Geometry of Freezing Impacts Ice Composition: Implications for Icy Satellites

Physicochemical impurities and heterogeneities in planetary ice shells (e.g., salts, oxidants, reductants, biosignatures, pores) likely impact important ice-ocean world processes and material properties. These include multiscale geophysical processes (mantle convection, subduction/subsumption, ice shell hydrology), ice shell transport capabilities (material/heat transport, biosignature expression), ice shell properties (dielectrics, strength, rheology, melting points), and subsurface ocean habitability (via ocean-surface redox cycling). Furthermore, the interpretation and required sensitivity of remote sensing measurements as a method to understand the interiors of ice-ocean worlds depends critically on linking ice shell characteristics to underlying reservoir properties (e.g., ocean composition/habitability, life detection limits).

It has been shown that the reactive transport dynamics occurring at multiphase ice-ocean/brine solidification interfaces control the physicochemical properties of the resultant ice. While contemporary studies have begun to incorporate the multiphase physics required to describe these interfaces, investigations have currently been limited to simplified planar geometries with unidirectional temperature gradients (cold upper boundary, warm lower boundary). Given the likelihood that there exist much more geometrically and thermally complex environments within planetary ice shells (e.g., nonplanar ice-ocean interfaces, fractures, sills, dikes, lenses) it is crucial to assess the physicochemical evolution of such features to determine the role they play in the geophysical and geochemical evolution of ice-ocean worlds.

To that end, we have used the 2D multiphase reactive transport model SOFTBALL to simulate the evolution of two geometrically unique end members: 1) the top-down and bottom-up solidification of an isolated sill intruded into the shallow ice shell of Europa, and 2) the horizontal (edge to center) solidification of fluid filled basal fractures extending from the ice-ocean interface up into the ice shell of Europa. We will present the 2D spatiotemporally evolving physicochemical profiles of these systems, compare their properties and structures to analog magmatic and metal alloy systems across multiple scales (mm to km), highlight the impact of distinct ocean compositions on the resultant profiles, and discuss the implications our results have for the geophysics, habitability, geology, observation, and ocean-surface material transport capabilities of planetary ice shells.