Photochemical Oxygen in Non-1 Bar CO2 Atmospheres of Terrestrial Exoplanets

TL;DR

This study models how surface pressure influences the buildup of abiotic oxygen in CO2-rich atmospheres of exoplanets, revealing conditions under which detectable oxygen levels can form without life.

Contribution

It introduces a pressure-dependent photochemical model for CO2 atmospheres, expanding previous models that assumed 1 bar surface pressure, and explores a range of outgassing scenarios.

Findings

Oxygen buildup decreases with increasing pressure at high outgassing rates.

Detectable abiotic O2 can form in 10-bar atmospheres with Venus-like volcanic activity.

Surface pressure significantly affects the stability and accumulation of atmospheric oxygen.

Abstract

Atmospheric chemistry models have shown molecular oxygen can build up in CO2-dominated atmospheres on potentially habitable exoplanets without an input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO2-dominated atmospheres with the surface pressure ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We use a one-dimensional photochemistry model to calculate the abundance of O2 and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20x Earth-like activity. The model maintains the redox balance of the atmosphere and the ocean, and includes the pressure dependency of outgassing on the surface pressure. Our calculations show that the surface pressure is a controlling parameter in the photochemical stability and oxygen buildup of CO2-dominated atmospheres. The mixing ratio of O2 monotonically decreases as the surface pressure increases at the very high outgassing rates, whereas it increases as the surface pressure increases at the lower-than-Earth outgassing rates. Abiotic O2 can only build up to the detectable level, defined as 1e-3 in volume mixing ratio, in 10-bar atmospheres with the Venus-like volcanic activity rate and the reduced outgassing rate of H2 due to the high surface pressure. Our results support the search for biological activities and habitability via atmospheric O2 on terrestrial planets in the habitable zone of Sun-like stars.