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Richard G. French (Wellesley C.), Daniel Graham (Middlebury C.), Jeff Cuzzi, Luke Dones, Jack J. Lissauer (NASA/Ames)
We have embarked on a long-term survey of Saturn's rings using the Hubble Space Telescope to determine the variation of ring brightness (I/F) over the full range of phase angles (\alpha<6\circ) and ring opening angles (B<25\circ) accessible from the Earth. To date, we have obtained nearly 100 high-resolution (~q300 km/pix) Planetary Camera images with WFPC2, primarily using standard UBVRI filters. These observations span nearly three oppositions, during which B has increased from 4\circ during the October 1996 opposition to nearly 16\circ in 1998. There is a strong opposition surge in ring brightness near zero phase, and the photometric accuracy and high spatial resolution of the WFPC2 images make it possible to measure the opposition effect for quite narrow individually selected radial ranges in the A, B, and C rings.
The strength and shape of the opposition surge depend in a complex way on both individual and collective properties of the ring particles. For example, particle surface roughness and single-scattering albedo help to determine the importance of wave-optical coherent backscatter and of regolith shadowing, whereas the optical depth and spatial distribution of particles into a monolayer or many layers determine the character of mutual particle shadowing and multiple scattering. The opposition effect in the low optical depth C ring, seen for the first time in these observations, is particularly useful in helping to disentangle these sometimes correlated effects.
We will compare the observations to Irvine's [{\it J.~Geophys.~Res.} {\bf 71,} 2931, 1966] shadowing model for multilayered media, Hapke's [{\it Theory of Reflectance and Emittance Spectroscopy}, Cambridge Pr., 1993] bi-directional reflectance shadowing model, Mishchenko's [{\it Astrophys.~J.} {\bf 411,} 351, 1993] model for coherent backscatter, and Helfenstein {\it et al.}'s [{\it Icarus} {\bf 128,} 2, 1997] composite model including shadowing and coherent backscatter effects, modified for layered ring geometry.