37th DPS Meeting, 4-9 September 2005
Session 11 Comets
Oral, Monday, September 5, 2005, 4:20-6:00pm, Law LG19

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[11.05] What Polarimetry and Thermal Infrared Spectra Tell Us About Two Types of Dust in Comets

L. Kolokolova (University of Maryland, USA), H. Kimura (Institute of Low Temperature Science, Hokkaido University, Japan), N. Kiselev (Institute of Astronomy, Kharkiv National University, Ukraine)

Observations of cometary dust indicate two types of comets whose characteristics are summarized below:
Low dust/gas ratio
High dust/gas ratio
Low polarization
High polarization
Weak or absent infrared silicate feature
Strong infrared silicate feature
Periodic comets with perihelia <0.8AU
New comets or periodic ones with perihelia >0.8AU

Recently Jockers et al.(A&A, in press), Kiselev et al. (2004, Icarus 168) and Jewitt (2004, AJ 128) have shown that in Class 1 comets the polarization increases as approaching the nucleus, reaching the values typical for Class 2 comets. This supports the idea that the dust in both classes of comets has intrinsically similar polarization, and the overall low polarization of Class 1 comets is due to depolarization by gas contamination at low dust concentrations far from the nucleus. We study what characteristics of the dust can affect its spatial distribution and provide the difference in the infrared spectra of cometary dust. We simulate the infrared spectra of comet particles considering them as Ballistic Particle-Cluster (BPC) and Ballistic Cluster-Cluster (BCC) Aggregates that consist of submicron spherical grains. The results for more compact BPCA show less pronounced silicate feature than for more porous BCCA; for large BPCA (>100 micron) the silicate feature completely vanishes. We conclude that the dust in Class 2 comets consists of loosely packed aggregates, whereas the dust in Class 1 comets is characterized by large compact particles that tend to concentrate near the nucleus. This may indicate predominance of particles from the surface mantle in Class 1 comets, whereas in Class 2 comets we see fresh or slightly-processed dust that is in accordance with the orbital characteristics of the comets. We expect this to be confirmed by the Deep Impact observations.

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