Large volumes of eutectic brines resistant to freezing at low pressures: implications for effusive flows on icy worlds

Icy worlds such as Europa, Enceladus and Ceres show evidence for subsurface liquids reaching the surface, either as plumes or effusive flows. Regions where this has occurred serve as potential archives of subsurface chemistry and habitability, making them prime targets for future missions. Subsurface fluids on these bodies may range in salinity from dilute to eutectic compositions, with brines approaching eutectic concentrations expected to be more common in the shallow subsurface due to their longevity at low temperatures. Despite their importance, little is understood about how highly saline fluids evolve if exposed to surface conditions.

We exposed large quantities (~50 kg) of NaCl and MgSO₄ brines at eutectic concentrations to pressures below their triple points and observed their physical behavior and thermal evolution. We found that eutectic brines, if emplaced into low-pressure environments, resist evaporatively driven freezing through the formation of salts at their surface which acts to strongly decrease evaporation rate. Furthermore, instead of evolving towards the eutectic point and thus complete solidification, the salinity and temperature of the brines instead asymptotically approached their hydrate liquidus at a concentration approximately 3% above the eutectic concentration. After 120-300 minutes, both brines approached steady-state whereby salts precipitated at the surface and sank, to be replaced by fresh surficial salts. Our findings indicate that eutectic liquids could be relatively long-lived in low-pressure environments. Furthermore, although emplaced brines at icy worlds may freeze conductively from below, ice formation should not be expected in the upper 10s of cm simulated by these experiments. Instead, we predict the systems should continue to evaporate and precipitate hydrates until dryness, meaning that regions where eutectic brines have been emplaced could be indicated by salt lags rather than salt-bearing ices. Our findings provide a new perspective on surface processes involving the extrusion of high-salinity liquids into low-pressure environments and the possible longevity of liquid water under non-equilibrium scenarios on planetary surfaces.