometary emissions and the physical properties of those particles argue for the increasing application of modern technology to their study. First, continued efforts to predict these events must be made to allow their observation. Long-term video and radar monitoring for the identification of new meteor streams should be conducted. Campaigns should focus on predicted meteor outbursts with high-speed imagers and photometers, and head-echo of meteors using high-power radar, particularly focusing on small meteoroid masses, should be conducted to assess their fragmentation properties. Information on meteoroid composition could be greatly improved by the use of cooled CCD cameras for slitless optical spectroscopy, and the development of instruments that would focus on individual emission lines/bands and the indirect detection of organics. Together these should not only provide information on properties, but also allow us to assess differences among comets that might relate to different formation conditions and locations as well as ages. 8. FINAL NOTE The ability to associate IDPs collected at Earth s orbit with specific sources is of great value in that it provides sample return information on these sources that might otherwise be impractical to obtain as well as provide great insight into the origin and evolution of those sources and the solar system in general. The increase in our very diverse means of studying interplanetary dust, including atmospheric collection, in situ studies by spacecraft, remote observations at visual and thermal wavelengths, and collisional

14 690 Comets II and dynamical modeling have substantially increased the complexity (and interest) of the problem. The interplanetary dust complex is not in a steady-state condition. Evidence today bolsters the significant episodic infusion of dust by asteroid collisions, a greater potential cometary source due to the discovery of their large particle emissions, and the contribution at some level of dust from the Kuiper belt. Studies of these sources and interplanetary dust are increasingly interrelated. We need to study both sources and dust in order to have a more complete understanding of all. Acknowledgments. This chapter benefited from the detailed reviews of D. Brownlee and N. McBride, to whom the authors express their appreciation. REFERENCES Asher D. J., Bailey M. E., and Emel yanenko V. V. (1999) Resonant meteoroids from Comet Tempel-Tuttle in Mon. Not. R. Astron. Soc., 304, L53 L56. Backman D. E., Fajardo-Acosta S. B., Stencel R. E., and Stauffer J. R. (1997) Dust disks around main sequence stars. Astrophys. Space Sci., 255, Baggaley W. J. (2000) Advanced meteor orbit radar observations of interstellar meteoroids. J. Geophys. Res., 105, Bottke W. F., Morbidelli A., Jedicke R., Petit J-M., Levison H., Michel P., and Metcalfe T. S. (2002a) Debiased orbital and size distributions of the near-earth objects. Icarus, 156, Bottke W. F., Vokrouhlický D., Rubicam D. P., and Broz M. (2002b) The effect of Yarkovsky thermal forces on the dynamical evolution of asteroids and meteoroids. In Asteroids III (W. F. Bottke Jr. et al., eds.), pp Univ. of Arizona, Tucson. Brownlee D. E. (1985) Cosmic dust: Collection and research. Annu. Rev. Earth Planet. Sci., 13, Brownlee D. E., Hörz F., Vedder J. F., Gault D. E., and Hartung J. B. (1973) Some physical parameters of micrometeoroids. Proc. Lunar Sci. Conf. 4th, pp Brownlee D. E., Tsou P., Clark B., Hanner M. S., Hörz F., Kissel J., McDonnell J. A. M., Newburn R. L., Sanford S., Sekanina Z., Tuzzolino A. J., and Zolensky M. (2000) Stardust: A comet sample return mission. Meteoritics & Planet. Sci., 35, A35. Burns J. A., Lamy P. L., and Soter S. (1979) Radiation