Dynamics of Titan's high-pressure ice layer

The presence of Ar-40 in Titan's atmosphere and the replenishment of methane argue for the exchange between the interior and the atmosphere. These observations triggered the present study that aims to determine the conditions under which the high-pressure (HP) ice layer, likely present between the deep ocean and the silicate core, poses a barrier for the exchange of volatiles. We model heat and water transport through this convecting HP ice layer using a two-phase numerical model of solid ice-liquid water mixture. We observe that for a large range of heat fluxes from the silicate core and HP ice layer thicknesses, a few percent of liquid water forms at the interface with the silicates. Liquid water being less dense than the HP ice, it creates additional buoyancy, thus facilitating the transport of volatiles towards the ocean. Our results also show that convection is characterized by the presence of hot and the absence of cold plumes. We derive a scaling law that describes the dependence of a critical heat flux for the onset of melting at the silicates interface on the thickness of the HP ice layer and the ice viscosity. We also study the processes at the interface with the base of the ocean where a few tens of kilometers thick layer of temperate (partially molten) ice is present. We find a scaling law for its thickness that depends mainly on the ice viscosity and the density difference between the ice and water. Water from this partially molten, temperate layer flows into the ocean thus completing the connection with the silicate core. The water flux depends primarily on the amount of heat supplied from the silicates. Future evolution models that will use the scaling laws derived in this study will place bounds on the timing of these exchange processes. Using Cassini data and reasonable values of HP ice viscosity and silicate heat flux, we predict melting at the silicates/HP ice interface at present time. (C) 2020 Elsevier B.V. All rights reserved.