Light Scattering in Saturn's Rings: Basic Disk Properties and the A Ring Azimuthal Asymmetry

We present the results of an ongoing program to investigate the nature of Saturn's particle disk by comparing its photometric behavior to numerical ray-tracing simulations of light scattering into computer generated patches of ring particles of prescibed photometric and particle properties. A main goal is to explain the physical ring conditions underlying the azimuthal brightness asymmetry in Saturn's rings, in particular the A ring. To investigate this phenomenon, we begin by using an N-body parallel tree code, to simulate a 3-D patch of colliding, self-gravitating ring particles in Saturn's gravity field. Typical runs include N ~ 10⁵ particles, in the size range r = 0.15m - 2m. For realistic particle properties (ie, mass density, packing fraction, coefficient of restitution, etc.), transient spiral density wakes are observed in these simulations (Salo, 1992; Richardson et al. 1994). A ray-tracing code is then used to shoot light rays into these particle patches; multiple scattering, mutual shadowing, obscuration, realistic albedo and phase functions, and illumination by both the Sun and the lit hemisphere of Saturn are considered. In these experiments, we have thus far successfully modelled the asymmetry's shape and position in Saturn's A ring; the amplitude is not yet reproduced with the present set of particle properties. Work is continuing in this area. Our light scattering experiments have also produced two additional, significant results. First, we find that -- regardless of the existence of wakes -- physically thinner rings are brighter/darker than thick ones at low/high phase. Therefore, vertical thickness must now be considered, along with particle albedo and particle size distribution, as a viable parameter in explaining photometric observations of Saturn's rings. Second, we have numerically modelled the opposition effect seen in the rings. Interparticle shadowing appears to account for approximately half of the opposition surge observed in Saturn's rings in our models. Other contributors to this well known effect are likely to be present.