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Yu. Shkuratov, A. Occharenko, D. Stankevich (Kharkov State University), R. M. Nelson, W. D. Smythe (Jet Propulsion Laboratory), K. Muinonen (Helsinki Astron. Obs), J. Piironen (Helsinki Astron Obs.), S. Keranen (Helsinki Astron. Obs.), B. W. Hapke (University of Pittsburgh), V. Rosenbush (MAO NASU), P. Helfenstein (Cornell University)
To understand the polarimetric features of the Jovian and Saturnian satellites and rings, using available and anticipated Cassini data, laboratory simulated measurements and theoretical light-scattering modeling are needed. We present here results of polarimetric measurements of prepared samples with different structure and albedo. The studies were carried out by the laboratory polarimeter of Kharkov observatory at the wavelength 650 nm and the phase angles 0.1-3.5 degs. This polarimeter was calibrated with the analogous JPL device. We found the following: 1. Although surfaces formed by smoked deposits of MgO and carbon soot have extremely different albedos, they each feature prominent, similarly-shaped negative polarization branches (NPB) with a minimum near 1.5 degs and depth near 0.8 Meudon.1929. v. 8. No 1). 2. Fresh snow consisting of large enough particles (about 50 x 100 mcm) does not reveal NPB at all. 3. A strong particle-size dependence of NPB parameters for powdered dielectric surfaces was found. We studied particle-size separates of Al2O3 (0.1, 0.3, 0.5, 1, 4, 7, 12, and 31 mcm). At the sizes more than 0.5 mcm the NPB depth does not exceed 0.2 the 0.1 fraction. 4. Surfaces with complex submicron-scale structure show NPB regardless of whether the surfaces are powdered (e.g., SiO2 or Al2O3) or solid with a fine microcrystaline structure (frozen glasses). 5. If porosity of dielectric powdered surface decreases, the NPB becomes deeper. 6. All samples have shown dependence of polarization degree on the latitude and longitude change of illuminating/observing geometry. Polarimetric computer simulations of coherent backscattering for media consisting of small scatterers reveal NPB with similar minima and depths to those seen in the measurements.