A focus on Chemical Equilibria of Saline Aqueous Solutions Freezing under Ceres’ Surface

On Ceres’ surface, the region including Kupalo and Juling was selected, in this study, for the geological aspects characterizing the area. Spectral data revealed that, at surface, the two craters show the presence of different carbonates: Na-carbonates (Kupalo) and Ca-Mg-carbonates (Juling). We think that the distinct geology at surface may be the result of the freezing of different subsurface aqueous reservoirs, characterized by different compositions and initial P-T conditions. The first step was to characterize the initial composition of aqueous solutions which, after freezing/precipitation processes, by using FREZCHEM code, delivered salts which can better approximate the surface compositions found by spectral analysis. Our potential brines were alkaline aqueous solutions, and they differ for the Na- and Cl- amounts (higher at Kupalo, rather than Juling solutions). The formation of hydrated sodium carbonate (natron, Na2CO3·10H2O) is highly favored in Na-enriched solution (e.g., Kupalo); instead, it could form only after water ice precipitation in Na-depleted solutions (e.g., in Juling). Natrite (Na2CO3) was found in Kupalo’s surface layers, and, in our simulations, it did not directly precipitate from the solutions. We suggest that it could have been formed from natron and nahcolite (Na2CO3·10H2O and NaHCO3 respectively) at 1 bar of total pressure. Furthermore, we discussed the stability of the carbonates formed including MgCO3 (magnesite) and CaMg(CO3)2 (dolomite). Finally, we investigated on the transport dynamics of brines suggesting that, already in the subsurface, there could be the physical processes to bring solid material (precipitated from the solutions) to the surface.