Constraints on Primordial Hydrosphere Development in Io and Europa from Interior Thermal Models

The Galilean moons exhibit a radial gradient in bulk density that decreases with increasing distance from Jupiter, reflecting a corresponding increase in their ice-to-rock ratios. The origin of this compositional gradient remains a matter of debate. Competing hypotheses attribute it either to primordial conditions within Jupiter's circumplanetary disk or to divergent evolutionary pathways. In the latter framework, Io and Europa may have experienced substantial volatile loss early in their history, possibly driven by the formation of transient atmospheres and surface oceans that were subsequently lost through atmospheric escape processes facilitated by their relatively low surface gravities (Bierson & Nimmo 2020; Bierson et al. 2023).

To explain Europa's preserved ice content, one hypothesis is that its subsurface ocean formed later in its evolution as a result of metamorphic dehydration of accreted hydrous silicates. Fluids released during this process could have migrated upward, leading to the formation of an ice shell. However, this scenario raises a critical question: if Io also accreted hydrous minerals, what mechanism allowed it to completely lose its volatiles while Europa retained a significant fraction?

To explore this divergence, we model the early thermal evolution of both satellites, incorporating surface and internal temperature profiles as well as hydrodynamic escape processes to quantify potential volatile losses. We consider a range of formation scenarios, paying particular attention to the controversial existence of a primordial ocean on Io and the plausibility of Europa's ocean-ice shell system being formed solely by silicate dehydration.