Scaling of convection in high-Pressure ice layers of large icy moons and implications for habitability

The existence of a high pressure ice layer between the silicate core and the liquid ocean in large icy moons and ocean worlds is usually seen as a barrier to habitability, preventing the compounds needed for life to flow into the ocean. More recently, three studies from Choblet et al [1] and Kalousová et al [2, 3] challenged that hypothesis and showed that, in certain conditions, exchanges were possible between the core and the ocean, allowing transport of salts toward the ocean. Here, we consider an effect not taken into account in these previous studies: the possibility of mass exchange between the ice and ocean layers by phase change. Convective stresses in the solid create a topography of the interface which can be erased by melting and freezing if flow on the liquid side is efficient. This effect is included in a convection model as a phase change boundary condition, allowing a non-zero vertical velocity at the surface of the HP ice layer, which has a significant impact on the flow dynamics and enables exchanges with the ocean by fusion and crystallization of the ice at the top interface, even without partial melting in the bulk of the ice layer. These exchanges are directly linked to the melting capacity of the ice at the interface between the HP ice layer and the core, depending on the Rayleigh number and the efficiency of convection. Then, considering this new condition at the interface between the HP ice layer and the liquid ocean, we propose a scaling of the bottom temperature and the vertical velocity. Applied to a specific celestial body, as Ganymede or Titan, it would be the first step to conclude about its habitability.

 

References:

[1] G. Choblet, G. Tobie, C. Sotin, K. Kalousová, O. Grasset (2017). Heat transport in the high-pressure ice mantle of large icy moons. Icarus, 285, 252-262

[2] K. Kalousová, C. Sotin, G. Choblet, G. Tobie, O. Grasset (2018). Two-phase convection in Ganymede’s high-pressure ice layer — Implications for its geological evolution. Icarus, 299, 133-147

[3] K.Kalousová, C. Sotin (2018). Melting in High-Pressure Ice Layers of Large Ocean Worlds—Implications for Volatiles Transport. Geophys. Res. Lett., 45, 8096-8103.