Effects of salts on the exchanges through high-pressure ice layers of large ocean worlds

Abstract

The habitability of large icy moons has long been disputed due to the presence of an high-pressure (HP) ice layer located between the core and the internal liquid ocean, preventing a direct contact and facilitated exchanges between both. In our previous study (Lebec et al. 2023), we have shown that under certain conditions and considering the phase change at the upper boundary of the HP ice shell, heat and mass transfer can occur and be efficient. Now, the aim of this work is to study the effects of impurities inside the HP ices on the flow dynamics and the mass transfer efficiency from the core to the ocean. These salts can enter the ice layer either by diffusion, which is a very inefficient process, or by partial melting at the bottom of the ice layer, if the temperature is high enough. In that case, the liquid water can interact with the core and get enriched in salts. This salty water, which can be lighter than high-pressure ices, could rise through the ice shell and refreeze. The resulting salts/ice mixture can be transported into the ocean by convection in the solid state but could also impede it due to its density higher than that of pure ice. The efficiency of convection depends on the buoyancy number Bsalts, which is the ratio of density increase associated to salts compared to the decrease associated to temperature. To explore the impact of salts, we model a flux of salts from the core and the advection of these salts using Lagrangian tracers in order to study the effects of the Rayleigh number, the buoyancy number and K, which is the partition coefficient of the salts between the ocean and the HP ices at the top interface, on the heat and salts transfer efficiency through the HP ice layer, as well as the implications for the evolution of salt concentration in the ocean of large icy moons and ocean worlds. We present a scaling of the bottom temperature and the top radial velocity wtop as function of the Rayleigh number Raq for various values of the buoyancy number and a scaling of the effective flux of salts across the top interface with the ocean as function of the buoyancy number for various values of the partition coefficient. We perform a numerical application for Ganymede and show that depending on the values of the system parameters, a significant percentage of the salts entering the HP ice layer from the core can efficiently flow into the ocean.