Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres

High-resolution near-infrared observations of the Occator bright areas on the dwarf planet Ceres suggest that the bright material is mostly made up of endogenous sodium carbonate. Ceres carbonates catch the eye NASA's Dawn orbiter probe has revealed localized bright areas on the surface of the dwarf asteroid-belt planet Ceres, most prominently in the Occator crater. These features were tentatively interpreted as containing a large amount of hydrated magnesium sulfates. Now Maria Cristina De Sanctis et al . present high-resolution near-infrared spectra of the Occator bright areas that suggest that the bright material consists mostly of endogenous sodium carbonate, mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. The authors propose that these compounds are residues from the crystallization of brines, following upwelling through nearby fracture systems, together with entrained altered solids that reached the surface from below. Such a model requires a heat source, which may have been transient, triggered by impact heating for instance. Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today. The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater 1 . These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate 1 , in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates 2 , 3 , 4 . Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus 5 . The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (