Interplanetary dust particles collected from the stratosphere: physical, chemical, and mineralogical properties and implications for their sources

The suggestion that significant quantities of interplanetary dust are produced by both main-belt asteroids and comets is based on the Infrared Astronomical Satellite detection of dust trails or bands associated with these objects. Gravitational focusing strongly biases all near-Earth collections of interplanetary dust in favor of particles with the lowest geocentric velocities, that is the dust from main-belt asteroids spiraling into the Sun under the influence of Poynting-Robertson radiation drag.

The major dust bands in the main-belt appear to be associated with the catastrophic disruptions which produced the Eos, Themis and Koronis families of asteroids. If dust particles are produced in the catastrophic collision process, then Poynting-Robertson radiation drag is such an efficient transport mechanism from the main-belt to 1 AU that near-Earth collections of interplanetary dust should include, and perhaps be dominated by, this material. The physical, chemical and mineralogical properties of this asteroidal dust can provide constraints on the properties of the asteroidal parent bodies.

Interplanetary dust particles from 5 to 100 μm in diameter have been recovered from the stratosphere of the Earth by NASA sampling aircraft since the mid1970s. The densities of a large fraction of these interplanetary dust particles are significantly lower than the densities of their constituent silicate mineral phases, indicating significant porosities. Direct examination of ultra-microtome thin-sections of interplanetary dust particles also shows significant porosities. The majority of the particles are chemically and mineralogically similar to, but not identical to, the carbonaceous chondrite meteorites.

Most stony interplanetary dust particles have carbon contents exceeding those of Allende, a carbonaceous chondrite meteorite having a low albedo. The population of interplanetary dust does not appear to exhibit the full range of compositional diversity inferred from reflection spectroscopy of the main-belt asteroids. In particular, higher albedo particles corresponding to S-type asteroids are underrepresented or absent from the stratospheric collections, and primitive carbonaceous particles seem to be overrepresented in the stratospheric collections compared to the fraction of mainbelt asteroids classified as primitive. This suggests that much of the interplanetary dust may be generated by a stochastic process, probably preferentially sampling a few most recent collisional events.     

Opportunities for the stratospheric collection of dust from short‐period comets

Abstract— We have identified four comets which have produced low‐velocity Earth‐crossing dust streams within the past century: 7P/Pons‐Winnecke, 26P/Grigg‐Skjellerup, 73P/Schwassmann‐Wachmann 3, and 103P/Hartley 2. These comets have had the rare characteristics of low eccentricity, low inclination orbits with nodes very close to 1 AU. Dust from these comets is directly injected into Earth‐crossing orbits by radiation pressure, unlike the great majority of interplanetary dust particles collected in the stratosphere which spend millennia in space prior to Earth‐encounter. Complete dust streams from these comets form within a few decades, and appreciable amounts of dust are accreted by the Earth each year regardless of the positions of the parent comets. Dust from these comets could be collected in the stratosphere and identified by its short space exposure age, as indicated by low abundances of implanted solar‐wind noble gases and/or lack of solar flare tracks. Dust from Grigg‐Skjellerup probably has the highest concentration at Earth orbit. We estimate that the proportion of dust from this comet will reach at least several percent of the background interplanetary dust flux in the >40 μm size range during April 23–24 of 2003.     

End-states of short-period comets and their role in maintaining the zodiacal dust cloud

Physical lifetimes and end-states of short-period comets are analysed in connection with the problem of the maintainance of the zodiacal dust cloud. In particular, the problem of the comet-asteroid relationship is addressed. Recent studies of the physical properties of Apollo-Amor asteroids and short-period comets (e.g., Hartmann et al., 1987) show significant differences between them, suggesting that they are distinct classes of objects. A few percent of the active SP comets might become asteroidal-like bodies in comet-type orbits due to the buildup of dust mantles. The remainder probably disintegrate as they consume their volatile content so their debris can only be observed as fireballs when they meet the Earth. Unobservable faint SP comets — i.e., comets so small (m 10¹⁴ g) that quickly disintegrate before being detected, might be a complementary source of dust material. They might be completely sublimated even at rather large heliocentric distances (r - 3 AU). Yet the released dust grains can reach the vicinity of the Sun by Poynting-Robertson drag. The mass associated with unobservable SP comets with perihelion distances q 3 AU might be comparable to that computed for the sample of observed SP co-mets with q 1.5 AU. It is concluded that SP comets (from the large to the unobservable small ones) may supply an average of several tons/sec of meteoric matter to the zodiacal dust cloud.     

