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

Abstract

Saturn’s moon Enceladus harbors a global, subsurface liquid ocean beneath an icy crust that actively erupts water jets from fissures in its south pole. Data returned from the Cassini mission have identified 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. 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 amino acids with different side chains (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 organic–salt solutions likely undergo some degree of vitrification upon flash freezing, especially for the chloride-bearing species. The subsequent annealing of the vitreous samples reveals a preferential association of the amino acids with crystalline salt hydrates, while minimal negative to no correlation is found between them and water ice. In particular, both amino acids exhibit stronger affinities for natron (Na _2 CO _3 •10H _2 O) than hydrohalite (NaCl•2H _2 O). This suggests that solute–solute interaction likely dominates in these frozen systems, especially when 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 analysis.

Keywords

• Enceladus
• Ice spectroscopy
• Experimental techniques
• Astrochemistry
• Planetary science