Subsurface ocean formation in icy moons: exploring the timing and the diversity of differentiation pathways

The formation of subsurface oceans in icy moons is a key process shaping their internal structure, thermal evolution, and potential habitability. These oceans typically emerge during the early stages of internal differentiation, when water released by ice melting begins to segregate and creates porosity within an initially homogeneous matrix of ices, silicates, and metals. The onset of this critical transition depends on the intensity and timing of radiogenic heating associated with the decay of U, Th and K, which is controled by their initial abundance, the body's size, the accretion history. The present-day structure and composition of subsurface oceans are largely conditioned by the duration of the differentiation phase, and by the ongoing mobilization of salts and potential organic compounds from the surrounding layers.

This study describes the numerical approach developed to model the multiphase evolution of initially undifferentiated icy cores. The initial structure is made of two solid phases: the refractory component made of silicates, iron bearing minerals and carbon rich molecules, and a solid ice phase. As the interior heats up, a fraction of the ice melts leading to compaction of the solid phases and migration of the liquid phase towards the ocean. The model self-consistently tracks the co-evolution of solid and liquid phases by solving coupled conservation equations for mass, momentum and energy in a multi-phase framework. This allows us to compute the evolution of porosity, melt migration and matrix compaction in a self-consistent way. It incorporates a wide range of flexible input parameters spanning structural dimensions, initial phase distributions, thermal properties, rheology, physical processes, and heating conditions. This approach allows us to identify key regimes and transitions in the path to differentiation, including the timing and extent of internal melting, the redistribution of fluids, and the emergence of layered structures. We investigate a range of evolutionary scenarios relevant to icy satellites, from small moons experiencing intense early heating due to rapid accretion and impact-driven melting, to larger bodies with slower, pebble-like accretion and significant internal heating.

Preliminary results suggest that the core of large icy moons could differentiate into a hydrosphere and a refractory core in a few tens to a few hundreds of millions of years (Figure 1). However, we need to include two complexities that are not yet implemented: the pressure dependence of the ice melting temperature and the presence of high-pressure ices at the interface between the core and the hydrosphere. Such studies will provide context for the interpretation of missions such as JUICE and Europa Clipper, and for a better understanding of ocean worlds evolution throughout the Solar System.

Figure 1: Typical evolution of phases distribution and temperature profile of a Ganymede-like moon.