forces on small particles in the solar system. Icarus, 40, Ceplecha Z. (1977) Meteoroid populations and orbits. In Comets, Asteroids, Meteorites: Interrelations, Evolution and Origins (A. H. Delsemme, ed.), pp Univ. of Toledo, Ohio. Ceplecha Z. (1992) Influx of interplanetary bodies onto Earth. Astron. Astrophys., 263, Ceplecha Z. (1994) Impacts of meteoroids larger than 1 m into the Earth s atmosphere. Astron. Astrophys., 286, Davies J. K., Green S. F., Stewart B. C., Meadows A. J., and Aumann H. H. (1984) The IRAS fast-moving object search. Nature, 309, Davies J. K., Sykes M. V., Reach W. T., Boulanger F., Sibille F., and Cesarsky C. J. (1997) ISOCAM observations of the comet P/Kopff dust trail. Icarus, 127, Dermott S. F., Nicholson P. D., Burns J. A., and Houck J. R. (1984) On the origin of the IRAS solar system dust bands. Nature, 312, Dermott S. F., Nicholson P. D., and Wolven B. (1986) Preliminary analysis of the IRAS solar system dust data. In Asteroids, Comets, Meteors II (C.-I. Lagerkvist et al., eds.), pp Uppsala, Sweden. Dermott S. F., Jayaraman S., Xu Y. L., Gustafson B. Å. S., and Liou J.-C. (1994) A circumsolar ring of asteroidal dust in resonant lock with the Earth. Nature, 369, 719. Dermott S. F., Grogan K., Durda D. D., Jayaraman S., Kehoe T. J. J., Kortenkamp S. J., and Wyatt M. C. (2001) Orbital evolution of interplanetary dust. In Interplanetary Dust (E. Grün et al., eds.), pp Springer-Verlag, Berlin. Dikarev V., Grün E., Landgraf M., Baggaley E. J., and Galligan D. (2002) Interplanetary dust model: From micron-sized dust to meteors. In Proc. Meteoroids 2001 Conf., pp ESA SP-495, Noordwijk, The Netherlands. Dohnanyi J. S. (1970) On the origin and distribution of meteoroids. J. Geophys. Res., 75, Eaton N., Davies J. K., and Green S. F. (1984) The anomalous dust tail of comet P/Tempel 2. Mon. Not. R. Astron. Soc., 211, 15P 19P. Fechtig H., Leinert C., and Berg O. E. (2001) Historical perspectives. In Interplanetary Dust (E. Grün et al., eds.), pp Springer-Verlag, Berlin. Flynn G. J. (1989) Atmospheric entry heating: A criterion to distinguish between asteroid and cometary sources of interplanetary dust. Icarus, 77, Flynn G. J. (1994) Does the Kuiper Belt contribute significantly to the zodiacal cloud and the stratospheric interplanetary dust? (abstract). In Lunar and Planetary Science XXV, pp Lunar and Planetary Institute, Houston. Frisch P. C., Dorschner J., Geiss J., Greenberg J.M., Grün E., Landgraf M., Hoppe P., Jones A. P., Kratschmer W., Linde T. J., Morfill G. E., Reach W. T., Slavin J., Svestka J., Witt A., and Zank G. P. (1999) Dust in the local interstellar wind. Astrophys. J., 525, Fujiwara Y., Ueda M., Sugimoto M., Sagayama T., Satake M., and Furoue A. (2001) TV observations of the 1998 Giacobinid meteor shower in Japan, In Proc. Meteoroids 2001 Conf., pp ESA SP-495, Noordwijk, The Netherlands. Fulle M. (2004) Motion of cometary dust. In Comets II (M. C. Festou et al., eds.), this volume. Univ. of Arizona, Tucson. Galligan D. P. and Baggaley W. J. (2001) Probing the structure of the interplanetary dust cloud using the AMOR meteoroid orbit radar. In Proc. Meteoroids 2001 Conf., pp ESA SP- 495, Noordwijk, The Netherlands. Greaves J. S., Holland W. S., Moriarty-Schieven G., Jenness T., Dent W. R. F., Zuckerman B., McCarthy C., Webb R. A., Butner H. M., Gear W. K., and Walker H. J. (1998) A dust ring around epsilon Eridani: Analog to the young solar system. Astrophys. J. Lett., 506, L133 L137. Green S. F., Davies J. K., Eaton N., Stewart B. C., and Meadows A. J. (1985) The detection of fast-moving asteroids and comets by IRAS. Icarus, 64, Grogan K., Dermott S. F., and Durda D. D. (2001) The size-frequency distribution of the zodiacal cloud: Evidence from the solar system dust bands. Icarus, 152, Grün E., Kissel J., and Hoffmann H.-J. (1976) Dust emission from comet Kohoutek (1973 f) at large distance from the Sun. In Interplanetary Dust and Zodiacal Light (H. Elsässer and H. Fechtig, eds.), pp Springer-Verlag, Berlin. Grün E., Pailer N., Fechtig H., and Kissel J. (1980) Orbital and physical characteristics of micrometeoroids in the inner solar

