IMPACT-DRIVEN HYDROTHERMAL VOLATILE REDISTRIBUTION AT OCCATOR CRATER ON CERES AS A COMPARATIVE PLANETARY PROCESS FROM DAWN STEREO MAPPING (Invited Presentation)

Spectacular Dawn XM2 imaging of 92-km diameter Occator crater on Ceres at 3.5 to 8 m pixel scales provides an opportunity to test hypotheses based on earlier LAMO mapping at ~35 m. Among these are origins of the lobate floor deposits (e.g., post-impact volcanism or impact melt), and of bright carbonate-rich deposits (e.g., ballistic emplacement or lateral flow) . Extensive stereo imaging has proved key. Lobate floor deposits at Occator resemble impact melt deposits on the Moon on a macro-scale (large blocks, draping over terraces, flows moving uphill, etc.) but have features unique to Ceres, mostly related to the “wet” ice-rich composition of the crust. Surface features on floor deposits are consistent with the physical and geochemical evolution of a cooling and solidifying impact “melt/mud” sheet composed of hydrated silicate, carbonates and salt blended with water. Mantling by solidified water- and salt-rich mud-like impact melts with scattered endogenic pits, troughs, and bright mounds is indicative widespread impact melt coatings and of outgassing of volatiles and periglacial-style activity during solidification. Scattered irregular pits and low bright mounds are the best candidates for groundwater exsolution and ‘pingo-like’ processes but are not dominant. Surprisingly the morphology and distribution of such features is very different from water-rich Mars, where volatile related features take the form of large densely packed floor pits (as well as rock outcrops of altered minerology). The hydrothermal carbonates on the eastern floor (Vinalia Facula) have complex topographic relationships, with contiguous areas ‘lining’ inter-ridge depressions on the solidified impact melt deposits but other outcrops draped over ridges. These suggest that bright carbonate deposits on the floor are a few 10’s of m thick at most and formed from locally limited brine effusion at multitudes of individual vents, which coalesced in several larger centers where effusion was greater. Unlike terrestrial complex craters, we observe no large-scale hydrothermal activity near the rim scarp, indicating that such activity was concentrated near crater center on Ceres.