Amino Acid-Mediated Formation of CO2 in Flash-Frozen Ceres Brines

The recent discovery of organic matter exposed in patches on the surface of Ceres has sparked considerable interest in its prospect as an emerging astrobiological target. The surface of the dwarf planet is also characterized by a pervasive display of salt minerals, which are suggestive of briny fluids at depth and ongoing geological activity that resulted in their emplacement. While these salt-rich subsurface liquid reservoirs could present enticing conditions for prebiotic chemistry to emerge, the fundamental relationship between dissolved organic materials and associated salt minerals upon exposure to Ceres’ surface environment has not yet been explored. This work investigates the chemistry of putative organic-bearing brines relevant to Ceres (containing sodium, ammonium, carbonate, and chloride) upon freezing using infrared (IR) reflectance and Raman spectroscopies, specifically focusing on two representative amino acids (Gly, Asp) and an N-containing aliphatic compound (hexylamine). The results indicate that the presence of these organic species in the brines leads to the formation of isolated CO2 molecules in water clusters upon flash freezing to liquid nitrogen temperatures. In particular, the two amino acids produced significantly more CO2 compared to the hexylamine-brine mixture. Slow-freezing conditions, on the other hand, do not exhibit such a behavior. Isotopic substitution experiments subsequently confirm that the bicarbonate anions in solution, rather than the amino acids, are the source of CO2. Raman measurements of these samples hint at a possible reaction pathway involving the formation of a carbamate intermediate. These results exemplify the potentially important role of organic-salt interactions in Ceres’ evolution, as well as providing novel insights into the fate of organic matter and future detection strategies of organic deposits on ocean worlds.