On the albedo of the interplanetary dust

The zodiacal light brightness and measured spatial density of the interplanetary dust lead to a mean geometric albedo of 0.24 for the dust particles near 1 AU; whereas the composition of collected micrometeroids suggests a geometric albedo ?0.1. The data do not support the very low albedo (?0.01) proposed by A. F. Cook [Icarus33 (1978), 349–360]. The evidence is against a change in the mean particle albedo between 0.1 and 2 AU. Beyond 2 AU the data are unclear and a change in albedo is not ruled out.     

A tale of two very different comets: ISO and MSX measurements of dust emission from 126P/IRAS (1996) and 2P/Encke (1997)

We present the characteristics of the dust comae of two comets, 126P/IRAS, a member of the Halley family (a near-isotropic comet), and 2P/Encke, an ecliptic comet. We have primarily used mid- and far-infrared data obtained by the ISOPHOT instrument aboard the Infrared Space Observatory (ISO) in 1996 and 1997, and mid-infrared data obtained by the SPIRIT III instrument aboard the Midcourse Space Experiment (MSX) in 1996. We find that the dust grains emitted by the two comets have markedly different thermal and physical properties. P/IRAS's dust grain size distribution appears to be similar to that of fellow family member 1P/Halley, with grains smaller than 5 microns dominating by surface area, whereas P/Encke emits a much higher fraction of big (20 μm and higher) grains, with the grain mass distribution being similar to that which is inferred for the interplanetary dust population. P/Encke's dearth of micron-scale grains accounts for its visible-wavelength classification as a “gassy” comet. These conclusions are based on analyses of both imaging and spectrophotometry of the two comets; this combination provides a powerful way to constrain cometary dust properties. Specifically, P/IRAS was observed preperihelion while 1.71 AU from the Sun, and seen to have a 15-arcmin long mid-infrared dust tail pointing in the antisolar direction. No sunward spike was seen despite the vantage point being nearly in the comet's orbital plane. The tail's total mass at the time was about 8×10⁹ kg. The spectral energy distribution (SED) is best fit by a modified greybody with temperature T=265±15 K and emissivity ε proportional to a steep power law in wavelength λ: ε∝λ^−α, where α=0.50±0.20(2σ). This temperature is elevated with respect to the expected equilibrium temperature for this heliocentric distance. The dust mass loss rate was between 150-600 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 3.3, and the albedo of the dust was 0.15±0.03. Carbonaceous material is depleted in the comet's dust by a factor of 2-3, paralleling the C₂ depletion in P/IRAS's gas coma. P/Encke, on the other hand, observed while 1.17 AU from the Sun, had an SED that is best fit by a Planck function with T=270±15 K and no emissivity falloff. The dust mass loss rate was 70-280 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 2.3, and the albedo of the dust was about 0.06±0.02. These conclusions are consistent with the strongly curved dust tail and bright dust trail seen by Reach et al. (2000; Icarus 148, 80) in their ISO 12-μm imaging of P/Encke. The observed differences in the P/IRAS and P/Encke dust are most likely due to the less evolved and insolated state of the P/IRAS nuclear surface. If the dust emission behavior of P/Encke is typical of other ecliptic comets, then comets are the major supplier of the interplanetary dust cloud.     

