Search for sulfates on the surface of Ceres

1C. Bu,1G. Rodriguez Lopez,1C. A. Dukes,2O. Ruesch,2L. A. McFadden,3J.-Y. Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13024]
1Laboratory for Astrophysics and Surface Physics, University of Virginia, Charlottesville, Virginia, USA
2NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

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 MgSO4·6H2O by X-ray powder diffraction. Long-term (>40 h), continuous exposure of MgSO4·nH2O (n = 0, 6, 7) to low pressure (10−3–10−6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO4·6H2O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO4·6H2O 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 (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.

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