Structure and Dynamics of Cold Water Super-Earths: The Case of Occluded CH4 and its Outgassing

TL;DR

This paper investigates how methane transport and phase changes in water-rich super-Earths influence their interior dynamics, crustal regimes, and atmospheric composition, highlighting the role of methane in planetary thermodynamics and tectonics.

Contribution

It introduces a new phase (filled ice) due to methane, models planetary crustal regimes, and links tectonic activity to methane outgassing in super-Earths.

Findings

Methane inclusion creates filled ice phase, affecting interior temperature.

Methane clathrate formation inhibits subterranean oceans.

Tectonic modes influence methane outgassing rates.

Abstract

We study the transport of methane in the external water envelopes surrounding water-rich super-Earths and estimate its outgassing into the atmosphere. We investigate the influence of methane on the thermodynamics and mechanics of the water mantle. We find that including methane in the water matrix introduces a new phase (filled ice) resulting in hotter planetary interiors. This effect renders the super-ionic and reticulating phases accessible to relatively low mass planets lacking a H/He atmosphere. We model the thermal and structural profile of the planetary crust and discuss five possible crustal regimes. The formation of methane clathrate in the subsurface is shown to inhibit the formation of a subterranean ocean. This effect results in increased stresses on the lithosphere making modes of ice plate tectonics possible. The dynamics of the tectonic plates are analysed. We derive overturn and resurfacing time scales as well as the melt fraction underneath spreading centers. Ice mantle dynamics is found to be important for assessing the composition of the atmosphere. We formulate the relation between the outgassing flux of methane and the tectonic mode dynamics. We give numerical estimates for the global outgassing rate of methane into the atmosphere.