Equilibrium composition between liquid and clathrate reservoirs on Titan

TL;DR

This paper models the interaction between subsurface hydrocarbon reservoirs and clathrate formation on Titan, predicting how molecular compositions differ based on clathrate structure and formation processes.

Contribution

It introduces a statistical-thermodynamic model to analyze the composition of clathrates formed from Titan's subsurface hydrocarbons, highlighting molecular fractionation effects.

Findings

Clathrate formation causes significant molecular fractionation.

Structure I clathrates favor propane-rich reservoirs.

Structure II clathrates favor ethane-rich reservoirs.

Abstract

Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by the Cassini spacecraft to cover the polar regions of Titan. A significant fraction of these lakes or seas could possibly be interconnected with subsurface liquid reservoirs of alkanes. In this paper, we investigate the interplay that would happen between a reservoir of liquid hydrocarbons located in Titan's subsurface and a hypothetical clathrate reservoir that progressively forms if the liquid mixture diffuses throughout a preexisting porous icy layer. To do so, we use a statistical-thermodynamic model in order to compute the composition of the clathrate reservoir that forms as a result of the progressive entrapping of the liquid mixture. This study shows that clathrate formation strongly fractionates the molecules between the liquid and the solid phases. Depending on whether the structure I or structure II clathrate forms, the present model predicts that the liquid reservoirs would be mainly composed of either propane or ethane, respectively. The other molecules present in the liquid are trapped in clathrates. Any river or lake emanating from subsurface liquid reservoirs that significantly interacted with clathrate reservoirs should present such composition. On the other hand, lakes and rivers sourced by precipitation should contain higher fractions of methane and nitrogen, as well as minor traces of argon and carbon monoxide.