Large volumes of eutectic brines quasi-stable under near-vacuum conditions: implications for transient liquids on icy world surfaces

Introduction: Icy worlds such as Europa, Enceladus and Ceres show evidence for subsurface liquids reaching the surface, either as plumes or effusive flows. Regions where liquids have been emplaced serve as potential archives of subsurface chemistry and habitability, making them prime targets for future missions. Fluids of a wide range of salt concentrations are expected at icy worlds, reflecting different potential liquid reservoirs and crysopheric processes. Despite their importance, little is understood about how saline fluids evolve if exposed to surface conditions. Previous experimental work has been limited to small scales, while models consider only equilibrium states or neglect phase changes. New large-scale experiments are therefore needed to better predict the behaviour of brine extrusions on icy worlds. We exposed large quantities (~50 kg) of NaCl and MgSO₄ brines at concentrations near their eutectics to pressures below their triple points and observed their physical and thermal evolution.

Results and Discussion: Both brines initially boiled during depressurization, which drove rapid cooling due to the loss of latent heat. However, vigorous boiling was soon interrupted by the formation of a thin ‘raft’ of salt crystals at the liquid-vapour interface. We found that the presence of the salt raft strongly limited evaporation, effectively halting further evaporative cooling of the brines.

Instead of gradually approaching the eutectic point, and hence complete solidification, the temperatures and bulk salinities of both experiments asymptotically approached the liquidus of their respective hydrates (hydrohalite, NaCl.2H₂O, in the NaCl case; meridianiite, MgSO₄.11H2O, in the MgSO₄ case) at concentrations approximately 3% higher than the eutectic. Ice was never observed to form in the NaCl experiment, and the temperature never decreased below 264 K. Instead, the system approached a steady state whereby hydrohalite formed at the surface, and then gradually sank, to be replaced by more surficial hydrohalite. In the MgSO₄ experiment, the eutectic point was initially reached, triggering massive ice and hydrate precipitation. However, the system quickly rebounded to a temperature above the eutectic, where a similar steady-state scenario was achieved; the experiment gradually approached a temperature of ~271 K, similarly buffered by the continual precipitation and sinking of hydrates.

Our results show that eutectic brines, if emplaced into low-pressure environments, resist evaporatively- driven freezing. Due to the formation of a salt raft which slows evaporation, eutectic liquids could be relatively long-lived in low-pressure environments, providing opportunity for them to pond and infiltrate pre-existing topography. 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 high salinity brines have been emplaced could be indicated by salt evaporites rather than salt-bearing ices. For example, evaporation of high salinity brines offers a potential route to form the observed ice-free salt deposits in the Ceres faculae.

Our findings provide a new perspective on the possible longevity of liquid water under non-equilibrium scenarios on planetary surfaces, and indicate that quasi-stable transient liquids can play an important role in surface processes on icy worlds.  

References: [1] Quick et al. (2017) Icarus 284; [2] Postberg et al. (2009) Nature 459; [3] De Sanctis et al. (2019) Icarus 320; [4] Chivers et al. (2023) Planet. Sci. J. 4; [5] Steinbrügge et al. (2020) GRL 47