Geometry of freezing impacts ice composition: implications for icy satellites

Abstract

Non‐ice impurities within the ice shells of ocean worlds (e.g., Europa, Enceladus, Titan, Ganymede) are believed to play a fundamental role in their geophysics and habitability and may become a surface expression of subsurface ocean properties. Heterogeneous entrainment and distribution of impurities within planetary ice shells have been proposed as mechanisms that can drive ice shell overturns, generate diverse geological features, and facilitate ocean‐surface material transport critical for maintaining a habitable subsurface ocean. However, current models of ice shell composition suggest that impurity rejection at the ice‐ocean interface of thick contemporary ice shells will be exceptionally efficient, resulting in relatively pure, homogeneous ice. As such, additional mechanisms capable of facilitating enhanced and heterogeneous impurity entrainment are needed to reconcile the observed physicochemical diversity of planetary ice shells. Here we investigate the potential for hydrologic features within planetary ice shells (sills and basal fractures), and the unique freezing geometries they promote, to provide such a mechanism. By simulating the two‐dimensional thermal and physicochemical evolution of these hydrological features as they solidify, we demonstrate that bottom‐up solidification at sill floors and horizontal solidification at fracture walls generate distinct ice compositions and provide mechanisms for both enhanced and heterogeneous impurity entrainment. We compare our results with magmatic and metallurgic analogs that exhibit similar micro‐ and macroscale chemical zonation patterns during solidification. Our results suggest variations in ice‐ocean/brine interface geometry could play a fundamental role in introducing compositional heterogeneities into planetary ice shells and cryoconcentrating impurities in (re)frozen hydrologic features.