Differential Partitioning Behavior of Chondritic Organics in Enceladus Analog Ices

Aqueous mixtures of inorganic and organic solutes display complex chemical behavior upon freezing. These systems are analogous to the ocean waters of icy moons in the outer solar system, and play an important role in impacting the habitability and astrobiological potential of microenvironments contained in the ice shells. In this study, putative Enceladus ice cores are synthesized in the presence of a range of organic compounds typically found in carbonaceous chondrite meteorites (glycerol, glyceric acid, 2,3-dihydroxybenzoic acid) and their spatial distribution investigated using Raman imaging. The results demonstrate an intricate interplay among the water ice, organic, and salt phases in dictating the partitioning of these components within the ice. Specifically, glycerol and glyceric acid are found to preferentially associate with hydrohalite, the primary salt hydrate that forms upon freezing (carbonates are also observed in these samples). However, the 2,3-dihydroxybenzoic acid (DHBA) system is found to exhibit a drastically different outcome, where the DHBA molecules agglomerate into organic-rich pockets instead of being co-located with the salt hydrates. Based on the molecular interactions previously demonstrated in the crystal structure of DHBA, we assign this behavior to the hydrophobic effects from its aromatic ring and the presence of intramolecular hydrogen bonding, coupled with the slow-freezing condition that typically expels impurities from the ice and allows the DHBA molecules to come together. The findings provide insights into the chemical composition of brine channels, which hold important implications for the search of organics in ocean world ice shells. Particularly, while salt-rich zones may present enticing targets to look for simple organics, larger and complex species with hydrophobic aromatic rings may be occluded elsewhere in the ice, potentially requiring a more dedicated sampling/detection strategy.