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K. Lumme (Observatory, University of Helsinki)
Cosmic dust covers the surfaces of atmosphereless bodies, is the reason for zodiacal light, and is an important incredient in the cometary comae. The extensive data sets show systematical differences in linear polarization between these three different environments. The polarization maximum is inversely proportional to the wavelength for the regoliths and interplanetary dust while the opposite is true for the comets. The phase angle of the minimum of the negative polarization branch for the regoliths is systematically smaller than that of the comets.
To understand these observational effects we have been collecting and creating various light scattering codes for this work. Multiwavelength data sets of comets range from the UV-band to the J-band. We can show that if the cometary particles are (as a first approximation) elongated cylinders with the aspect ratio l/d about three and a mean volume equivalent sphere radius about 0.4 \mu m and obey power law size distribution with \gamma about -3 all the data can be fitted with the rms error about 1%. The refractive index varies with the wavelength about its mean value 1.6+i*0.1 explaining the color dependency of polarization. We cannot yet claim that this model is unique.
To explain the reversed polarization maximum vs. wavelength and the position of the minimum polarization of the planetary regoliths we invoke two mechanisms. Because the regoliths are almost infinite layers in terms of the optical thickness both multiple scattering and coherent backscattering become important. The first of these can nicely explain the wavelength dependency because the single scattering albedo decreases with the wavelength. As recently shown by Mishchenko (2000) coherent backscattering causes a strongly asymmetric form for the negative branch with the minimum close to the exact opposition which could explain the observed effect described above.