Interplanetary dust detected by the Cassini CDA Chemical Analyser

During its cruise phase, prior to encountering Jupiter, the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft returned time of flight mass spectra (TOF MS) of two interplanetary dust particles. Both particles were found to be iron-rich, with possible traces of hydrogen, carbon, nickel, chromium, manganese, titanium, vanadium and minor silicates. Carbon, hydrogen, oxygen and potassium are also present as possible contaminants of the impact target of CDA. Silicates and magnesium do not feature predominantly in the spectra; this is surprising considering the expected dominance of silicate-rich minerals in interplanetary dust particles. The particle masses are and . The corresponding radii ranges for the particles, assuming densities from 7874-2500 kg m⁻³ are 0.7-4 μm and 2.6-6.8 μm, respectively. With the same density assumptions the β values (ratio of radiation pressure to gravitational force) are estimated as 0.027-0.21 and 0.016-0.06 respectively, allowing possible orbits to be calculated. The resulting orbits are bound and prograde with semi-major axes, eccentricities and inclinations in the region of 0.3-1.26 AU, 0.4-1.0 and 0-60° for the first particle and 0.8-2.5 AU, 0.2-0.9 and 0-30° for the second. The more probable orbits within these ranges indicate that the first particle is in an Aten-like orbit, whilst the second particle is in an Apollo-like orbit, despite both grains having very similar, predominantly metallic compositions. Other possible orbital solutions for both particles encompass orbits which more closely resemble those of Jupiter-family comets.     

DuneXpress

The DuneXpress observatory will characterize interstellar and interplanetary dust in-situ, in order to provide crucial information not achievable with remote sensing astronomical methods. Galactic interstellar dust constitutes the solid phase of matter from which stars and planetary systems form. Interplanetary dust, from comets and asteroids, represents remnant material from bodies at different stages of early solar system evolution. Thus, studies of interstellar and interplanetary dust with DuneXpress in Earth orbit will provide a comparison between the composition of the interstellar medium and primitive planetary objects. Hence DuneXpress will provide insights into the physical conditions during planetary system formation. This comparison of interstellar and interplanetary dust addresses directly themes of highest priority in astrophysics and solar system science, which are described in ESA’s Cosmic Vision. The discoveries of interstellar dust in the outer and inner solar system during the last decade suggest an innovative approach to the characterization of cosmic dust. DuneXpress establishes the next logical step beyond NASA’s Stardust mission, with four major advancements in cosmic dust research: (1) analysis of the elemental and isotopic composition of individual interstellar grains passing through the solar system, (2) determination of the size distribution of interstellar dust at 1 AU from 10^− 14 to 10^− 9 g, (3) characterization of the interstellar dust flow through the planetary system, (4) establish the interrelation of interplanetary dust with comets and asteroids. Additionally, in supporting the dust science objectives, DuneXpress will characterize dust charging in the solar wind and in the Earth’s magnetotail. The science payload consists of two dust telescopes of a total of 0.1 m² sensitive area, three dust cameras totaling 0.4 m² sensitive area, and a nano-dust detector. The dust telescopes measure high-resolution mass spectra of both positive and negative ions released upon impact of dust particles. The dust cameras employ different detection methods and are optimized for (1) large area impact detection and trajectory analysis of submicron sized and larger dust grains, (2) the determination of physical properties, such as flux, mass, speed, and electrical charge. A nano-dust detector searches for nanometer-sized dust particles in interplanetary space. A plasma monitor supports the dust charge measurements, thereby, providing additional information on the dust particles. About 1,000 grains are expected to be recorded by this payload every year, with 20% of these grains providing elemental composition. During the mission submicron to micron-sized interstellar grains are expected to be recorded in statistically significant numbers. DuneXpress will open a new window to dusty universe that will provide unprecedented information on cosmic dust and on the objects from which it is derived.     

