Weiss, J.W., Porco, C.C., Richardson, D.C., Dones, L., and Spitale, J.N. (2008). "Saturn's A-Ring Azimuthal Asymmetry Observed at High Solar Phase: Implications for Particle Properties" American Astronomical Society, DPS meeting #40, #29.01.


Abstract
Ring particle physical properties (such as internal mass density and particle size distribution), photometric properties (such as albedo and phase function) and elastic properties (such as coefficient of restitution) can be investigated by coupling an N-body dynamical code that simulates a patch of a ring system like Saturn's with a geometric ray-tracing code to model ring photometry. The azimuthal brightness asymmetry in Saturn's A ring, numerically demonstrated long ago to be attributable to the presence of self-gravity wakes (e.g., Dones and Porco, 1989), has been modeled in this way for low-solar-phase and lit-face illumination/viewing geometries. Using recent (including Cassini-era) constraints on many of the particle properties, we have made use of both Voyager and Cassini low-solar-phase images to find (Porco et al. 2008, Astronomical Journal, Submitted) that the particles in Saturn's mid-A ring, where the azimuthal asymmetry reaches its maximum variation, have a lower coefficient of restitution and therefore suffer greater energy loss than previously believed.

We are expanding our study of this phenomenon to high-solar-phase geometries, observed by Cassini, where sunlight scattered from the planet (a.k.a. planetshine) contributes a significant fraction to the total ring brightness. Modeling these observations with our ray-tracing code requires an accurate planetshine model. We will use an improved photometric model for the globe of Saturn, derived from Cassini images, to examine the azimuthal asymmetry in the mid-A ring as observed in high-solar-phase images. This presentation will compare the new results thus derived for the particles' photometric and elastic properties to our previous low-phase results.

We acknowledge the financial support of the Cassini Project.