Melting in High-Pressure Ice Layers of Large Ocean Worlds-Implications for Volatiles Transport

A high-pressure ice layer controls the exchange of heat and material between the silicate core and the ocean of Ganymede and Titan. We have shown (Kalousova et al., 2018, ) that a temperate (partially molten) layer is always present at the ocean interface. Another temperate layer with a few percent of water may be present at the silicates interface for low values of Rayleigh number. We derive scaling laws to predict the critical value under which this temperate layer exists and the amount of generated melt. The presence of liquid water in contact with silicates was probably limited to the early history, providing a pathway for the transfer of salts and volatiles like Ar-40 to the ocean. We also derive scaling laws for the water outflow velocity and for the top temperate layer thickness. These laws can be used to model the global thermal and compositional evolution of large ocean worlds. Plain Language Summary Ocean worlds, where a deep global ocean is present below the icy crust, provide an interesting habitable environment where life may exist. On Enceladus, which is small, and Europa, where the H2O/silicate (water/rock) ratio is small, the global ocean is in direct contact with the silicates. On Titan and Ganymede, where this ratio is large, a layer of high-pressure (HP) ice is present between the ocean and the rocky core. This paper shows that early in their evolution, the lower part of this HP ice layer was temperate (porous ice with water in the pores). Such a temperate layer enables a silicates-ocean exchange of salts and volatiles such as Ar-40 that was measured in Titan's atmosphere by the Cassini mission. It also provides a potentially habitable environment in Ganymede, the largest moon in the solar system that will be studied by the ESA JUICE mission.