Compositional gradient of Titan’s subsurface fluid due to thermo-gravitational effects

Abstract:

Titan is the only object in the Solar System other than Earth that has substantial surface liquid. The liquid is mainly methane in northern lakes and becomes richer with ethane towards the equator and can be modeled as a ternary mixture of nitrogen/methane/ethane in a two-phase vapor-liquid equilibrium with the atmosphere (Tan & Kargel, Fluid Phase Equilib. 2018, 458, 153).

The upper crust of Titan is likely porous and fractured, which allows liquid flow under a pressure gradient (Mitchell JGR 2008, 113, E08015). Such a subsurface hydrocarbon reservoir is needed to replenish methane upon its photocatalytic reactions in the upper atmosphere; the lakes alone are insufficient. So far, constant composition is applied for the gravity-induced pressure gradient. In fact, the composition must change with the pressure gradient, which is an issue that has long been known with oil reservoirs on Earth (Muskat, Phys. Rev. 1930, 35, 1384). The change in turn affects the pressure gradient, so that the inclusion of the gravitational effect is required for precise modeling.

However, gravity is not the only factor that affects the compositional gradient with depth. More realistic modeling should include the effects due to thermo-diffusion driven by the geothermal gradient, which has been thought to be important in the compositional grading in Earth’s oil reservoirs (Galliero et al., npj Microgravity 2017, 3, 20). In the model for Titan’s subsurface alkanology (Vance et al., LPSC 2012, #2939), the temperature gradient was estimated based on thermally conductive water-ice crust.

In a very deep subsurface of Titan, it is likely that the pressure and temperature gradients would alter the composition of the subsurface liquid. This work presents how these gradients affect the compositional grading in the liquid. Considering the exotic phase behavior of Titan’s fluid and its sensitivity to pressure and temperature (Tan et al., Icarus 2015, 250, 64), the results may reveal interesting features unknown before. Since the fluid may freeze at high pressures in the deep crust, CRYOCHEM, which is an equation of state that can deal with solid solutions, is used for this application so that more realistic phase equilibria due to simultaneous chemical-thermal-gravitational effects can be investigated.