15 Sykes et al.: Interplanetary Dust Complex and Comets 691 system as observed by Helios 1. Planet. Space Sci., 28, Grün E., Zook H. A., Fechtig H., and Giese R. H. (1985) Collisional balance of the meteoritic complex. Icarus, 62, Grün E., Gebhard J., Bar-Nun A., Benkhoff J., Düren H., Eich G., Hische R., Huebner W. F., Keller H. U., Klees G., Kochan H., Kölzer G., Kroker H., Kührt E., Lämmerzahl P., Lorenz E., Markiewicz W. J., Möhlmann D., Oehler A., Scholz J., Seidensticker K. J., Roessler K., Schwehm G., Steiner G., Thiel K., and Thomas H. (1993a) Development of a dust mantle on the surface of an insolated ice-dust mixture: Results from the KOSI-9 experiment. J. Geophys. Res., 98, Grün E., Zook H. A., Baguhl M., Balogh A., Bame S. J., Fechtig H., Forsyth R., Hanner M. S., Horanyi M., Kissel J., Lindblad B.-A., Linkert D., Linkert G., Mann I., McDonnell J. A. M., Morfill G. E., Phillips J. L., Polanskey C., Schwehm G., Siddique N., Staubach P., Svestka J., and Taylor A. (1993b) Discovery of jovian dust streams and interstellar grains by the Ulysses spacecraft. Nature, 362, Grün E., Gustafson B. Å. S., Mann I., Baguhl M., Morfill G. E., Staubach P., Taylor A., and Zook H. A. (1994) Interstellar dust in the heliosphere. Astron. Astrophys., 286, Grün E., Landgraf M., Horanyi M., Kissel J., Krüger H., Srama R., Svedhem H., and Withnel P. (2000) Techniques for galactic dust measurements in the heliosphere. J. Geophys. Res., 105, Grün E., Gustafson B. Å. S., Dermott S. F., and Fechtig H., eds. (2001) Interplanetary Dust. Springer-Verlag, Berlin. 804 pp. Gurnett D. A., Averkamp T. F., Scarf F. L., and Grün E. (1986) Dust particles detected near Giacobini-Zinner by the ICE Plasma Wave Instrument. Geophys. Res. Lett., 13, Gustafson B. Å. S. (1989) Geminid meteoroids traced to cometary activity on Phaethon. Astron. Astrophys., 225, Gustafson B. Å., Misconi N. Y., and Rusk E. T. (1987) Interplanetary dust dynamics. III. Dust released from P/Encke: Distribution with respect to the zodiacal cloud. Icarus, 72, Habing J. J., Dominik C., Jourdain de Muizon M., Laureijs R. J., Kessler M. F., Leech K., Metcalfe L., Salama A., Siebenmorgen R., Trams N., and Bouchet P. (2001) Incidence and survival of remnant disks around main-sequence stars. Astron. Astrophys., 365, Halliday I. (1988) Geminid fireballs and the peculiar asteroid 3200 Phaeton. Icarus, 76, Hauser M. G., Gillett F. C., Low F. J., Gautier T. N., Beichman C. A., Neugebauer G., Aumann H. H., Baud B., Boggess N., Emerson J. P., Houck J. R., Soifer B. T., and Walker R. G. (1984) IRAS observations of the diffuse infrared background. Astrophys. J. Lett., 278, L15 L18. Hauser M. G., Arendt R. G., Kelsall T., Dwek E., Odegard N., Weiland J. L., Freudenreich H. T., Reach W. T., Silverberg R. F., Moseley S. H., Pei Y. C., Lubin P., Mather J. C., Shafer R. A., Smoot G. F., Weiss R., Wilkinson D. T., and Wright E. L. (1998) The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background. I. Limits and detections. Astrophys. J., 508, Heap S. R., Lindler D. J., Lanz T. M., Cornett R. H., Robert H., Hubeny I., Maran S. P., and Woodgate B. (2000) Space Telescope Imaging Spectrograph coronagraphic observations of Beta Pictoris. Astrophys. J., 539, Hoffmann H.