Although most meteorites come from asteroids, some could originate from short-period comets. The identification of comet-asteroid transition object 4015 Wilson-Harrington as the first Apollo object known to have a cometary origin, confirms that short-period comets leave behind nonvolatile residues, from which meteoroids with low Earth-encounter velocities can originate. Studies of cometary fireballs suggest that some cometary meteoroids have high enough strengths to have survived atmospheric entry as cometary meteorites, which have either not been found or recognized. The presence of unequilibrated anhydrous silicates in cometary dust represents a significant difference between cometary material and CI and CM meteorites.

This is a description of the beginning of a systematic investigation into the optical properties of dust structures that are likely to be representative of interplanetary dust. I delineate the development of a physical dust model to parameterize the optically important characteristics of the dust. The result is a system with two refractive indexes in an aggregate structure of varying porosity - a challenging model for most current light scattering theories. Experimental data is needed to investigate the scattering by these structures and to test new theoretical solutions (e.g., Xu 1995) as they develop. I give a brief description of the new microwave analog scattering laboratory that has been developed for this purpose at the Laboratory for Astrophysics of the University of Florida's Astronomy Department. Finally, laboratory data is shown in support of dense aggregate models for interplanetary dust.

We have set up an experimental device to optically study the scattering properties of dust particles. Measurements over the 8 — 174° interval of scattering angles are performed on a continuously flowing dust loaded jet illuminated by a polarized red HeNe laser beam. The scattering is averaged over the population of the dust particles in the jet, which can be determined independently, and give the “volume scattering function” for the two directions of polarization directly. While results for spherical particles are in good agreement with Mie theory, those for arbitrary particles show conspicuous deviations.

Polarimetric measurements of the light scattered by irregular dust particles are essential to interpret observations of solar system dust in terms of its physical properties. We developed a iight scattering unit to retrieve polarimetric phase curves of dust samples in microgravity conditions. Preliminary results suggest that the values for the maximum polarization are higher under 1 “ g ” than under 0 “ g ” This can be compared to ground-based measurements which exhibit higher values for packed dust than for sifted dust. The unit is operational and is used to help design a related orbital experiment.

For a long time, the dominant scattering theory used in radiative transfer and scattering calculations has been Mie theory, which is the complete solution to the problems of light scattering by single, isotropic, and homogeneous spheres. However, cosmic dust collections show that most of the largest sized interplanetary dust particles may be porous, inhomogeneous, and aggregated and may have quite different scattering properties. Arbitrary configurations of aggregated spheres may provide a reasonable first approximation to realistic light-scattering models of interplanetary dust particles. In the last few decades, progress has been made in developing light scattering theory for interacting spheres, The development of the addition theorems for scalar and vector spherical wave functions (Friedman & Russek, 1954; Stein, 1961; Cruzan, 1962) opened up a new area in the theoretical study of multisphere scattering problems.

The first scattering coefficient (a1-term) in Mie theory is introduced to determine the dipole polarizability used in the discrete dipole approximation (DDA) to calculate the scattering properties of the cluster of spheres in order to improve our understanding of interplanetary dust particles. In this method, each sphere in a cluster is replaced by a single dipole. The accuracy of this method is tested for a few spheres in contact. It is confirmed that the a1-term method is superior to other types of DDA and is particularly suitable for the case when the particles are placed randomly. By using this method, it becomes possible to treat large cluster of spheres, i.e., a size parameter of the target X(≡ 2πreq/λ) ∽ 50, where req is a volume equivalent radius of the target, and λ is the wavelength of incident wave.

For particles grown in the two limiting cases of coagulation (ballistic particle-cluster agglomeration and ballistic cluster-cluster agglomeration), lower and upper limits of the extinction at wavelengths from 1 μm to 1 mm are derived. The particle sizes are in the Rayleigh limit and the number of constituent grains in each particle is constant. Effective medium theories, the discrete dipole approximation and the discrete multipole method are applied to compute the optical behaviour of the coagulated particles. The spectral representation for inhomogeneous media is employed to investigate topology effects systematically.

Halley probes (Giotto, Vega) have shown that cometary grains can be extremely dark and might have fluffy structures. These probes have also detected submicron sized particles in the coma of the comet. In view of these observations and to interpret the observed data on several other comets we calculate the angular distribution of the scattered intensity, polarization and albedo for porous and compact particles. For these calculations, we apply discrete dipole approximation (DDA) first to the spheroidal particles, assumed to be made of small scattering elements (dipoles). Then we reduce the number of dipoles systematically to model the porous grains. We study the phase function, linear polarization, and albedo as a function of grain size and packing density.

We use the DDA method to study scattering properties of aggregates. Based on this aggregate model, the temperatures of cometary dust particles will be close to those of the equivalent total volume sphere when the aggregate porosity is ≤ 0.64.

