Saturn's rings: Optical reflectance polarimetry

The distribution of optical linear polarization over Saturn's rings was measured telescopically. Ring B, after correction for the polarization produced by illumination by Saturn's globe, was found to show two effects: (1) A polarization resulting from direct solar illumination, for which the phase angle and wavelength dependences are indicative of at least millimeter-sized high-albedo blocks with rough surfaces, and bright submicrometer-sized particles, probably icy grains, attached to their surfaces. (2) Another component of polarization, everywhere parallel to the direction of the globe center, or systematically slightly tilted, is probably caused by anisotropic multiple scattering of light between aligned grains, possibly due to the wakes of dynamical ephemeral bodies that may exist in the rings. Ring A was found to show similar polarization due to direct solar illumination, suggesting similar aggregated blocks, although rather enriched with submicrometer coating particles. Azimuthal brightness variations suggest the presence of particles aligned as for ring B, but with no comparable component of polarization parallel to the direction of the globe center. Instead, a complex and patchy field of locally produced linear polarization was observed, variable in degree and in azimuth from place to place over ring A; this may reflect the vigorous and multistage process of accretion, wake formation, and disaggregation, as dynamically modeled by H. Salo (1992, Nature 359, 619-621), and which is specific to ring A. (C) 1996 Academic Press, Inc.