-J., Fechtig H., Grün E., and Kissel J. (1976) Particles from comet Kohoutek detected by the Micrometeoroid Experiment on HEOS 2. In The Study of Comets (B. Donn et al., eds.), pp NASA SP-39, Washington, DC. Holland W. S., Greaves J. S., Zuckerman B., Webb R. A., and McCarthy C. (1998) Submillimetre images of dusty debris around nearby stars. Nature, 392, Hong S. S. (1985) Henyey-Greenstein representation of the mean volume scattering phase function for zodiacal dust. Astron. Astrophys., 146, Humes D. H. (1980) Results of Pioneer 10 and 11 meteoroid experiments: Interplanetary and near-saturn. J. Geophys. Res., 85, Ishiguro M., Watanabe J., Usui F., Tanigawa T., Kinoshita D., Suzuki J., Nakamura R., Ueno M., and Mukai T. (2002) First detection of an optical dust trail along the orbit of 22P/Kopff. Astrophys. J. Lett., 572, L117 L120. Jenniskens P. (1994) Good prospects for α-monocerotid outburst in WGN, 23, Jenniskens P. (1995) Meteor stream activity. II. Meteor outbursts. Astron. Astrophys., 295, Jenniskens P. (1997) Meteor stream activity. IV. Meteor outbursts and the reflex motion of the Sun. Astron. Astrophys., 317, Jenniskens P. (2001) Model of a one-revolution comet dust trail from Leonid outburst observations. WGN, 29, Jenniskens P. (2002) More on the dust trails of comet 55P/Tempel- Tuttle from 2001 Leonid shower flux measurements. In Proceedings of Asteroids, Comets, Meteors ACM 2002 (B. Warmbein, ed.), pp ESA SP-500, Noordwijk, The Netherlands. Jenniskens P., Crawford C., Butow S. J., Nugent D., Koop M., Holman D., Houston J., Jobse K., Kronk G., and Beatty K. (2000) Lorentz shaped dust trail cross section from new hybrid visual and video meteor counting technique. Earth Moon Planets, 82 83, Jessberger E. K. and Kissel J. (1991) Chemical properties of cometary dust and a note on carbon isotopes. In Comets in the Post- Halley Era (R. L. Newburn Jr. et al., eds.), pp Kluwer, Dordrecht. Jessberger E. K., Kissel J., Fechtig H., and Krüger F. R. (1987) On the average chemical composition of cometary dust. In Physical Processes in Comets, Stars, and Active Galaxies (W. Hillebrandt et al., eds.), pp Springer-Verlag, Heidelberg. Joswiak D. J., Brownlee D. E., Pepin R. O., and Schlutter D. J. (2000) Characteristics of asteroidal and cometary IDPs obtained from stratospheric collectors: Summary of measured He release temperatures, velocities, and descriptive mineralogy (abstract). In Lunar and Planetary Science XXXI, Abstract #1500. Lunar and Planetary Institute, Houston (CD-ROM). Kelsall T., Weiland J. L., Franz B. A., Reach W. T., Arendt R. G., Dwek E., Freudenreich H. T., Hauser M. G., Moseley S. H., Odegard N. P., Silverberg R. F., and Wright E. L. (1998) The COBE Diffuse Infrared Background Experiment: Search for the cosmic infrared background. II. Model of the interplanetary dust cloud. Astrophys. J., 508, Kissel J. (1986) The Giotto particulate impact analyzer. In The Giotto Mission Its Scientific Investigations (R. Reinhard and B. Battrick, eds.), pp ESA SP-1077, Noordwijk, The Netherlands. Kissel J. and Krüger F. R. (1987) The organic component in dust from comet Halley as measured by the PUMA mass spectrometer onboard Vega 1. Nature, 326, Kissel J., Brownlee D. E., Büchler K., Clark B. C., Fechtig H., Grün E., Hornung K., Igenbergs E. B., Jessberger E. K.,