The relationship of active comets, “extinct” comets,and dark asteroids

Spectrophotometric data on groups of asteroids in different types of orbits reveal different distributions of spectral properties, depending on whether the orbits are cometary or noncometary. In a list of 10 asteroids frequently suggested on purely dynamical grounds to be extinct or dormant comets, all have properties suggestive of spectral classes D, P, or C. Preliminary IRAS albedo results support this. Objects in these classes are very dark, reddish-black to neutral-black, and prevalent among the Trojans and outer belt. Two comets observed at low activity (visible nuclei) also have properties more consistent with D asteroids than any other class (very low reported geometric albedos of 0.02 and red colors). Consistent with these results are very low albedos reported for materials in more than a dozen comets; they average 0.05. Also, sampled cometary dust particles appear to consist of dark carbonaceous materials. Dramatically different are a control group of 13 Aten/Apollo/Amor objects selected from noncometary orbits. Most are in moderate-albedo classes: 8 or 9 appear to be of class S, and only 1 is in a low-albedo class (C). These are probably mostly objects perturbed out of the inner asteroid belt. The preponderence of S's in the noncometary group, together with the preponderence of ordinary chondrites among meteorites, may be evidence that such meteorites came from S asteroids. The data indicate that extinct, dormant, inactive, and minimally active comet nuclei have low albedos (p_v=a few percent) and very red to moderately red colors. As a group, their spectra are more similar to those of outer Solar System asteroids of classes D, P, and C, than to those of inner belt classes, though the observations are frequently not yet complete enough to assign definitively a spectral class. The results, taken together, support the view that dynamically identified “extinct comet candidates” are indeed outer Solar System objects probably of cometary origin. The results also support a scenario of Solar System formation in which dark carbonaceous dust dominated the spectrophotometric properties of planetesimals formed from about 2.7 AU out to at least the Trojan region at 5.2 AU. From 2.7 to at least 5.2 AU, and from class C to class D, the color of this dust reddens, apparently due to increasing amounts of red organic condensates. Comets are probably also colored to different degrees, by dust of this type, and may in some cases be even redder than D asteroids.     

Trapping and dynamical evolution of interplanetary dust particles in Earth’s quasi-satellite resonance

We used numerical simulations to model the orbital evolution of interplanetary dust particles (IDPs) evolving inward past Earth’s orbit under the influence of radiation pressure, Poynting–Robertson light drag (PR drag), solar wind drag, and gravitational perturbations from the planets. A series of β values (where β is the ratio of the force from radiation pressure to that of central gravity) were used ranging from 0.0025 up to 0.02. Assuming a composition consistent with astronomical silicate and a particle density of 2.5 g cm⁻³ these β values correspond to dust particle diameters ranging from 200 μm down to 25 μm. As the dust particle orbits decay past 1 AU between 4% (for β = 0.02, or 25 μm) and 40% (for β = 0.0025, or 200 μm) of the population became trapped in 1:1 co-orbital resonance with Earth. In addition to traditional horseshoe type co-orbitals, we found about a quarter of the co-orbital IDPs became trapped as so-called quasi-satellites. Quasi-satellite IDPs always remain relatively near to Earth (within 0.1–0.3 AU, or 10–30 Hill radii, R_H) and undergo two close-encounters with Earth each year. While resonant perturbations from Earth halt the decay in semi-major axis of quasi-satellite IDPs their orbital eccentricities continue to decrease under the influence of PR drag and solar wind drag, forcing the IDPs onto more Earth-like orbits. This has dramatic consequences for the relative velocity and distance of closest approach between Earth and the quasi-satellite IDPs. After 10⁴–10⁵ years in the quasi-satellite resonance dust particles are typically less than 10R_H from Earth and consistently coming within about 3R_H. In the late stages of evolution, as the dust particles are escaping the 1:1 resonance, quasi-satellite IDPs can have deep close-encounters with Earth significantly below R_H. Removing the effects of Earth’s gravitational acceleration reveals that encounter velocities (i.e., velocities “at infinity”) between quasi-satellite IDPs and Earth during these close-encounters are just a few hundred meters per second or slower, well below the average values of 2–4 km s⁻¹ for non-resonant Earth-crossing IDPs with similar initial orbits. These low encounter velocities lead to a factor of 10–100 increase in Earth’s gravitationally enhanced impact cross-section (σ_grav) for quasi-satellite IDPs compared to similar non-resonant IDPs. The enhancement in σ_grav between quasi-satellite IDPs and cometary Earth-crossing IDPs is even more pronounced, favoring accretion of quasi-satellite dust particles by a factor of 100–3000 over the cometary IDPs. This suggests that quasi-satellite dust particles may dominate the flux of large (25–200 μm) IDPs entering Earth’s atmosphere. Furthermore, because quasi-satellite trapping is known to be directly correlated with the host planet’s orbital eccentricity the accretion of quasi-satellite dust likely ebbs and flows on 10⁵ year time scales synchronized with Earth’s orbital evolution.     

