Volatile Transport inside Super-Earths by Entrapment in the Water Ice Matrix

TL;DR

This study investigates the potential for methane to be trapped in water ice within super-Earths, using a modified thermodynamic model to assess stability conditions relevant to planetary interiors.

Contribution

It introduces a modified van der Waals & Platteeuw model to evaluate the stability of methane filled ice Ih in water-rich super-Earths.

Findings

Methane filled ice Ih may be stable at high pressures and temperatures.

Filled ice Ih could exist at the core-water mantle boundary of water planets.

Implications for volatile transport and atmospheric composition in water-rich exoplanets.

Abstract

Whether volatiles can be entrapped in a background matrix composing planetary envelopes and be dragged via convection to the surface is a key question in understanding atmospheric fluxes, cycles and composition. In this paper we consider super-Earths with an extensive water mantle (i.e. water planets), and the possibility of entrapment of methane in their extensive water ice envelopes. We adopt the theory developed by van der Waals & Platteeuw (1959) for modelling solid solutions, often used for modelling clathrate hydrates, and modify it in order to estimate the thermodynamic stability field of a new phase, called methane filled ice Ih. We find that in comparison to water ice VII the filled ice Ih structure may be stable not only at the high pressures but also at the high temperatures expected at the core-water mantle transition boundary of water planets.