16 692 Comets II Krüger F. R., Kuczera H., McDonnell J. A. M., Morfill G. E., Rahe J., Schwehm G. H., Sekanina Z., Utterback N. G., Völk H., and Zook H. A. (1986) Composition of comet Halley dust particles from Giotto observations. Nature, 321, Kondrat eva E. D. and Reznikov E. A. (1985) Comet Tempel- Tuttle and the Leonid meteor swarm. Solar Sys. Res., 31, Kresák L. (1993) Cometary dust trails and meteor storms. Astron. Astrophys., 279, Kuchner M. J. and Holman M. J. (2003) The geometry of resonant signatures in debris disks with planets. Astrophys. J., 588, Lagage P.-O. and Pantin E. (1994) Dust depletion in the inner disk of Beta-Pictoris as a possible indicator of planets. Nature, 369, 628. Landgraf M. (2000) Modeling the motion and distribution of interstellar dust inside the heliosphere. J. Geophys. Res., 105, Landgraf M., Liou J.-C., Zook H. A., and Grün E. (2002) Origins of solar system dust beyond Jupiter. Astrophys. J., 123, Laureijs R. J., Jourdain de Muizon M., Leech K., Siebenmorgen R., Dominik C., Habing H. J., Trams N., and Kessler M. F. (2002) A 25 micron search for Vega-like disks around mainsequence stars. Astron. Astrophys., 387, Leinert C., Richter I., Pitz E., and Planck B. (1981) The zodiacal light from 1.0 to 0.3 AU as observed by the HELIOS space probes. Astron. Astrophys., 103, Liou J.-C. and Zook H. A. (1996) Comets as a source of low eccentricity and low inclination interplanetary dust particles. Icarus, 123, Liou J.-C. and Zook H. A. (1999) Signatures of the giant planets imprinted on the Edgeworth-Kuiper belt dust disk. Astron. J., 118, Liou J. C., Dermott S. F., and Xu Y. L. (1995) The contribution of cometary dust to the zodiacal cloud. Planet. Space Sci., 43, Liou J.-C., Zook H. A., and Dermott S. F. (1996) Kuiper Belt dust grains as a source of interplanetary dust particles. Icarus, 124, Lisse C. (2002) On the role of dust mass loss in the evolution of comets and dusty disk systems. Earth Moon Planets, 90, Love S. G. and Brownlee D. E. (1994) Peak atmospheric entry temperature of meteorites. Meteoritics, 29, Love S. G., Joswiak D. J., and Brownlee D. E. (1993) Densities of 5 15 microns interplanetary dust particles (abstract). In Lunar and Planetary Science XXIV, pp Lunar and Planetary Institute, Houston. Low F. J., Beitema D. A., Gautier T. N., Gillett F. C., Beichman C. A., Neugebauer G., Young E., Aumann H. H., Boggess N., Emerson J. P., Habing H. J., Hauser M. G., Houck J. R., Rowan-Robinson M., Soifer B. T., Walker R. G., and Wesselius P. R. (1984) Infrared cirrus: New components of the extended infrared emission. Astrophys. J. Lett., 278, L19 L22. Lowry S. C., Weissman P. R., Sykes M. V., and Reach W. T. (2003) Observations of periodic comet 2P/Encke: Properties of the nucleus and first visual-wavelength detection of its dust trail (abstract). In Lunar and Planetary Science XXXIV, Abstract #2056. Lunar and Planetary Institute, Houston (CD- ROM). Lyytinen E. J. (1999) Meteor predictions for the years with the satellite model of comets. Meta Res. Bull., 8, Marsden B. G. (1974) Comets. Annu. Rev. Astron. Astrophys., 12, Mazets E. P., Sagdeev R. Z., Aptekar R. L., Golenetskii S. V., Guryan Y. A., Dyachkov A. V., Ilyinskii V. N., Panov V. N., Petrov G. G., Savvin A. V., Sokolov I. A., Frederiks D. D., Khavenson N. G., Shapiro V. D., and Shevchenko V. I. (1987) Dust in comet P/Halley from VEGA observations. Astron. Astrophys., 187, McBride N., Green S. F., Levasseur-Regourd A. C., Goidet-Deve B., and Renard J.-B. (1977) The inner dust coma of comet 26P/ Grigg-Skjellerup: Multiple jets and nucleus fragments? Mon. Not. R. Astron. Soc., 289, McDonnell J. A. M. and Gardner D. J. (1998) Meteorite morphology and densities: Decoding satellite impact data. Icarus, 133, McDonnell J. A. M., Alexander W. M., Burton W.M, Bussoletti E., Evans G. C., Evans S. T., Firth J. G., Grard R. J. L., Green S. F., Grün E., Hanner M. S., Hughes D. W., Igenbergs E., Kissel J., Kuczera H., Lindblad B. A., Langevin Y., Mandeville J. C., Pankiewicz G. S. A., Perry C. H., Schwehm G., Sekanina Z., Stevenson T. J., Turner R. F., Weishaupt U., Wallis M. K., and Zarnecki