Bulk composition and thermal evolution constrain the formation of organics in Ceres' subsurface ocean via geochemical modeling

Abstract

Ceres is the largest object in the asteroid belt and the only dwarf planet in the inner solar system. In 2015, carbon, and organic compounds, were found by the Dawn mission in high abundance in the surface of Ceres. Here, we use thermodynamic modeling with the goal of constraining the speciation, stability, and abundance of organic compounds formed via abiotic reactions in the early subsurface ocean of Ceres and its mud-bearing mantle. We vary environmental conditions such as temperature, pH, reduction potential, solution composition, and pressure to analyze the variables that lead to optimal formation of organics. Primary results predict that in-situ organic production is negligible for most cases in the subsurface ocean if Ceres primarily accreted CI carbonaceous chondrites yet may be more significant if Ceres formed from cometary material. Carbonate concentration is 3–6 orders of magnitude higher than organics in the chondritic models, while a cometary composition favors significant alcohol and carboxylic acid derivative production, among other organic species. Results also indicate that temperature and pH are drivers of organic formation by water-rock equilibrium, with temperature having the greatest effect. Further analysis reveals that a mixture of ≲ 80 wt% CI chondrite and ≳ 20 wt% cometary material is favorable to in situ organic production of reduced organics. Observational constraints from the Dawn mission indicate that our model results could be representative of the organic observations on the surface. While our models favor organic production in Ceres' ocean with moderate amounts of cometary material, further studies into alternative mechanisms of production and concentration on the surface of Ceres are needed.