Laboratory exploration of mineral precipitates from Europa’s subsurface ocean

Abstract

The precipitation of hydrated phases from a chondritic-like Na-Mg-Ca-SO4-Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid (360
K hour-1, T = 250-80 K, t = 3 hours) and ultra-slow (0.3 K day-1, T= 273-245K, t = 242 days) freezing conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models, however after ~ 50 days at 245 K, the formation of the highly hydrated sulphate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2-H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (=4 minutes, ?T = 2 K) and, although remaining present throughout, varied in their relative proportions with
decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, bl¨odite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. Results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.