J. C. (1987) The dust distribution within the inner coma of comet P/Halley (1982i): Encounter by Giotto s impact detectors. Astron. Astrophys., 187, McDonnell J. A. M., Lamy P. L., and Pankiewicz G. S. (1991) Physical properties of cometary dust. In Comets in the Post- Halley Era (R. L. Newburn Jr. et al., eds.), pp Kluwer, Dordrecht. McDonnell J. A. M., McBride N. M., Beard R., Bussoletti E., Colangeli L., Eberhardt P., Firth J. G., Grard R., Green S. F., Greenberg J. M., Grün E., Hughes D. W., Keller H. U., Kissel J., Lindblad B. A., Mandeville J. C., Perry C. H., Rembor K., Rickman H., Schwehm G. H., Roessler K., Schwehm G., Turner R. F., Wallis M. K., and Zarnecki J. C. (1993) Dust particle impacts during the Giotto encounter with comet Grigg- Skjellerup. Nature, 362, McNaught R. H. and Asher D. J. (1999) Leonid dust trails and meteor storms. WGN, 27, Minor Planet Center (2003) The Distribution of Minor Planets. Available on line at MPDistribution.html. Moro-Martín A. and Malhotra R. (2003) Dynamical models of Kuiper Belt dust in the inner and outer solar system. Astron. J., 125, Nagel K. and Fechtig H. (1980) Diameter to depth dependence of impact craters. Planet. Space Sci., 28, Nesvorný D., Bottke W. F., Levison H., and Dones L. (2002) The recent breakup of an asteroid in the main-belt region. Nature, 417, Nesvorný D., Levison H. F., Bottke W. F., and Dones L. (2003) Recent origin of the solar system dust bands. Astrophys. J., 591, Neugebauer G., Habing H. J., van Duinen R., Aumann H. H., Baud B., Beichman C. A., Beintema D. A., Boggess N., Clegg P. E., de Jong T., Emerson J. P., Gautier T. N., Gillett F. C., Harris S., Hauser M. G., Houck J. R., Jennings R. E., Low F. J., Marsden P. L., Miley G., Olnon F. M., Pottasch S. R., Raimond E., Rowan-Robinson M., Soifer B. T., Walker R. G., Wesselius P. R., and Young E. (1984) The Infrared Astronomical Satellite (IRAS) mission. Astrophys. J. Lett., 278, L1 L6. Öpik E. (1951) Astronomy and the bottom of the sea. Irish Astron. J., 1, Pailer N. and Grün E. (1980) The penetration limit of thin films.

17 Sykes et al.: Interplanetary Dust Complex and Comets 693 Planet. Space Sci., 28, Quillen A. C. and Thorndike S. (2002) Structure in the Epsilon Eridani dusty disk caused by mean motion resonances with a 0.3 eccentricity planet at periastron. Astrophys. J. Lett., 578, L149 L152. Rabinowitz D. and Scotti J. (1991) Periodic Comet Faye (1991n). IAU Circular No Reach W. T. (1988) Zodiacal emission. I. Dust near the Earth s orbit. Astrophys. J., 335, Reach W. T. (1992) Zodiacal emission. III. Dust near the asteroid belt. Astrophys. J., 392, Reach W. T., Abergel A., Boulanger F., Desert F.-X., Perault M., Bernard J.-P., Blommaert J., Cesarsky C., Cesarsky D., Metcalfe L., Puget J.-L., Sibille F., and Vigroux L. (1996) Midinfrared spectrum of the zodiacal light. Astron. Astrophys., 315, L381 L384. Reach W. T., Franz B. A., and Weiland J. L. (1997) The threedimensional structure of the zodiacal dust bands. Icarus, 127, Reach W. T., Sykes M. V., Lien D., and Davies J. K. (2000) The formation of Encke meteoroids and dust trail. Icarus, 148, Reach W. T., Morris P., Boulanger F., and Okumura K. (2003) The mid-infrared spectrum of the zodiacal and exozodiacal light. Icarus, 164, Silverberg R. F., Hauser M. G., Boggess N. W., Kelsall T. J., Moseley S. H., and Murdock T. L. (1993) Design of the Diffuse Infrared Background Experiment (DIRBE) on COBE. In Infrared Spaceborne Remote Sensing (M. S. Scholl, ed.), pp SPIE Conference Proceedings Vol. 2019, Tempe, Arizona. Simpson J. A., Rabinowitz D., Tuzzolino A. J., Ksanfomality L. V., and Sagdeev R. Z. (1987) The dust coma of comet Halley: Measurements on the VEGA-1 and 2 