Ice-Shell Dynamics of Ganymede and Europa and Their Impact on Heat Flux, Brines, and Lithospheric Strength

Icy moons and their cryo- and hydrospheres are central to the search for subsurface habitable environments in the Solar System (e.g., [1]). Their outer ice shells are of particular importance because they are the most accessible targets for exploration, act as a conduit between the surface and the subsurface ocean, and may themselves host potential niches for life. Understanding the thermal, dynamic, and mechanical state of these ice shells is therefore essential for interpreting spacecraft observations and assessing astrobiological potential.

In this work, we investigate solid-state convection in ice shells spanning a wide range of thicknesses, focusing on thin shells (10–50 km) representative of Europa and thicker shells (50–170 km) characteristic of Ganymede. Ice shell dynamics are modeled using the GAIA convection code [2]. Building on recent studies [3,4], we incorporate temperature-dependent thermal conductivity, temperature- and pressure-dependent thermal expansivity (α), and a composite flow law for ice that accounts for multiple deformation mechanisms [5]. For Europa-like scenarios, we additionally include tidal heating, which constitutes a major internal heat source [6].

Our analysis systematically explores the influence of constant ice grain size, which directly controls viscosity and is a key parameter governing ice shell dynamics. For each combination of shell thickness and grain size, we assess the convective regime and characterize the resulting thermal structure. We find that Europa-like ice shells remain convective for grain sizes up to approximately 1 mm, whereas Ganymede-like ice shells can sustain convection for grain sizes on the order of several centimeters. In the case of Europa, however, this threshold strongly depends on the magnitude of tidal heating: enhanced tidal dissipation significantly promotes convection and allows convective behavior to persist even for larger grain sizes.

For moderately convecting ice shells, the stagnant lid thickness is typically on the order of 30% of the total ice shell thickness. Heat fluxes at both the surface and the ice–ocean interface increase with decreasing shell thickness, while basal heat fluxes show pronounced lateral variability linked to convective flow patterns. We further investigate the stability of brines within the ice shell and find that NaCl- and NH₃-rich brines can persist throughout the convective domain, with NH₃-bearing brines potentially remaining stable even within the stagnant lid, depending on the convective regime.

Finally, we evaluate the lithospheric strength of the ice shell, which is relevant for future exploration concepts such as ice-penetrating melt probes [7]. Overall, our results provide constraints on the dynamic, thermal, and mechanical state of Europa’s and Ganymede’s ice shells and support the interpretation of data from current and upcoming missions.

References:

[1] Coustenis & Encrenaz et al., 2013. [2] Hüttig et al., 2013. [3] Carnahan et al. 2021. [4] Harel et al. 2020. [5] Goldsby and Kohlstedt, 2001. [6] Tobie et al., 2003. [7] Rhoden et al. 2026.