Planetary Ice-Ocean Interfaces: Structure, Dynamics, and Astrobiological Implications

Abstract:

Ocean worlds have been identified as high priority astrobiology targets due to the unique link between life and liquid water. Furthermore, youthful surface terrain on many icy bodies indicates they support active geophysical cycles that may facilitate ocean-ice shell-surface transport and/or near surface water reservoirs that could provide observables for upcoming missions tasked with understanding the geophysics and assessing the habitability of ice-ocean worlds (e.g. Europa Clipper).

Accurately interpreting spacecraft observations requires constraining the relationship between empirical characteristics of the ice shells of icy worlds and their interior dynamics. On Earth, the composition, physical characteristics, and bioburden of ocean derived ices are related to their formation history and parent fluid composition. In such systems the ice-ocean interface, which exists as a multiphase permeable ice-brine slush (or mushy layer), plays a fundamental role in dictating the overlying ice’s properties and evolution. Inclusion of the physics governing these boundaries is a novel strategy in modeling planetary ice-ocean systems, and thus far has been limited to 1D approaches.

Here we present results from 2D simulations of an archetypal ice-ocean world system. We track the evolution of temperature, salinity, porosity, and brine velocity within a thickening ice shell enabling us to place improved constraints on a number of ice-ocean world properties, including: the composition of planetary ice shells, the thickness and hydraulic connectivity of ice-ocean interfaces, and heterogenous dynamics/structures in the interfacial mushy layer. We show that stable eutectic horizons are likely a common feature of ice-ocean worlds and that ocean composition plays an important role in governing the structure and dynamics of the interface, including the formation of gradient rich inverted chemical gardens. We discuss the geophysical and astrobiological implications of our results and highlight how they can be validated by and aid in the synthesis of instrument specific measurements.