Spatial Distribution of Glycine and Aspartic Acid in Frozen Enceladus Brines Relevant to Enceladus

Saturn’s moon Enceladus harbors a global, subsurface liquid ocean beneath an icy crust that actively erupts water jets from cracks in its south pole. Data returned from the Cassini mission have identified the presence of salts and organic matter within these ejected plume particles. Such combinations of water, organics, and salts present rather complex chemical environments that may hold direct implications for habitability. In particular, the fundamental behavior of organics in frozen brine systems upon exposure to relevant Enceladus surface conditions is an important aspect that has not been explored to date (e.g. how they organize and partition relative to the salt minerals within the ice matrix). The present work investigates this topic by characterizing the spatial distribution of two representative amino acids (glycine and aspartic acid) in a putative frozen Enceladus brine containing sodium, chloride, and carbonate ions via micro-Raman imaging. The results show that both amino acid-bearing solutions undergo some degree of vitrification upon flash freezing, especially for the chloride salt and glycine. Subsequent annealing of the vitreous samples reveals preferential association of the amino acids with crystalline salt hydrates rather than with water ice. The highest correlation is found between aspartic acid and natron (Na2CO3•10H2O), while glycine and water ice exhibit the lowest. This suggests that solute-solute interaction likely dominates in these frozen systems, especially if charged species are present. The results imply that salt-rich ice particles can serve as concentrators of organic biomarkers, enhancing their potential for detection from remote sensing or in situ measurements.