The Insulating Effect of Methane Clathrate Crust on Titan's Thermal Evolution

Abstract:

Saturn's largest moon Titan is unique among other solar system moons for its dense, methane‐rich atmosphere where complex organic chemistry is ongoing. Titan is also the only solar system body, apart from Earth, with liquids present on its surface in the form of hydrocarbon lakes and seas. Finally, Titan is an ocean world as it harbors a subsurface water ocean. The outer ice shell of Titan is likely capped by a thick crust of methane clathrates that are very stable under Titan's surface conditions. The presence of this crust can have important consequences for Titan's thermal evolution since clathrates have about one order of magnitude lower thermal conductivity than water ice at Titan's surface temperature. Here, we perform two-dimensional numerical simulations in Cartesian geometry to investigate the effect of this insulating clathrate crust on convection in the ice shell. Without the clathrate crust, convection occurs below a stagnant lid that is a few tens of kilometers thick and limits the exchange between the surface and the deep ocean. We show that a 5-10 km thick clathrate crust reduces the thickness of the stagnant lid about threefold which increases the potential for the exchange between the deep ocean and the surface. Moreover, the insulating effect of clathrates dramatically decreases the amount of heat that is extracted from the ocean by convection. Therefore, the presence of clathrate crust limits the freezing rate of Titan's ocean.