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J. N. Cuzzi (Ames Research Center), R. G. French (Wellesley College), L. Dones (S. W. R. I.), P. R. Estrada (Cornell), M. R. Showalter (Stanford)
Saturn's rings were observed with the HST-WFPC2 Planetary Camera between 1996 and 1999 as the ring opening angle increased from 4 to 20 degrees, for phase angles between 0.3 and 6 degrees at each opening angle. Images were obtained in the UBVRI filters (F336W, F439W, F555W, F675W, and F814W) and occasionally in the F255W, F785LP, and F1042M filters. (a) In the 300--600 nm spectral range where the rings are red, the A and B ring redness increases by about 25% as the phase angle increases from 0.3 to 6 degrees. This effect is larger than the phase reddening which characterizes other icy objects. However, there is no tendency for color to vary with ring opening angle at a given phase angle, and there is no phase variation of color where the spectrum is flat. We infer that multiple intraparticle scattering, either in a regolith or between facets of an unusually rough surface, is important in these geometries, but that multiple interparticle scattering in a vertically extended ring layer is not. Ring particle reflectivity is being modeled by Poulet et al. (this meeting). (b) Ring-average spectra are compatible with earlier groundbased work; however, ring spectra vary noticeably with region, and radial profiles of color ratio show considerable structure. Color-color plots (ie. G/V vs. V/UV) using both HST and Voyager data indicate a mixing-type trend line connecting the dense, reddish B ring with the innermost, optically thinnest C ring. This behavior is consistent with compositional evolution under meteoroid bombardment (Cuzzi and Estrada 1998, Icarus 132, 1-35). On smaller scales, color-color and color-optical depth correlations contain very intriguing and puzzling features. Each main ring region has its own unique characteristic dependence of color on radius and optical depth, with radial dependence being much stronger than optical depth dependence.