Terrestrial constraints on the distribution of brine in Europa’s ice shell

Jupiter’s ice-covered moon Europa is considered a prime target in the search for habitable worlds within our Solar System. Freezing at the ice-ocean interface (accretion) entrains material from the ocean, including salts and potential biosignatures. In this work, we first examine how accreted ice on Earth can be used to estimate the bulk composition of Europa’s ice shell through a review of published ice core data, sampling low temperature gradient environments. We find that ice forming beneath ice shelves can serve as a valid analog for ice accreting beneath the ice shells of Europa and Enceladus and that the mechanism of ice accretion (frazil vs. congelation) will serve as the primary factor governing the amount of salt entrained in the ice shell from the ocean. We then introduce a framework to model the brine, salt, and ice volume fraction for a given composition as a function of bulk salinity and temperature by translating the output of the aqueous geochemistry software to polynomial functions of temperature. Using this framework, we build models for two endmember (NaCl and MgSO₄) and two multi-ion “analog” endmember (chloride-dominated and sulfate-dominated) ice shell compositions. Motivated by the observation that a percolation threshold could ultimately govern the stable bulk salinity of a congelation ice shell, we use these models to study the efficiency of salt entrainment as a function of ocean salinity, composition, and the effective critical porosity, a new parameter introduced in this work. We find that the efficiency of salt entrainment in a congelation ice shell is minimally influenced by composition and ocean salinity and is ultimately governed by the effective critical porosity. Finally, we study the habitability of ice shell brine pockets and find that they are not geochemically prohibitive to life as we know it. Furthermore, we suggest that brine volume fraction, as a proxy for nutrient transport and recycling, may be a critical factor governing the habitability of Europa's ice shell and use it to define different classes of potential habitats within Europa’s ice shell.