Brine Volume Fraction as a Habitability Metric for Europa's Ice Shell

Abstract

Jupiter's icy moon Europa is a high-priority target for exploration in the search for habitable worlds beyond Earth. Although the global sub-ice ocean represents Europa's most compelling potential habitat, habitable niches could extend from the ocean into the ice shell interior. Brine systems within Europa's ice shell have been hypothesized to represent potential habitats; however studies focused on their distribution and characteristics have been relatively limited (e.g.,

Water dynamics in eutectic solutions of sodium chloride and magnesium sulfate: implications for life in Europa's subsurface ocean and ice shell

“…Recent simulations have shown that the pockets of salty liquid water (brines) existing in the ice shell of Europa can reach high salinity levels, which are still compatible with life. 38,39 Our NMR data show the higher mobility of water molecules in the highly concentrated eutectic NaCl compared with the MgSO 4 solution in temperatures pertinent to Europa's ocean and brine pockets near the ice shell–ocean interface. Considering the important role of water mobility in life-related processes, our data highlight the higher habitability potential of a NaCl-rich compared with a MgSO 4 -rich Europa ocean.…”

Stable Brine Layers beneath Europa’s Chaos

“…Over the last 100 kyr of our simulation, a stable multiphase mushy layer developed between the ice and residual brine. So long as the mushy layer stays relatively hydraulically connected (f > 0.06) to the underlying brine, nutrients within the brine can be efficiently exchanged (Wolfenbarger et al 2022), providing a chemical gradient rich substrate for putative life to thrive in, much like it does beneath sea ice, ice shelves, and in perennially ice-covered saline lakes (Murray et al 2012;Bowman et al 2016). Though, hydraulic connection to an underlying brine is not a necessity for sustaining life or preserving it (e.g., Wolfenbarger et al 2022).…”

“…So long as the mushy layer stays relatively hydraulically connected (f > 0.06) to the underlying brine, nutrients within the brine can be efficiently exchanged (Wolfenbarger et al 2022), providing a chemical gradient rich substrate for putative life to thrive in, much like it does beneath sea ice, ice shelves, and in perennially ice-covered saline lakes (Murray et al 2012;Bowman et al 2016). Though, hydraulic connection to an underlying brine is not a necessity for sustaining life or preserving it (e.g., Wolfenbarger et al 2022). While habitability is intimately related to salinity, other physicochemical properties of the solution are likely more important, such as its water activity (e.g., Stevenson et al 2015) and chaotropicity (e.g., Pontefract et al 2017).…”

“…Measurements of the ionic composition of shallow water reservoirs by future remotesensing missions may depend on whether such reservoirs are plume sources (Walker & Schmidt 2018;Chivers et al 2021). For these reasons, shallow water reservoirs, relict or quasistable, may be the most accessible habitable niche for future remote and subsurface missions aimed at the detection of life (e.g., Oleson et al 2019;Bryson et al 2020;Schmidt et al 2022;Wolfenbarger et al 2022).…”

Salt Distribution from Freezing Intrusions in Ice Shells on Ocean Worlds: Application to Europa

“…Water activities are found to be the same for equilibrium and fractional crystallization if the temperature is the same. Therefore, activities corresponding to the minimum temperature simulated across the equilibrium and fractional crystallization of each solution (shown in Table 2 and Figure potential for delivery of nutrients to these potential habitats (Wolfenbarger et al 2022c).…”