Small-Scale Structures in Saturn's C Ring and Cassini Division

The small-scale structure of Saturn's optically thick A and B rings is dominated by self-gravity wakes (e.g. Colwell et al. 2009, Saturn from Cassini-Huygens, Springer, 375; Jerousek et al. 2016, Icarus, 279, 36). These ephemeral structures result from the competition between the mutual gravitational attraction of ring particles and tidal shear acting to pull them apart (Julian and Toomre 1966, Astrophys. J., 146, 810). Self-gravity wakes are elongated and canted to the orbital direction by about 20-25 degrees. This preferential orientation and the spacing between the self-gravity wakes results in a geometry-dependent transparency of the rings that is observed in stellar occultations. The surface mass density of the Cassini Division and the C ring is lower than that of the A and B rings so that the most unstable length scale for gravitational collapse is comparable in size to individual ring particles. Thus, self-gravity wakes are not anticipated. However, one signature of self-gravity wakes is an increase in apparent normal optical depth with increasing occultation elevation angle, B. This is observed for all values of B in the A and B rings but not in the C ring and Cassini Division. Here we report on stellar occultation measurements of the C ring ramp and plateaus and the Cassini Division ramp and triple band structure made by the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP), the Cassini Radio Science Subsystem (RSS), and the Cassini Visual and Infrared Mapping Spectrometer (VIMS) over a broad range of viewing geometries. The normal optical depth of the C ring and Cassini Division does increase at small values of B. These variations can be caused by the finite vertical thickness of the rings and the distribution of under-dense or empty regions between particles and clumps. We present our results on ring vertical thickness and particle clump characteristics in the C ring and Cassini Division from Cassini occultation data.