Search for sulfates on the surface of Ceres

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile-rich protoplanets like Ceres. Sulfates, present on water-bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres' UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres' cryogenic, low-pressure environment and the consequent spectral effects, using UV-Vis-IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO(4)6H(2)O by X-ray powder diffraction. Long-term (>40h), continuous exposure of MgSO(4)nH(2)O (n=0, 6, 7) to low pressure (10(-3)-10(-6)Torr) causes material decomposition and strong UV absorption below 0.5m. Our measurements suggest that MgSO(4)6H(2)O grains (45-83m) dehydrate to 2% of the original 1.9m water band area over similar to 0.3Ma at 200K on Ceres and after similar to 42Ma for 147K. These rates, inferred from an Avrami dehydration model, preclude MgSO(4)6H(2)O as a component of Ceres' surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5-17m) suggests sulfates cannot be excluded from Ceres' mineralogy.