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Composition of Ceres’ Bright Spots

Wednesday, September 9, 2015

Abstract:  The dwarf planet Ceres, located at a mean solar distance of ~2.8 Astronomical Units, is the largest (diameter ~950 km) object in the main asteroid belt between Mars and Jupiter. Several evolution models suggest a differentiated body with potential geologic activity. One of the objectives of the Dawn mission during the Ceres encounter is to search for signs of past or present geological activity, including processes that might be linked to observations of transient water vapour events. One of the most striking features of Ceres’ surface are localized bright areas, which are commonly associated with impact craters. Of particular interest is a bright pit on the floor of a 90.5 km diameter crater named Occator that shows signs of activity in the form of water ice sublimation. I will present evidence that the Ceres bright spots are hydrated salt deposits, using a combination of Dawn Framing Camera (FC) multispectral observations, laboratory spectroscopy, and geochemical data from carbonaceous chondrite leaching experiments. Based on previous spectroscopic mineral identifications, a range of candidate high albedo materials were investigated including ice, Mg-carbonates, brucite, saponite and ammonium saponite, (Mg,Na) sulphate salts, and (Mg,Na) halide salts. Of these, the best matches are to mixtures of hydrated Mg sulfates along with dark ‘average Ceres material’, which may be broadly analogous to aqueously altered carbonaceous chondrite. The bright spots may be forming as a result of sublimation of water from brines exposed near the surface, leaving behind a chemical lag deposit of former solutes, which are predicted on experimental and theoretical grounds to be dominated by MgSO4 hydrates.

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