spacecraft. Astron. Astrophys., 187, Srama R., Grün E., and the Cassini Dust Science Team (1996) The Cosmic Dust Analyzer for the Cassini mission to Saturn. In Physics, Chemistry, and Dynamics of Interplanetary Dust (B. Å. S. Gustafson and M. S. Hanner, eds.), pp ASP Conference Series 104, Astronomical Society of the Pacific, San Francisco. Srama R., Bradley J. G., Grün E., Ahrens T. J., Auer S., Cruise M., Fechtig H., Graps A., Havnes O., Heck A., Helfert S., Igenbergs E., Jessberger E. K., Johnson T. V., Kempf S., Krüger H., Lamy P., Landgraf M., Linkert D., Lura F., McDonnell J. A. M., Mohlmann D., Morfill G. E., Schwehm G. H., Stübig M., Svestka J., Tuzzolino A. J., Wäsch R., and Zook H. A. (2004) The Cassini Cosmic Dust Analyser. Space Sci. Rev., in press. Stern S. A. (1996) Signatures of collisions in the Kuiper disk. Astron. Astrophys., 310, Stewart B. C., Davies J. K., and Green S. F. (1984) IRAS fast mover program. J. Brit. Interplan. Soc., 37, Sykes M. V. (1987) The albedo of large refractory particles in P/ Tempel 2. Bull. Am. Astron. Soc., 19, Sykes M. V. (1988) IRAS observations of extended zodiacal structures. Astrophys. J. Lett., 334, L55 L58. Sykes M. V. (1990) Zodiacal dust bands: Their relation to asteroid families. Icarus, 84, Sykes M. V. and Greenberg R. (1986) The formation and origin of the IRAS zodiacal dust bands as a consequence of single collisions between asteroids. Icarus, 65, Sykes M. V. and Walker R. G. (1992) Cometary dust trails. I. Survey. Icarus, 95, Sykes M. V., Lebofsky L. A., Hunten D. M., and Low F. (1986) The discovery of dust trails in the orbits of periodic comets. Science, 232, Sykes M. V., Lien D. J., and Walker R. G. (1990) The Tempel 2 dust trail. Icarus, 86, Sykes M. V., Cutri R., Moynihan P., and Plath J. (1994) A parallactic mini-survey of the infrared sky. Bull. Am. Astron. Soc., 26, Tsurutani B. T., Clay D. R., Zhang L. D., Dasgupta B., Brinza D., Henry M., Mendis A., Moses S., Glassmeier K.-H., Musmann G., and Richter I. (2003) Dust impacts at comet P/Borrelly. Geophys. Res. Lett., 30(22), SSC 1-1 to 1-4. Vaisberg O. L., Smirnov V., Omel Chenko A., Gorn L., and Iovlev M. (1987) Spatial and mass distribution of low-mass dust particles (m < g) in comet P/Halley s coma. Astron. Astrophys., 187, Vedder J. F. and Mandeville J.-C. (1974) Microcraters formed in glass by projectiles of various densities. J. Geophys. Res., 79, Walker R. G., Sykes M. V., and Lien D. J. (1989) Thermal properties of dust trail particles. Bull. Am. Astron. Soc., 21, 967. Weinberger A. J., Becklin E. E., and Zuckerman B. (2003) The first spactially resolved mid-infrared spectroscopy of β Pictoris. Astrophys. J. Lett., 584, L33 L37. Werner M. W., Reach W. T., and Rieke M. (2001) Studies of the cosmic infrared background with the Space Infrared Telescope Facility (SIRTF). In The Extragalactic Infrared Background and Its Cosmological Implications (M. Harwit, ed.), p IAU Symposium 204. Whipple F. L. (1955) A comet model. III. The zodiacal light. Astrophys. J., 121, Wilner D. J., Holman M. J., Kuchner M. J., and Ho P. T. P. (2002) Structure in the dusty debris around Vega. Astrophys. J. Lett., 569, L115 L119. Wyatt M. C. and Dent W. R. F. (2002) Collisional processes in extrasolar planetesimal discs dust clumps in Fomalhaut s debris disc. Mon. Not. R. Astron. Soc., 334, Wyatt S. P. and Whipple F. L. (1950) The Poynting-Robertson effect on meteor orbits. Astrophys. J., 111, Yamamoto S. and Mukai T. (1998) Dust production by impacts of interstellar dust on Edgeworth-Kuiper Belt objects. Astron. Astrophys., 329, Yeomans D. K. (1991) Comets: A Chronological History of Observation, Science, Myth, and Folklore. Wiley, New York. 485 pp.

18 694 Comets II