Icy realms compared: Global ice shell dynamics of Ganymede and Europa

Kurzfassung

Icy moons and their cryo-/hydrospheres are central to the search for subsurface habitable environments in the solar system (e.g., [1]). While the structure of internal ice and liquid water layers varies with the moon’s size, an outer ice shell is a common feature. Smaller moons, like Europa, typically have a thin ice shell overlaying a liquid ocean, whereas larger moons, like Ganymede, possess a thicker ice shell, burying the ocean deeper beneath the surface. The outer ice layer is particularly significant: it is the most accessible for exploration, serves as a conduit between the surface and subsurface ocean, and may itself harbor niches for life. Understanding its thermal and dynamic state is essential for interpreting mission data and assessing astrobiological potential. In this work, we compare thin (10–40 km, e.g., Europa) and thick (50–200 km, e.g., Ganymede) ice shells, focusing on their impact on thermal and dynamic properties. We model ice shell dynamics using the GAIA convection code [2], building on recent studies [3,4] to incorporate temperature-dependent thermal conductivity (k), temperature- and pressure-dependent thermal expansivity ($\alpha$), and a complex rheology. We also examine tidal heating, a significant factor for Europa [5]. Our analysis explores various ice grain sizes, which influence the viscosity—a critical parameter for ice shell dynamics. Key model outputs that can be tested with future measurements include elastic thickness, brittle-to-ductile transition, boundary heat flux, and potential formation of brines. Furthermore, scaling laws relating heat loss and convection vigor, as well as the creep mechanism that dominates the deformation help us to characterize the ice shell dynamic regime (i.e., conductive, weakly convective, or highly convective). By distinguishing dynamic regimes, we aim to advance our understanding of icy worlds, the heat and material transport through their icy shells, and their potential for habitability.