Light Scattering and Compositional Analysis of Saturn’s Rings Using Cassini UVIS Spectral Observations

Abstract:

Saturn’s ring particles are roughly centimeter to several-meter-sized objects that are covered with a regolith of icy grains. In general the composition of the regolith of an object can be investigated by comparing photometric models to reflectance spectra of the surface. However, since the rings as a whole are not a solid surface and the A and B ring particles clump together into self-gravity wakes, the light scattering behavior of the rings has a complex dependence on the geometry of the observations. In order to deal with this we first determine the ring particle albedo for the A, B, and C rings and the Cassini Division at discrete far ultraviolet wavelengths across the water ice absorption edge at 165 nm. We next use the retrieved albedos to compare to spectral models that use available optical constants in the FUV for water ice and various plausible non-water-ice contaminants such as organic tholins, silicates, and other ices.. We retrieve the ring particle albedo by comparing observations of the lit face of the rings to a single scattering Chandrasekhar model. We then compare the observations to photometric models where we assume that the non-water-ice constituents are either embedded in water ice grains so that the effective optical constants are a linear combination of constituent optical constants or that the regolith consists of discrete grains of water ice and non-ice constituents. We find that the best fit of the compositional models to the ring particle albedo is for water ice abundances greater than 95% and water ice grain sizes of a few microns.