Fleeting but not Forgotten: the Imprint of Escaping Hydrogen Atmospheres on Super-Earth Interiors

TL;DR

This paper investigates how transient hydrogen atmospheres on super-Earths leave lasting chemical and density signatures, including interior oxidation and water production, which can be observed with JWST.

Contribution

It introduces a model of chemical equilibrium showing hydrogen sequestration into super-Earth interiors and predicts observable density and atmospheric composition signatures.

Findings

Hydrogen is efficiently sequestered into the interior, producing water.

Super-Earths have bulk densities around 5.0 g/cm³, consistent with observations.

Hydrogen atmospheres become steam-dominated by mass but hydrogen-rich by mole fraction.

Abstract

Small, close-in exoplanets are divided into two sub-populations: super-Earths and sub-Neptunes. Most super-Earths are thought to have lost their primordially accreted hydrogen-dominated atmospheres via thermally driven winds. We consider the global chemical equilibrium of super-Earths and the lasting impacts of their fleeting hydrogen atmospheres. We find that hydrogen is efficiently sequestered into the interior, oxidising iron and endogenously producing $\sim0.5-1.0\%$ water by mass. As the atmospheres of super-Earths are continuously sculpted by mass loss and chemical equilibration, they remain hydrogen-dominated by mole (number) fraction but become steam-dominated by mass, which may be observable with JWST for planets transitioning across the radius valley. One of the main effects of efficient sequestration of hydrogen into the interior is to produce an under-dense bulk interior compared to that of Earth. We predict bulk densities of super-Earths to be $\sim 5.0 \text{ g cm}^{-3}$ for a $1M_\oplus$ planet, which is consistent with high-precision mass measurements and also population-level inference analyses from atmospheric escape models.