Stability of hydrated carbonates on Ceres

Abstract

Deposits of carbonates have been observed and definitively identified by Dawn's Visible Near-Infrared Mapping Spectrometer (VIR), particularly in the faculae that lie within the central portion of Occator, Oxo, and Haulani craters, implying geologically recent cryo-volcanism or extrusion with sub-surface CO2 and H2O. Carbonate composition varies from primarily sodium at the Cerealia and Vinalia Faculae and at Oxo crater, where carbonate deposits are most abundant, to magnesium and calcium for most other bright regions. The formation of hydrated salts is expected from the aqueous alteration of silicates; however, VIR measurements of the faculae show no water signature, potentially the result of dehydration after exposure to Ceres’ surface conditions. We investigate the stability and decomposition pathway for hydrated sodium-carbonate, natron (Na2CO3.10H2O), grains in the laboratory under Ceres’ cryogenic, low-pressure environment by UV–vis–NIR reflectance spectroscopy and X-ray powder diffraction. H2O-loss begins simultaneously with vacuum-exposure, altering natron's spectral signature by attenuation of the water bands, enhancement of the carbonate features, and concurrent reduction of the NIR blue spectral slope. We find that the water absorption features in natron reduce below VIR's detection limit (< 2%) within a time scale of < 6 days at temperatures ≥ 200 K, eliminating hydrous sodium carbonate from Ceres’ surface mineralogy in the equatorial region and the mid-latitudes without continuous rehydration. A temperature-dependent systematic shift of the 1.9-µm band center to lower wavelengths is observed with vacumm-exposure time at 200 and 240 K. In Ceres’ polar-regions (∼120 K), natron retains water longer, depleting the 1.9-µm water band to < 2% within a few hundred years (∼300 years). No significant changes in the visible relative reflectance or spectral slope result from vacuum-exposure of hydrous or anhydrous sodium carbonate, which does not match the observed red-slope in the Framing Camera (FC) measurements for Occator Crater's faculae. In the UV, extended exposure to vacuum in all sodium carbonates examined here causes significant reddening due to an increase in short-range crystallographic defects and reduction of the conduction band energy; in addition, the development of new UV electronic transition features at ∼ 275 and 235 nm is observed in hydrous and anhydrous sodium-carbonates with vacuum-processing. Similar transitions in carbonates and organics may contribute to the unidentified 280 nm absorption feature on Ceres.