Volatile atmospheres of lava worlds

TL;DR

This study models the composition and structure of secondary atmospheres formed by volatile outgassing from magma oceans on rocky exoplanets, revealing how planetary size, magma ocean depth, and redox conditions influence atmospheric chemistry and size.

Contribution

It introduces a coupled chemical and structural model to predict the composition and transit radius of lava world atmospheres based on planetary and magma ocean parameters.

Findings

Small planets tend to have H2-rich atmospheres due to reduced magma oceans.

Larger planets and deeper magma oceans produce CO, CO2, N2, and SO2 dominated atmospheres.

Atmospheric outgassing of N and C is robust, while S and H depend on redox state and planetary size.

Abstract

A magma ocean (MO) is thought to be a ubiquitous stage in the early evolution of rocky planets and exoplanets. During the lifetime of the MO, exchanges between the interior and exterior envelopes of the planet are very efficient. In particular, volatile elements that initially are contained in the solid part of the planet can be released and form a secondary outgassed atmosphere. We determine trends in the H-C-N-O-S composition and thickness of these secondary atmospheres for varying planetary sizes and MO extents, and the oxygen fugacity of MOs, which provides the main control for the atmospheric chemistry. We used a model with coupled chemical gas-gas and silicate melt-gas equilibria and mass conservation to predict the composition of an atmosphere at equilibrium with the MO depending on the planet size and the extent and redox state of the MO. We used a self-consistent mass-radius model for the rocky core to inform the structure of the planet, which we combined with an atmosphere model to predict the transit radius of lava worlds. We find that MOs (especially the shallow ones) on small planets are generally more reduced, and are thus dominated by H2-rich atmospheres (whose outgassing is strengthened at low planetary mass), while larger planets and deeper MOs vary from CO to CO2-N2-SO2 atmospheres, with increasing fO2 . In the former case, the low molecular mass of the atmosphere combined with the low gravity of the planets yields a large vertical extension of the atmosphere, while in the latter cases, secondary outgassed atmospheres on super-Earths are likely significantly shrunk. Both N and C are largely outgassed regardless of the conditions, while the S and H outgassing is strongly dependent on the fO2 , as well as on the planetary mass and MO extent for the latter.