SATURNS RINGS - OPTICAL REFLECTANCE POLARIMETRY

The distribution of optical linear polarization over Saturn's rings wa s measured telescopically. Ring B, after correction for the polarizati on produced by illumination by Saturn's globe, was found to show two e ffects: (1) A polarization resulting from direct solar illumination, f or 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 par allel to the direction of the globe center, or systematically slightly tilted, is probably caused by anisotropic multiple scattering of ligh t between aligned grains, possibly due to the wakes of dynamical ephem eral bodies that may exist in the rings. Ring A was found to show simi lar polarization due to direct solar illumination, suggesting similar aggregated blocks, although rather enriched with submicrometer coating particles. Azimuthal brightness variations suggest the presence of pa rticles aligned as for ring B, but with no comparable component of pol arization parallel to the direction of the globe center. Instead, a co mplex and patchy field of locally produced linear polarization was obs erved, 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) 199 6 Academic Press, Inc.