ENCELADUS PLUME GRAINS: UV EXPOSURE AGES FOR SURFACE ORGANICS

The saturnian moon Enceladus presents a remarkable opportunity in our solar system for searching for evidence of life, given its habitable ocean and active plume that deposits organic-bearing ocean material onto the icy surface. Organic ocean material could be sampled by a lander mission on the surface of Enceladus. It is of interest to understand the amount of relatively pristine, unaltered organics present on the surface, given the UV and plasma environment. Here we provide estimates of exposure ages of plume deposits - and the organic material therein - using the UV spectral signature, as observed using the Cassini Ultraviolet Imaging Spectrograph and the Hubble Space Telescope, of Enceladus to understand the penetration depth of solar UV photons into the icy regolith. We investigate the resultant effective exposure ages for various regions on Enceladus, considering the rate of resurfacing by plume fallout. In high plume fallout regions near the south pole, we find that plume grains are buried by fresher grains within years, resulting in low levels of exposure of surface grains to solar UV, which penetrates only ~100 microns. Regions at latitudes south of ~40°S can have exposure ages <100 years, translating to relatively high abundances of pristine organic material preserved in the icy regolith.