HST Search for Irradiated NaCl on the Surface of Ceres

Dwarf planet Ceres is the largest asteroid-belt body and may have sustained a global ocean for hundreds of millions to perhaps billions of years. Some thermal models suggest that a deep brine-rich layer may still be possible beneath its surface today [1]. 

Occator (approx. 92 km diameter) is a crater on Ceres' surface and is unique for its “bright spots”, known as the Cerealia and Vinalia Faculae. These bright spots are geologically young, high-albedo (7x background Ceres) salt deposits probably formed by some mechanism of brine mobilisation [2,3,4], making them an ideal window into Ceres' interior and its past or present brines. Infrared spectroscopy has shown that the composition of the faculae includes sodium carbonates and ammonium chlorides consistent with an ascent from the deep brine layer [5,6], but not all salts are as readily detectable at infrared wavelengths. In particular, halite (anhydrous NaCl) is featureless in the infrared in its pure form, but develops characteristic absorption features at visible wavelengths (0.46 µm and 0.72 µm) from lattice impurities (called “colour centers”) when exposed to radiation [7,8]. One study found potential spectral evidence for these colour centers in Dawn VIR data at Cerealia Facula, Ahuna Mons, and several impact craters, but the visible channel of the VIR instrument suffered from significant calibration uncertainties, and clear colour-center absorptions remain ambiguous across published reductions [9]. 

We will present new visible-wavelength spectra of the Occator crater region on Ceres obtained with the STIS instrument on the Hubble Space Telescope (HST). We compare our spectra to Dawn observations and past global-scale HST observations and analyse our spectra for halite colour centers, placing constraints on the presence of halite within Occator crater and thus the chemistry of the faculae.

 

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