Global Color Variations on Saturn's Icy Satellites, and New Evidence for Rhea's Ring

Global color maps of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus, constructed using calibrated Cassini IR, green, and UV images, reveal distinct color asymmetries and equatorial features on these satellites. Variations are most strongly pronounced in IR/UV color ratios, which reveal strong sinusoidal enhancements in IR/UV centered on the trailing hemispheres of Tethys, Dione and Rhea. This peak is strongest on Dione and broadest on Rhea, and correlates with the well-known trailing hemispheric albedo darkening and reduced water-ice absorption bands seen by VIMS. In contrast, an (albeit weaker) increase in IR/UV is also centered on the leading hemisphere, but is not correlated with albedo. These patterns are likely related, at least in part, to plasma bombardment. Enceladus has a similar color asymmetry, but it is weaker and offset 45-50° to the west relative to the patterns seen on the other satellites, whereas Mimas has no such albedo or color asymmetry. Wide banding along the equators of the leading hemispheres of Tethys and (newly discovered) on Mimas have low IR/UV ratios and are roughly 250-300 km wide N-S and lens-shaped in map projection. The equatorial orientation suggests a role for ring material, possibly the E-ring. For Rhea, we confirm the unusual equatorial color features of Jones et al. (Fall 2009 AGU, abs. #32A-04). These form a very narrow, discontinuous, great-circle chain of discreet, low-IR/UV deposits over >180° of arc, and tilted 1.8° to the equator. The deposits occur preferentially on the east-facing walls of older impact craters, and we hypothesize the deposits represent near-grazing impacts (at 450 m/s) of small Rhea ring bodies. East-facing impacts indicate a retrograde ring, and the 1.8° tilt indicates a small amount of polar wander since formation, or perhaps, a minor error in Rhea's pole position.