Enceladus and Tethys: Ultraviolet clues to surface composition & surface processing

Cassini's Ultraviolet Imaging Spectrograph (UVIS) is sensitive to the uppermost portion of the regoliths of the icy Saturnian moons, where interactions with E-ring grains and plasma processing are important. Organics are present in at least 30% of E ring grains (Postberg et al., 2008) and are likely transported to the surfaces of the satellites orbiting Saturn within the E ring. Plasma bombardment on the trailing hemispheres of the satellites can further process these organic species. Enceladus' surface exhibits visible color variations (Schenk et al., 2011), evidence of plume fall-out zones and zones where plume fall-out is not as heavy (and where E ring grain bombardment dominates). In this study, we investigate far-UV spectral and photometric differences in the Enceladus plume fallout and non-fallout regions to study compositional and structural differences, and we also study compositional and photometric variations in regions on Tethys' trailing and leading hemispheres to understand spectral effects of organics, E ring bombardment and plasma bombardment.