The Interplanetary Dust Complex and Comets

Transkrypt

Sykes et al.: Interplanetary Dust Complex and Comets
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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 1866. 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.
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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 PoyntingRobertson 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 8 × 109 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 summa-
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.
rizing 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 20–200 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 = nor –1.34exp[–4.14g(ζ)0.942]
where
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(a)
ζ = z/r
and
(b)
g(ζ) = ζ2/0.378 for ζ < 0.189
and
g(ζ) = ζ – 0.0945 for ζ ≥ 0.189
The smooth cloud is azimuthally symmetric, with its midplane tilted by 2.03° from the ecliptic and its center offset
from the Sun by 0.013 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
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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 [104–107 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 0.83. 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
104 kg. A hump in the mass distribution at mh signals that
there is an excess of particles more massive than mh, 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
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 ~ 10 –8–10–6 kg) over photographic
meteors (10 –6–1 kg) to fireballs (1–10 6 kg). “Asteroidal”
meteoroids have high densities (~3 g/cm3) 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/
cm3). “Long-period comet”-like meteoroids have almost
parabolic orbits, random inclinations, and very low densities (0.2–0.6 g/cm3). 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
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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/cm3 (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/cm3 (McDonnell
and Gardner, 1998), somewhere in between canonical cometary and asteroidal values.
3.2.
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/
cm3, about half of which are below 2 g/cm3, 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.
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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/cm3 and exhibiting “fluffy,
porous, aggregate textures” and the low-velocity (asteroidal)
group having average density of 2.5 g/cm3 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.
3.3.
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 (eave ≤ 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/cm3
(Pailer and Grün, 1980), at least not for the smallest par-
ticles 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, eave ~ 0.7, and semimajor axes,
aave ~ 0.9. Grün et al. (1980) conclude that at least half the
eccentric particles should have densities below 1 g/cm3, 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 104 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 2 × 105 km down to about 8000 km from the nucleus
(Mazets et al., 1987; Simpson et al., 1987; Vaisberg et al.,
1987). ESA’s Giotto spacecraft flew closest (600 km) to 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-m2
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 2001. 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-
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 12C/13C 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).
4.
4.1.
683
tribution 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
βp ≥ (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)
HOW COMETS SUPPLY THE CLOUD
General
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 con-
Fig. 5. Maximum β (minimum radius, assuming a density of 1 g/
cm3) 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, βp = 0.5. Particles are assumed to have zero
albedo.
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.
4.2.
“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 10 4–105 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-
Dust Trails
4.2.1. 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
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.
TABLE 1.
685
Cometary dust trail information from Sykes and Walker (1992).
Name
θ
W
∆vp
Age
LM
D/G
Churyomov-Gerasimenko*
Encke
Gunn
Kopff
Pons-Winnecke
Schwassmann-Wachmann 1
Tempel 1†
Tempel 2
1
93
6
17
3
10
7
65
50
680
111
47
40
769
68
31
2
40
3
3
3
5
4
2
3–11
19–21
27–74
47–158
6–21
114–148
14–38
140–665
13.1
14.9
13.8
13.5
13.2
14.5
13.4
13.7
4.6
3.5
3.6
1.2
2.4
1.6
3.5
2.9
*Rosetta target.
† Deep Impact target.
Includes the observed angular extent, θ (deg.); the width, W (103 km); normal velocity assuming perihelion emission, ∆vp (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.
4.2.2. 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/cm3 (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.
4.2.3. 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 0.1–1. This translates to roughly equal
volumes of “rock” and “ice” in a comet nucleus (assuming
a rock density of 3 g/cm3 and an ice density of 1 g/cm3), 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.
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 8.4 × 108 kg/yr (Sykes
and Walker, 1992), the amount of material contributed to
the zodiacal dust complex (assuming 150 comets) would be
1.3 × 1011 kg/yr, a significant fraction of the ~2.9 × 1011 kg/
yr lost within 1 AU that needs to be replenished if the 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).
4.3.
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 ~2.6 × 1010 kg/return was measured.
4.4.
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
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 9.5 × 108 to 3.2 ×
1011 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 3.7 × 105
and 3.1 × 107 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/SchwassmannWachmann 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
5 × 107 g/s would need to be generated in the Kuiper belt
(between 0.01 and 6 mm in size), more than an order of magnitude below the minimum estimate of Stern (1996). The
required contributions from P/Halley-type comets were 3 ×
105 g/s and that from P/Schwassmann-Wachmann 1 comets
were 8 × 10 4 g/s. It is interesting to note that dust production from P/Schwassmann-Wachmann 1 itself was estimated
to be ~105 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 veloci-
687
ties 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 10 –18 kg
to more than 10–13 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-
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.
7.1.
THE FUTURE
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 m2, 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-m2 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.
7.2.
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
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 m2 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 m2, 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.
7.3.
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 inter-
689
planetary 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.
7.4.
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
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.
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Comets II

The Interplanetary Dust Complex and Comets

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