Porco, C., Weiss, J., Richardson, D.C., Dones, L. (2007). "Saturn's Ring Particles: Lossier than Previously Thought." American Astronomical Society, DPS meeting #39, #26.04.

We apply a sophisticated ray-tracing code to dynamically realistic computer simulations of a patch in Saturn's rings to model the elastic and photometric properties of the ring particles. In particular, we continue our efforts to reproduce the characteristics (i.e., the orbital phase, the shape, and amplitude) of the azimuthal brightness variation in Saturn's A ring that has long been known (eg., Dones and Porco 1989) to be caused by self-gravity wakes in the rings. In earlier dynamical simulations, we assumed a particle velocity-dependent coefficient of restitution, epsilon, appropriate for smooth ice spheres (Supulver et al., 1995), and failed to match the asymmetry amplitude by factors of several. We previously noted as the likely cause of this discrepancy the abundance of small particles between the wakes in the simulations and the consequent low wake/inter-wake contrast in optical depth. We report new results in which we match the overall ring brightness and the asymmetry amplitude and shape, as observed in low-solar-phase Cassini and Voyager images, by assuming a velocity-dependent restitution law that yields an epsilon ~2.5 times lower at the relative velocities of the inter-wake particles than that expected for smooth ice spheres. We simultaneously find a particle albedo and phase function consistent with those deduced from photometric analyses of Cassini images taken on approach to Saturn (Porco et al., 2005). These results suggest that the ring particle collisions in Saturn's A ring are considerably more lossy than previously expected, a result very likely due to particle surface roughness (Borderies et al., 1984).