Three‐dimensional shapes and Fe contents of Stardust impact tracks: A track formation model and estimation of comet Wild 2 coma dust particle densities

Abstract– We investigated three‐dimensional structures of comet Wild 2 coma particle impact tracks using synchrotron radiation (SR) X‐ray microtomography at SPring‐8 to elucidate the nature of comet Wild 2 coma dust particles captured in aerogel by understanding the capture process. All tracks have a similar entrance morphology, indicating a common track formation process near the entrance by impact shock propagation irrespective of impactor materials. Distributions of elements along the tracks were simultaneously measured using SR‐XRF. Iron is distributed throughout the tracks, but it tends to concentrate in the terminal grains and at the bottoms of bulbs. Based on these results, we propose an impact track formation process. We estimate the densities of cometary dust particles based on the hypothesis that the kinetic energy of impacting dust particles is proportional to the track volume. The density of 148 cometary dust particles we investigated ranges from 0.80 to 5.96 g cm^?3 with an average of 1.01 (±0.25) g cm^?3. Moreover, we suggest that less fragile crystalline particles account for approximately 5 vol% (20 wt%) of impacting particles. This value of crystalline particles corresponds to that of chondrules and CAIs, which were transported from the inner region of the solar system to the outer comet‐forming region. Our results also suggest the presence of volatile components, such as organic material and perhaps ice, in some bulbous tracks (type‐C).     

Structure of the Earth’s circumsolar dust ring

Interplanetary dust particles from comets and asteroids pervade the Solar System and become temporarily trapped into orbital resonances with Earth, leading to a circumsolar dust ring. Using the unique vantage point of the Spitzer Space Telescope from its Earth-trailing solar orbit, we have measured for the first time the azimuthal structure of the Earth’s resonant dust ring. There is a relative paucity of particles within 0.1 AU of the Earth, followed by an enhancement in a cloud that is centered 0.2 AU behind Earth with a width of 0.08 AU along the Earth’s orbit. The North ecliptic pole is ∼3% brighter at 8 μm wavelength when viewed from inside the enhancement. The presence of azimuthal asymmetries in debris disks around other stars is considered strong evidence for planets. By measuring the properties of the Earth’s resonant ring, we can provide “ground truth” to models for interactions of planets and debris disks, possibly leading to improved predictions for detectability of life-bearing planets. The low amplitude of the azimuthal asymmetry in the Earth’s circumsolar ring suggests significant contributions to the zodiacal light from particles that are large (>30 μm) or have large orbital eccentricity that makes capture into mean motion resonances inefficient.     

Accretion of dust by chondrules in a MHD-turbulent solar nebula

Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Compaction and fragmentation of dust aggregates are not included. Different chondrule volume densities ρ_c lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within ～1 gas scale-height of the nebula midplane. Chondrules can reach their asymptotic radius in 10–800 years, although short growth times due to very high ρ_c may not be altogether realistic. If the sticking efficiency Q of dust to chondrules depends on their relative speed δv, such that Q < 10^?2 whenever δv > v_stick ≈ 34 cm/s (with v_stick a critical sticking velocity), then longer growth times result due to the prevalence of high MRI-turbulent relative velocities. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height, and in a ～20% dispersion in radius values at every height bin. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula.     

