Composition and Density of Clathrate Hydrates in Ocean Worlds: Implications for Insulating Ice Shells

Understanding the formation of subsurface oceans and their survival is of fundamental importance to identify potentially habitable worlds. An important control on ocean evolution is the composition of the ice shell, specifically the presence and abundance of clathrate hydrates – crystalline inclusion compounds that form when water solidifies in the presence of gases under appropriate low temperature and high-pressure conditions. Because all common clathrate structures consist of at least 85% water, many clathrate physical properties are similar to those of water ice I_h. However, the differences in mechanical strength, thermal conductivity and density may have a significant effect on geologic processes of planetary environments. The stability, composition and distribution of clathrate hydrates in ocean worlds remain poorly understood.

This study examines the composition of mixed CH₄-CO₂ clathrate hydrates that could form in ocean worlds and assesses their potential to sink or float, contributing to the formation of a clathrate layer at the top or bottom of the internal ocean, and potentially facilitating their incorporation in the outer ice shell. Our calculations are mainly based on pure water systems, with some preliminary analyses incorporating inhibitors such as ammonia and salts. Using a thermodynamic model based on the statistical thermodynamic approach of Van der Waals and Platteeuw, we evaluate the density and composition of CH₄-CO₂ clathrate hydrates under conditions relevant to Europa, Titan, and Enceladus. Simulations are conducted around 273 K and at pressures ranging up to several hundred MPa.

By refining constraints on the presence and composition of clathrate hydrates in ice shells, this research contributes to understanding the conditions necessary for maintaining potentially habitable subsurface liquid water reservoirs. This work provides valuable insights for interior modeling of icy bodies and supports ongoing and future missions, including JUICE, Europa Clipper, and Dragonfly.