All extraterrestrial particles are heated during their passage through the Earth's atmosphere. The mineral assemblages that form during entry heating can be used to constrain the maximum temperature attained which, in turn, places limits on their entry velocity Entry velocity can be used to discriminate typical cometary IDPs from asteroidal particles.

A common feature of heated IDPs is the presence of magnetite (Mt) rims on the particle surface. These rims range from thin, discontinuous Mt layers on lobes or on small constituent grains within lightly heated IDPs, to thick, continuous, polycrystalline rims that completely surround many strongly heated particles. Petrographic evidence indicates that Mt rims form directly from individual phases in IDPs via oxidation of Fe from the host phases. Our data indicate that the temperature of magnetite formation can range from a minimum of ~600°C (by decarbonation of Fe-bearing carbonates and the breakdown of phyllosilicates) to T > 850°C (decomposition of laihunite). In extreme cases, μm-sized Mt plates armor the surface of partly melted chondritic IDPs. Magnetite rims are more well-developed on hydrated IDPs than on anhydrous particles. The development of magnetite rims correlates with other independent indicators of heating including loss of volatile trace elements and sulfur depletions.

Recent progress has been reported in the determination of the zodiacal thermal emission, brightness and polarization. These results, of interest to estimate the foreground sources in astrophysical observations, do not provide immediately information on the dust distribution, and on its optical and thermal properties. To infer local information about the bulk density, and the physical properties of the dust particles, it is necessary to compare the observations with realistic models or to invert the line-of-sight data. The latter approach typically suggests that the bulk density is (in the symmetry plane) inversely proportional to the solar distance, that the particles are not spheroidal, but rather irregular in shape, that their physical properties change with their distance to the Sun and their orbital inclination, and finally that they do not emit like a blackbody. The heterogeneity noticed in the cloud is due to various sources of dust particles, the size, shape or albedo of which evolve as a function of time, under collision and/or evaporation processes.

The COBE Diffuse Infrared Background Experiment has obtained some of the most extensive observations of the interplanetary dust (IPD) cloud ever assembled. For the 10 months of cryogenic operation, the brightness of the entire celestial, sphere was mapped with an 0.7° x 0.7° field of view at wavelengths of 1.25, 2.2, 3.5, 4.9, 12, 25, 60, 100, 140, and 240 μm, and the linear polarization was mapped at 1.25, 2.2, and 3.5 μm. Observations with reduced sensitivity continued at all wavelengths short of 12 μm for over 3 years after cryogen expiration. Throughout these observations, nearly 1/2 of the sky was mapped every day at elongation angles ranging from 64° to 124°. I describe the DIRBE and the general character of the infrared sky, outline the DIRBE team's approach to isolating the IPD signal, and review results of our initial studies of the zodiacal dust bands, the circumsolar dust ring, and the character of IPD cloud particles.

Yielding the inner continuation of the interplanetary dust cloud, the dust at about 0.3 AU and closer to the Sun is studied under observing conditions different from those of the Zodiacal light. The F-coronal brightness indicates its optical particle properties as well as its overall spatial distribution. The present knowledge is based on visible and near infrared F-coronal observations and may be improved from data of the SOHO satellite in the near future. Some dynamical effects become particulary important for sub-μm particles in the solar vicinity. However, these particles seem to have only a small effect on the observable corona brightness, but are more accessible to in-situ experiments.

The high photometric quality and full-sky coverage in the COBE DIRBE datasets make possible detailed studies of the interplanetary medium. This paper presents a preliminary derivation of the infrared scattering phase function of interplanetary dust. The ultimate purpose of these investigations is to use the DIRBE observations to constrain the composition, size and structure of interplanetary dust grains.

Impact data from the ULYSSES dust detector at 5 AU from the Sun have been interpreted as a flux of sub-micron interstellar dust particles (Grün et al, 1994) arriving from 252° ecliptic longitude and 2.5° ecliptic latitude. Following the motions of these particles under the influence of solar gravity, radiation pressure and electromagnetic forces, we present results from the modeling of the thermal emission from the resultant particle cloud, and conclude that the chances for the detection of such an interstellar signature in the COBE data are marginal at best.

The ability of the Earth to trap interplanetary grains into a dust ring lying along the terrestrial orbit was shown by numerical simulations and confirmed by infrared observations (IRAS, COBE). Such a ring could have its signature on the elongation dependence of the zodiacal brightness along the ecliptic, especially near 90° of the Sun. Indeed, the elongation dependence observed at Tenerife by Dumont and Sanchez (1975) shows that the space density of interplanetary dust slightly increases with increasing heliocentric distance, within the 2 or 3 hundredths of AU approaching Earth's orbit.

Over 700 photographs of the inner zodiacal light were taken from the Clementine spacecraft while it was in orbit around the Moon. These exposures were taken with the 28° x 43° field-of-view Star Tracker camera and the 4.2° x 5.6° field-of-view UV/Vis camera. The images were made while the Clementine spacecraft was on the dark side of the Moon such that the Sun was occulted. Most of the photos were taken at the highest possible sensitivity and longest exposure time (0.7 sec) in order to detect an expected weak lunar horizon glow. Consequently, many of the photos are over exposed where the zodiacal light is the brightest. However, a subset of photos were purposefully taken with a range of exposure times to reveal the entire inner zodiacal light structure, both in latitude and longitude, to within 1° of the Sun. These Star Tracker images show the lenticular shape of the inner zodiacal light. When work to correct the images to absolute photometry is concluded, the detailed structure of the entire inner zodiacal light will be derived.