Ion bombardment of interplanetary dust

The recent discovery of ion tracks in interplanetary dust and the increasing evidence for carbon and carburized materials in these objects are strongly suggestive that chemical processing by energetic charged-particle bombardment has occured during the dust lifetimes. The track density gives a measure of the total ion fluence experienced by the grains. We use this information and laboratory data on the modification of icy surfaces by incident ions and electrons to discuss the likelihood that chondritic interplanetary dust particles could have been proceed, by plasma bombardment, from aggregates of particles which had volatile and/or ogranic mantles. Such a processing would leave carbon and carburized deposits and can affect estimates of the temperature of formation of these dust grains.     

Three years of Ulysses dust data: 1993–1995

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79°). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraftreapproached the inner solar system, flew over the Suns south polar region in September 1994,crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Suns northpolar region in July 1995. We report on dust impact data obtained with the dust detector onboardUlysses between January 1993 and December 1995. We publish and analyse the complete dataset of 509 recorded impacts of dust particles with masses between 10⁻¹⁶ g–10⁻⁷ g. Together with 968 dust impacts from launch until the end of 1992 published earlier (Grün et al., 1995c), information about 1477 particles detected with theUlysses sensor between October 1990 and December 1995 is now available. The impact ratemeasured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day andvaried by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU arecompatible with particles of interstellar origin. Two populations of interplanetary particles havebeen recognized: big micrometer-sized particles close to the ecliptic plane and smallsub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is comparedwith a model for the flux of interstellar dust particles which gives relatively good agreement withthe observed impact rate. No change in the instruments noise characteristics or degradation of thechanneltron could be revealed during the three-year period.     

Optical properties of dust from Jupiter family comets

To try to define specific physical properties of the dust of Jupiter-family comets (JFCs), we compare the light scattered by them. Amongst the more than 1000 JFCs, less than 200 are numbered, 40 of them being rather bright. In the present work we use data from the latter. In situ observations of three nuclei show low albedo surfaces. The albedo of the dust particles in the coma is low, with generally a red colour. The A(α)fρ product is a measure of cometary activity and secular changes. Images of different regions (jets and fans) give indications on the nucleus rotation and position of the emitting areas, as compared to the position of the rotation axis. Differences in physical properties between the particles in different regions are pointed out by differences in the linear polarization of the scattered light and by spectral variations in brightness and polarization. Jupiter family comets are considered as dust-poor comets. Tails and trails’ studies give an estimation of the size distribution of the particles. However the dust production rates depend on the largest particles (up to centimetre size), which are mainly observed in the trails where large dark compact particles are found. These dark particles are also responsible for the high polarization in the inner most coma of some comets. The meaning, in terms of physical properties, of the linear polarization is discussed through different examples such as 2P/Encke, 9P/Tempel 1 or the fragments of 73P/Schwassmann-Wachmann 3. Cometary outbursts and splitting events show that the properties of the dust ejected from the interior of the nucleus are similar to the ones of more active comets (new or with larger semi-major axis).     

Physical properties of cometary and interplanetary dust

In the absence of numerous in situ studies, physical properties of cosmic dust may be derived from observations of their light scattering and thermal properties, through numerical simulations making use of realistic assumptions. Estimations about cometary and interplanetary dust composition, structure, size, as well as about their light scattering and thermal properties, are first summarized. We then present and discuss the numerical simulations we have performed with different types of particles: core-mantle submicron-sized elongated grains (having contributed to the formation of cometary dust), fractal aggregates of such grains (found in cometary comae and in the interplanetary dust cloud), and fractal aggregates of large dust grains (found in cometary dust trails).A very satisfactory fit to the numerous polarimetric observations of comet Hale-Bopp is obtained for a mixture with about 33–60% of organics in mass, with a power law size distribution with an index of (−3) and a radius of 20 μm for the upper cut-off. For the less-constrained polarimetric observations of interplanetary dust near 1 AU, a fit is obtained for a mixture with about 40% of organics in mass, with a similar size distribution and a radius of about 50 μm for the upper cut-off. The ensemble of results obtained for the interplanetary dust strongly suggest that its light scattering and thermal properties stem from the presence of compact and fluffy particles, with compositions ranging from silicates to more absorbing materials, whose contribution decreases with decreasing distance to the Sun.     

