CHEMISTRY OF FROZEN SODIUM–MAGNESIUM–SULFATE–CHLORIDE BRINES: IMPLICATIONS FOR SURFACE EXPRESSION OF EUROPA’S OCEAN COMPOSITION

Abstract

The composition of Europa’s subsurface ocean is a critical determinant of its habitability. However, our current understanding of the ocean composition is limited to its expression on the surface. This work investigates experimentally the composition of mixed sodium–magnesium–sulfate–chloride solutions when frozen to 100 K, simulating conditions that likely occur as ocean fluids are emplaced onto Europa’s surface. Micro-Raman spectroscopy is used to characterize phase composition of the frozen brines at 100 K. Our results show that solutions containing Na + , Cl − , Mg 2+ , and preferentially crystallize into Na 2 SO 4 and MgCl 2 hydrated minerals upon freezing, even at elevated [Mg 2+ ]/[Na + ] ratios. The detection of epsomite (MgSO 4 •7H 2 O) on Europa’s surface, if confirmed, may thus imply a relatively sodium-poor ocean composition or a radiolytic process that converts MgCl 2 to MgSO 4 as suggested by Brown & Hand. The formation of NaCl on the surface, while dependent upon a number of factors such as freezing rate, may indicate an ocean significantly more concentrated in sodium than in magnesium.