The scattering function of Hong's successful model for the brightness variation with elongation, B(ε), of the Zodiacal Light has been used to calculate profiles of Doppler shifted Fraunhofer lines. Substantial line shape changes with ε are predicted with an indication of a bifurcation at ε < 30°, so offering an explanation for reported F corona observations.

Observations of radial velocities in the F-corona in the elongation range from 3 to 6 solar radii were carried out on July 11, 1991 (Mexico, La Paz) by means of a Fabry-Perot spectrograph. The Fraunhofer lines in the spectral region of H and K Call were used. The average velocities to the West and to the East of the Sun within 30° of the ecliptic plane are +30 km/s and -27 km/s respectively. This confirms the main results of the F-corona radial velocity observations by Shcheglov et al. (1987), Shestakova (1987) on July 31, 1981 about predominance of prograde Keplerian motion of the dust near the ecliptic plane and the effective radius of dust grains (≈0.4μm)

We present the detection of a neutral helium line in observations of the solar corona obtained during the 1994 eclipse, and show that a coronal excitation process is unlikely to cause such a significant Hel emission that extends out to a distance of 3 solar radii (R⊙) from the center of the sun. Besides the scattered light of a solar prominence, as well as components of geocoronal helium, the observed emission line may come from the neutral helium components in the interplanetary medium. Recombination of ions on interplanetary dust particles may play a role for the formation of the neutral helium.

Motivated by new infrared observations, we examine the contribution of various size intervals of interplanetary dust particles to visible and infrared scattered radiances of the solar F-corona, employing Mie theory and particle size distributions based upon differing interpretations of lunar microcrater evidence and interplanetary flux measurements.

We have found from line-of-sight integration of scattered light and thermal emission that the appearance of a hump in the F-corona, which was observed in the near-infrared wavelengths by Peterson (1969) and MacQueen (1968), is very sensitive to the change of size distribution of circumsolar dust grains. Namely, the size distribution affects the balance of scattered sunlight and thermal emission. We suggest that a temporal variation in the size distribution of circumsolar dust grains is a potential origin of the temporal variation of the hump structure observed in the F-corona.

We discuss efforts to explain the dust distribution, as it was derived from the F-coronal brightness, mainly how to explain a constant number density near the Sun. It is shown that silicate particles have temperatures below the blackbody temperature if the impurity from absorbing material amounts to less than 0.1% in volume. This effect is especially significant for porous particles and may explain why particles can approach to less than 3 R⊙ from the Sun. Another effect may arise from particles on high eccentricity orbits and from the influence of collisional fragments. Whereas on the one hand heat resistant silicate particles may cause a stable dust cloud in the solar vicinity, on the other hand dynamical effects may cause a variation of the number density in the corona in short time scales.

The orbital motion of interplanetary dust grains in the sublimation zone near the Sun has been considered for graphite and silicate. Calculations showed that dust grains with initial radii s = 0.5 - 5 μm can form regions of enhanced concentration. The inner corona is slightly enriched with particles s = 0.3 - 0.6 μm due to the departure of the evaporated grains onto highly elliptic orbits. However, they may be not recognized due to their small contribution to the total brightness along the line-of-sight compared with the background of the more typical Zodiacal particles. The astrosilicate dust grains do not form zones of enhanced concentration. Finally, particles with initial radii from 0.3 to 4 μm leave the Solar system and become β-meteoroids.

Comet dust consists primarily of silicate and carbonaceous material. The carbonaceous material is not yet well-characterized. The 3.36 μm emission feature arises primarily from gas phase molecules, while a small feature at 3.29 μm may be due to aromatic hydrocarbons in the grains. Olivine, a high temperature condensate, is present in some comets. Polarization and albedo maps have shown that the coma is not uniform; an inner halo of large particles may be present.

This paper summarizes some recent progress in our understanding of the morphological diversity of dust comets. This diversity is a product of dust emission from discrete active areas on the nucleus surface and provides information on the comet's rotation state and source function. Advances in computer simulations of large-scale dust coma morphology are described and the diagnostic properties of various dusty features are emphasized. Also addressed are some of the issues of dust tail morphology and particle fragmentation. Finally, constraints on the mass, size, and velocity field of the dust population of comet Shoemaker-Levy 9 are set from the object's morphological observations in 1993-94.

Fragmentation dominates the ablation process of cometary type impactors during their atmospheric flight. Models that account for it imply that a kilometer-sized nuclear fragment of comet Shoemaker-Levy 9 has disintegrated almost completely by the time of its terminal explosion, at a pressure level of <1 bar. This result, consistent with impact scenarios based on precursor-event timings, suggests that the initial masses of two fragments, K and R, were between 7×1014 and 5×1015grams apiece.

A model to describe the intensity of dust scattered light observed by the spectrometer TKS during the Vega/Halley encounter is presented. Good agreement is obtained when using a particle radius dependent mass density. The color effect is reproduced if the refractive index of tholin is adopted.

The evolution of the polarization of light scattered by dust particles (e.g. cometary dust) with the phase angle is an indicator of the physical properties of the particles. For grains large compared to the wavelength, the maximum value of polarization Pmax increases with increasing size. A dichotomy is pointed out in the cometary poiarimetric phase curves, showing two ranges of values for Pmax. A comparison between laboratory measurements performed under Earth and microgravity conditions is presented. Pmax and the corresponding αmax appear to be smaller in microgravity than for ground based measurements. The comparison between ground based, microgravity and cometary results is then tentatively discussed in term of physical properties of the grains.