Composition and Evolution of Frozen Chloride Brines under the Surface Conditions of Europa

Chloride salts have been proposed to exist on the surface of Europa based on some geochemical predictions of the ocean composition as well as some interpretations of data from the Galileo spacecraft. To help elucidate our understanding of Europa’s surface composition, we have conducted a study of frozen chloride salt brines prepared under simulated Europa surface conditions (vacuum and temperature) using both near-infrared (NIR) and Raman spectroscopies. The latter was used to determine the hydration states of various chloride salts as a function of the temperature, while NIR spectroscopy of identically prepared samples was used to provide reference reflectance spectra of the identified hydrated salts. We further investigated the stability of the formed hydrated salts under vacuum and ultraviolet (UV) irradiation. Our results indicate that, at temperatures ranging from 80 to 233 K, (i) NaCl·2H2O is formed from the freezing of NaCl brines, (ii) freezing of KCl solutions does not form KCl hydrates, (iii) fr...