Formation of Vitreous Salt Hydrates Under Conditions Relevant to Europa

Technologies are being developed to enable a robotic spacecraft to directly access Europa's ocean by penetrating its kilometers-thick ice crust. Until such time comes, however, our ability to characterize it will be limited to observations of its expression on the surface. Further, the ability to place meaningful constraints on the composition of the source brines will depend on our understanding of the chemistry associated with their freezing and emplacement. Among the body of research focused on improving this understanding, recent laboratory studies have shown that Mg-bearing salts, previously proposed as constituents of Europa's surface, have a propensity to form vitreous hydrates when flash frozen from their parent brines. The possibility that such vitreous species could form on Europa is important as their presence will be a confounding factor in their detection and identification of the underlying salt hydrates. Further, since glass formation can protect biological molecules and even microorganisms from damage caused by crystallization, they may prove to be important targets of astrobiological interest. This work explores the conditions under which vitreous salt hydrates form in order to better assess their potential for forming on Europa. Specifically, solutions of Mg- and Na-bearing salts were frozen over a range of freezing rates and interrogated with Raman spectroscopy to determine whether vitreous or crystalline species (including hydration state) formed. These results show that vitreous salt hydrates can form at freezing rates as low as similar to 10 K minute(-1), thus expanding their potential for occurrence on Europa beyond extreme flash-freezing scenarios such as plume deposition.