Compositional Controls on the Distribution of Brine in Europa's Ice Shell

Abstract

The presence of liquid water has long guided the search for life beyond Earth (Des Marais et al., 2003). Once thought to be confined to the "Goldilocks zone," vast oceans have been inferred to exist beneath the thick ice shells of moons in the outer solar system (Hand et al., 2020). Although these sub-ice oceans represent the most compelling potential habitats in the outer solar system (Shematovich, 2018), they are covered by ice shells that can range from kilometers to hundreds of kilometers thick (Soderlund et al., 2020). Impurities within the ice shell, such as salts, acids, or organic compounds, could allow for liquid water to remain stable at temperatures well below that predicted by the pressure-melting curve of pure water (Marion et al., 2003(Marion et al., , 2005Ruiz et al., 2007). These impurities support the formation of intra-ice shell brine pockets, which could represent potential habitats that might be more accessible than the sub-ice ocean (Kargel et al., 2000;Marion et al., 2003).Estimating the distribution of brine in an ice shell represents an important step in studying the habitability of ocean worlds. The brine volume fraction governs a number of bulk ice thermophysical properties, such as density, thermal conductivity, specific heat capacity, and viscosity (Petrich & Eicken, 2017). These thermophysical properties in turn govern processes of surface-ice-ocean exchange (e.g., solid-state convection, subduction, diapirism),

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

“…In our evaluation of habitability metrics for Europa's ice shell, we assume the analog endmember ocean compositions of Wolfenbarger, Fox-Powell, et al (2022). In their model, they considered only impurities sourced from the ocean and adopted two endmember ocean compositions: (a) a chloride-dominated composition, analogous to terrestrial seawater, and (b) a sulfate-dominated composition, analogous to the modeled Europa K1a ocean of Zolotov and Shock (2001).…”

“…Figure 2 shows the brine volume fraction, V b /V, for Europa's ice shell considering a range of bulk salinities up to 100 ppt, adapted from Wolfenbarger, Fox-Powell, et al (2022) (see Section 3 for model assumptions). Our proposed classification indicates that ∼80% of the ice shell corresponds to relict potential habitats, since brine is not thermodynamically stable for temperatures below the shared eutectic of T = −32.2°C (240.95 K).…”

“…Practically, this assumption is not realistic because the ice will be in a state of progressive desalination due to the unstable brine density gradient in the region where nutrient-open habitats are present. Ultimately, this process will limit the stable bulk ice shell salinity to a fraction of the underlying ocean salinity Wolfenbarger, Fox-Powell, et al, 2022). Thus, the true vertical distribution of nutrient-open habitats in a conductive ice shell is governed by the fraction that is in an active state of desalination, where brine convection is occurring within the ice (i.e., the equilibrium mushy layer of Buffo et al (2021)).…”

“…Figure2. Brine volume fraction for Europa's ice shell assuming analog endmember compositions for the ocean and an ice shell of arbitrary thickness (assuming a fixed pressure of 1 atm), adapted fromWolfenbarger, Fox-Powell, et al (2022). The 0.06 contour depicts the temperature and bulk salinity where the brine volume fraction equals our effective critical porosity.…”

“…Figure 1.Habitability metrics for brine in the bottom 20% of a thermally conductive Europan ice shell assuming binary (NaCl, MgSO 4 ) and analog endmember (chloride-dominated, sulfate-dominated) ocean compositions and a fixed pressure of 1 atm. For panels (d-g), brine volume fraction is calculated using the model ofWolfenbarger, Fox-Powell, et al (2022), assuming a bulk ice salinity of 1 ppt (see Text S4 in Supporting Information S1). The solid black lines depict a water activity of 0.9 (a and d) and the temperature at which the water activity equals 0.9 (a-c).…”