On the origin of interplanetary dust within recorded history

Abstract— The possibility of an abrupt origin of interplanetary dust as a result of a collision between asteroids or an extraordinary comet is considered. If all interplanetary dust were produced in one event within recorded history, it would have been visible from the Earth with the unaided eye. The rate, surface area, and brightness of asteroid collision remnants are derived. Ancient Chinese records are searched for extraordinary comets and bright pointlike objects with small angular motion and concentration to the ecliptic.     

Physical properties of asteroid dust bands and their sources

Disruptive collisions in the main belt can liberate fragments from parent bodies ranging in size from several micrometers to tens of kilometers in diameter. These debris bodies group at initially similar orbital locations. Most asteroid-sized fragments remain at these locations and are presently observed as asteroid families. Small debris particles are quickly removed by Poynting-Robertson drag or comminution but their populations are replenished in the source locations by collisional cascade. Observations from the Infrared Astronomical Satellite (IRAS) showed that particles from particular families have thermal radiation signatures that appear as band pairs of infrared emission at roughly constant latitudes both above and below the Solar System plane. Here we apply a new physical model capable of linking the IRAS dust bands to families with characteristic inclinations. We use our results to constrain the physical properties of IRAS dust bands and their source families. Our results indicate that two prominent IRAS bands at inclinations ≈2.1° and ≈9.3° are byproducts of recent asteroid disruption events. The former is associated with a disruption of a ≈30-km asteroid occurring 5.8 Myr ago; this event gave birth to the Karin family. The latter came from the breakup of a large >100-km-diameter asteroid 8.3 Myr ago that produced the Veritas family. Using an N-body code, we tracked the dynamical evolution of ≈10⁶ particles, 1 μm to 1 cm in diameter, from both families. We then used these results in a Monte Carlo code to determine how small particles from each population undergo collisional evolution. By computing the thermal emission of particles, we were able to compare our results with IRAS observations. Our best-fit model results suggest the Karin and Veritas family particles contribute by 5-9% in 10-60-μm wavelengths to the zodiacal cloud's brightness within 50° latitudes around the ecliptic, and by 9-15% within 10° latitudes. The high brightness of the zodiacal cloud at large latitudes suggests that it is mainly produced by particles with higher inclinations than what would be expected for asteroidal particles produced by sources in the main belt. From these results, we infer that asteroidal dust represents a smaller fraction of the zodiacal cloud than previously thought. We estimate that the total mass accreted by the Earth in Karin and Veritas particles with diameters 20-400 μm is ≈15,000-20,000 tons per year (assuming 2 g cm⁻³ particles density). This is ≈30-50% of the terrestrial accretion rate of cosmic material measured by the Long Duration Exposure Facility. We hypothesize that up to ≈50% of our collected interplanetary dust particles and micrometeorites may be made up of particle species from the Veritas and Karin families. The Karin family IDPs should be about as abundant as Veritas family IDPs though this ratio may change if the contribution of third, near-ecliptic source is significant. Other sources of dust and/or large impact speeds must be invoked to explain the remaining ≈50-70%. The disproportional contribution of Karin/Veritas particles to the zodiacal cloud (only 5-9%) and to the terrestrial accretion rate (30-50%) suggests that the effects of gravitational focusing by the Earth enhance the accretion rate of Karin/Veritas particles relative to those in the background zodiacal cloud. From this result and from the latitudinal brightness of the zodiacal cloud, we infer that the zodiacal cloud emission may be dominated by high-speed cometary particles, while the terrestrial impactor flux contains a major contribution from asteroidal sources. Collisions and Poynting-Robertson drift produce the size-frequency distribution (SFD) of Karin and Veritas particles that becomes increasingly steeper closer to the Sun. At 1 AU, the SFD is relatively shallow for small particle diameters D (differential slope exponent of particles with D?100 μm is ≈2.2-2.5) and steep for D?100 μm. Most of the mass at 1 AU, as well as most of the cross-sectional area, is contributed by particles with D≈100-200 μm. Similar result has been found previously for the SFD of the zodiacal cloud particles at 1 AU. The fact that the SFD of Karin/Veritas particles is similar to that of the zodiacal cloud suggests that similar processes shaped these particle populations. We estimate that there are ≈5×¹⁰²⁴ Karin and ≈10²⁵ Veritas family particles with D>30 μm in the Solar System today. The IRAS observation of the dust bands may be satisfactorily modeled using ‘averaged’ SFDs that are constant with semimajor axis. These SFDs are best described by a broken power-law function with differential power index α≈2.1-2.4 for D?100 μm and by α?3.5 for 100 μm?D?1 cm. The total cross-sectional surface area of Veritas particles is a factor of ≈2 larger than the surface area of the particles producing the inner dust bands. The total volumes in Karin and Veritas family particles with 1 μm<D<1 cm correspond to D=11 km and D=14 km asteroids with equivalent masses ≈1.5×¹⁰¹⁸ g and ≈3.0×¹⁰¹⁸ g, respectively (assuming 2 g cm⁻³ bulk density). If the size-frequency and radial distribution of particles in the zodiacal cloud were similar to those in the asteroid dust bands, we estimate that the zodiacal cloud represents ∼3×¹⁰¹⁹ g of material (in particles with 1 μm<D<1 cm) at ±10° around the ecliptic and perhaps as much as ∼¹⁰²⁰ g in total. The later number corresponds to about a 23-km-radius sphere with 2 g cm⁻³ density.     