The photometric phase curves of Saturn's A and B rings exhibit a sharp peak in reflectance when the phase angle approaches zero, commonly known as the opposition effect. Recent work has suggested that the width and amplitude of the opposition effect may be consistent with coherent backscattering from wavelength-sized grains which cover the surfaces of macroscopic ring particles.

We present a new technique for computing the optical constants for partially disordered solids based on their crystalline optical constants. The technique assumes that the material is composed of a continuous distribution of oscillators (CDO) and that the degree of atomic disorder can be described by one, or at most two, scalar parameters. We apply the technique to an oft-mentioned solar system material, olivine, and show that its dielectric functions can be predicted for an arbitrary degree of disorder.

Three topics are briefly reviewed to examine evidence for the nature of dust grains and grain alteration by physical processes related to mainly sublimation. Namely, (1) a change of dust flux detected by in situ dust measurements beyond ~ 2 AU from the Sun suggests a disappearance of volatile ices due to sublimation below this distance, (2) an elongation angle dependence of Doppler shifts observed in the zodiacal light can be explained by the introduction of a dependence of the orbital velocity on the radiation pressure forces acting on the grains, taking into account a variation of dust structure with heliocentric distance, and (3) the variation of color temperature observed in cometary coma suggests a loss of carbon content from cometary dust grains due to sublimation. This piece of evidence agrees with the generation of carbon-bearing molecules from extended sources, as infered from observations in cometary comae.

Ground-based, remote and “in situ” observations show that dust plays an important role in the evolution of interstellar, circumstellar and interplanetary media. Despite the wealth of available astronomical data, a main item to be clarified concerns the actual chemico-physical composition of cosmic dust. A correct interpretation of observations must rely on a systematic comparison with results coming from laboratory simulations. In this paper we present an overview of recent results, obtained in our laboratory, on various forms of carbon-based and silicate grains of possible astrophysical interest.

It is probable that most molecules in space are formed on the surface of small solid grains in dense molecular clouds. Such grains have generally accumulated icy mantles inside dark clouds. We have investigated the dynamical behavior of a hydrogen atom impinging on the mantle consisting of amorphous water ice based on an MD (Molecular Dynamics) computer simulation to estimate the structure of the resulting grains with the icy mantles. We have found that the hydrogen atoms impinging on the surface of amorphous water ice were easily trapped in a dent of the surface and, consequently they are fixed there firmly. Our results, which neglect tunneling, suggest that the migration of hydrogen atoms over a large region of the surface of icy grains may be less common than is often assumed.

We present results of a numerical collisional model which shows that the slope index of the equilibrium size distribution is dependent upon the size-strength scaling properties of the colliding bodies. This implies that individual asteroid families or distinct taxonomic classes within the mainbelt asteroid population may evolve different equilibrium size distributions. Well constrained observations of the size distribution over particular size ranges may allow constraints to be placed on the impact strengths of particles much larger or smaller than are capable of being measured in laboratory impact experiments.

Computer simulations of the collisions of gas molecules with fluffy aggregates have been carried out to estimate gas drag forces in the free molecular flow limit. It is found that the gas drag force on fluffy aggregates can be approximated by that on an area-equivalent sphere in a wide range of relative velocities of the aggregate through ambient gas. The deviation from this approximation is weakly dependent on the relative velocity. This result leads to a significant increase of gas drag forces on fluffy aggregates compared to that on a mass-equivalent sphere.

The Galileo and Ulysses dust detectors can detect electric charges of dust particles. Dust particles entering the sensor (see, e.g., Grün et al. 1992) may be detected by the charge Qp that they induce to the charge grid. All suitably massive dust particles - charged or uncharged - are then detected by the cloud of ions and electrons they produce during the impact on the hemispherical target after the time of flight between the charge grid and the target. After separation in the electric field, ions and electrons are collected by separate electrodes and produce two pulses of opposite polarity. From the two pulse heights and the rise times, the mass and impact speed of the dust particle are derived.

In this review, the picture of the interstellar dust is outlined from the viewpoint of its role in galactic evolution. The modern concept of distinct dust populations is presented. The properties of the dust grains in the diffuse interstellar medium and in the molecular clouds are described. Some unsolved problems and future perspectives of interstellar dust research are pointed out.

The effects of preaggregation silicate crystallinity and postaggregation silicate crystallinity on the 11.3 μm structure in comet dust emission have been calculated. Of the order of 10 - 20% crystallinity provides the best agreement to date with observation but the fits are not yet fully satisfactory.

The bright 10 μm excess emission of β Pic stars allows detailed spectroscopy of their warm circumstellar solid material. The silicate features in β Pic and 51 Oph are broader and contain more structure than interstellar and most circumstellar emission features. The β Pic and 51 Oph features are remarkably similar to those in comets Halley, Bradfield 1987s, Levy 1990 XX, and Mueller 1993a which have 11.3 μm emission features characteristic of crystalline silicates. In contrast, other β Pic-type stars (ζ Lep and β UMa) show weak 10 μm emission from larger, amorphous dust grains. Some comets such as Austin 1990 V emit similar weak 10 μm features.