The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets

TL;DR

This paper introduces a comprehensive model of how primary atmospheres evolve into secondary atmospheres on rocky exoplanets, showing that primary atmosphere erosion often still allows habitability and can increase surface water.

Contribution

It presents a new self-consistent evolutionary model incorporating magma ocean solidification, climate, escape, and redox processes, applied to TRAPPIST-1 planets.

Findings

TRAPPIST-1e likely retains a substantial atmosphere.

TRAPPIST-1b's atmosphere is predicted to be thin and vulnerable.

Erosion of primary atmospheres often does not hinder habitability.

Abstract

Exoplanet exploration has revealed that many$\unicode{x2013}$perhaps most$\unicode{x2013}$terrestrial exoplanets formed with substantial H$_2$-rich envelopes, seemingly in contrast to solar system terrestrials, for which there is scant evidence of long-lived primary atmospheres. It is not known how a long-lived primary atmosphere might affect the subsequent habitability prospects of terrestrial exoplanets. Here, we present a new, self-consistent evolutionary model of the transition from primary to secondary atmospheres. The model incorporates all Fe-C-O-H-bearing species and simulates magma ocean solidification, radiative-convective climate, thermal escape, and mantle redox evolution. For our illustrative example TRAPPIST-1, our model strongly favors atmosphere retention for the habitable zone planet TRAPPIST-1e. In contrast, the same model predicts a comparatively thin atmosphere for the Venus-analog TRAPPIST-1b, which would be vulnerable to complete erosion via non-thermal escape and is consistent with JWST observations. More broadly, we conclude that the erosion of primary atmospheres typically does not preclude surface habitability, and frequently results in large surface water inventories due to the reduction of FeO by H$_2$.