The effect of a bounded interplanetary diffusion medium on the propagation of solar flare cosmic rays

An analysis of solar flare data indicates that the graph of log(nt ^3/(2–)) deviates late in the solar event from the straight line predicted for the infinite, unbounded interplanetary medium. It is shown by mathematical analysis, utilizing a model based on the radial diffusion coefficient D = Mr , with 1, that the deviation can be ascribed to the loss of flare particles through an external boundary at about 5–6 AU from the Sun. An inner region terminating at 5–6 AU, followed by an extensive region of increasingly less resistance to the diffusion of flare particles is also feasible and it is shown that measurements taken at the Earth cannot predict the extent of this outer region. The results are applicable to either the isotropic or highly anisotropic models. The constant diffusion model is shown to be inadequate since it requires a boundary 1.5 AU from the Sun. In view of the present and previous studies of solar flare data, it is asserted that the fundamental principle governing the diffusion of solar flare particles through interplanetary space is the radial diffusion coefficient mode of propagation.     

Comet classification with new methods for gas and dust spectroscopy

We present the results of a program of comet long-slit spectroscopy with the Kast Dual Spectrograph on the 3-m Shane Telescope at Lick Observatory. A total of 26 comets, from a variety of dynamical families, were observed on 39 different nights from 1996 to 2007. A new statistical method extracted the twilight sky from comet frames, because traditional sky subtraction techniques were inadequate. Because previously published Haser model parent and daughter scale lengths did not fit the data well, unbiased ranges of scale lengths were searched for the best-fitting pairs. Coma gas production rates for OH, CN, C₂, C₃, NH, NH₂, and OH confirmed the widely reported carbon-chain depletion for a sub-class of comets, most notably high-perihelion Jupiter-family comets observed at r_h > 1.5 AU, with different behaviors for C₂ and C₃. Our long-slit spectroscopy data was also adapted for the A(θ)fρ dust production parameter. The assumption that A(θ)fρ is constant throughout the nucleus was not upheld. High dust-to-gas ratios for comets with large perihelia were not a selection effect, and suggest that the dust was released earlier in the formation of the coma than the gas. The dust-to-gas ratio did not exhibit any evolutionary traces between different comet dynamical families. The comet survey illuminates the diversity among comets, including the unusually carbon poor Comet 96P/Machholz.