Ice–ocean interactions as a driver of basal ice roughness and ice shell evolution on Europa

Europa’s ice shell is thought to overlie a global liquid water ocean, with exchange of heat and momentum across the ice–ocean boundary playing a fundamental role in shaping the shell’s structure and dynamics. Previous studies have shown that heterogeneous ocean circulation and tidal heating can imprint spatially variable heat flux and thus melting and freezing at the ice–ocean interface. However, how the ocean’s thermal state and convective vigour influence the morphology of the ice–ocean interface itself remains poorly understood. Here, we investigate coupled ice–ocean dynamics relevant to Europa in a two-dimensional annulus geometry, with the phase transition between the liquid water ocean and the solid ice layer treated self-consistently through a phase-field approach. Spatially variable temperature anomalies are imposed following Lemasquerier et al. (2023), allowing us to explore the effects of heterogeneous tidal heating under different ocean thermal regimes. We find that colder oceans lead to thicker ice shells and systematically rougher ice–ocean interfaces, characterised by enhanced basal ice topography, which may lead to stronger lateral variability in melting and freezing rates. We also note that as the temperature anomaly between the poles and equator increases, heat transport becomes strongly asymmetric. This results in hemispheric-scale contrasts in ice thickness and the formation of two distinct ice hemispheres separated by a global ocean band confined to low latitudes. These conditions promote the development and persistence of basal roughness on spatial scales comparable to large-scale ice shell heterogeneity. Basal ice roughness and associated thickness variations are expected to strongly influence ice shell dynamics by driving lateral ice flow, promoting ice fabric development, and enhancing lateral stress focusing. On Europa, such mechanically heterogeneous ice shells may play a key role in localising deformation, modulating fracture patterns, and controlling pathways for ocean–ice exchange. Our results highlight the ice–ocean interface morphology as a critical, yet often overlooked, outcome of ice–ocean coupling, with important implications for the evolution and dynamics of Europa’s ice shell.