DPS 35th Meeting, 1-6 September 2003
Session 11. Planetary Rings I
Oral, Chairs: L. Spilker and C. B. Olkin, Wednesday, September 3, 2003, 1:30-3:00pm, DeAnza III

## [11.05] Saturn's Particle Disk and the A Ring Azimuthal Asymmetry

C.C. Porco, H.B. Throop (SwRI), D.C. Richardson (U. Maryland)

We present new results in an ongoing program to study the azimuthal brightness asymmetry observed in Saturn's A ring for the purpose of understanding the dynamic and collisional conditions characterizing this region of the rings and the physical properties of the particles inhabiting it. Our approach to this investigation entails shooting light rays (using a custom-built ray-tracing code) into computer-generated patches of particles of realistic physical and photometric properties, and collecting the rays which emerge and are seen by observers from a variety of geometries. These patches consist of either randomly distributed particles or of particles whose locations have been produced by a ring patch' code which realistically simulates hundreds of thousands of colliding self-gravitating particles in orbit around a planet. The results of these light-scattering experiments' are then compared, either with theory or data. Multiple scattering, mutual shadowing, obscuration, realistic albedos and phase functions, and illumination by both the Sun and the lit hemisphere of Saturn are considered. All effects have been now demonstrated to work properly.

In our previous experiments (Porco et al. 2001, B.A.A.S. 33 (3) 1091), we had successfully modelled the shape and longitudinal position of the asymmetry as seen at low phase on the lit side of Saturn's A ring, but the modelled amplitude was too small by a factor of ~3. We have explored this problem by varying, in turn, the internal density of the particles and both the tangential and normal coefficients of restitution, and find the latter to be a more promising direction to achieve the observed amplitude. We have also begun examination of the asymmetry as seen at high phase, on both lit and unlit sides of the rings. We will report on these results and their significance.

\noindent The work was supported, in part, by NASA's PGG program through #NAG5-11641.

Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.