ANATOMY OF A LARGE IMPACT CRATER ON A DWARF PLANET: XM2 INITIAL HIGH-RESOLUTION MAPPING OF OCCATOR CRATER FROM DAWN (Invited Presentation)

Spectacular Dawn XM2 imaging of 92-km diameter Occator crater on Ceres at <10 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), of bright carbonate-rich deposits (e.g., ballistic emplacement or lateral flow) and of the central dome (e.g., laccolithic “pingo-like” inflation or extrusion). Extensive stereo imaging has proved key. Steep overhangs on rim outcrops suggest a resistant surface layer. 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 likely related to the “wet” composition of the crust. Resistant-layer capped scarps, pit clusters, cryptic shallow ring structures, irregular mounds, and sinuous rilles are common across plains that can be flat, though pitted, or ridged. Our initial survey of surface features on floor deposits indicates they 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. Exposures in fresh post-impact fractures suggest bright carbonate deposits on the floor are a few 10’s of m thick at most. Carbonate deposits exhibit conflicting indication of directional deposition and overflow of ridges. Some bright spots include small domes or depressions, possible indications of late-stage activity within this complex crater. Prominent V-shaped fractures cross the central dome but no dark material is exposed in the interior, suggesting the bright deposit is rather thick there. Dome fracturing appears to have occurred some time ago, yet small pits on dome crest indicate some venting occurred more recently. No flow front at dome edge is yet recognized, suggesting that laccolithic or “pingo” inflation may be more likely.