Photochemical Oxygen in Non-1-bar CO_2 Atmospheres of Terrestrial Exoplanets

Abstract

Atmospheric chemistry models have shown that molecular oxygen can build up in CO_2-dominated atmospheres on potentially habitable exoplanets without input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO_2-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 O_2 and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20 times 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 CO_2-dominated atmospheres. The mixing ratio of O_2 monotonically decreases as the surface pressure increases at very high outgassing rates, whereas it increases as the surface pressure increases at lower-than-Earth outgassing rates. Abiotic O_2 can only build up to the detectable level, defined as 10^(−3) in volume mixing ratio, in 10-bar atmospheres with the Venus-like volcanic activity rate and the reduced outgassing rate of H_2 due to the high surface pressure. Our results support the search for biological activities and habitability via atmospheric O_2 on terrestrial planets in the habitable zone of Sun-like stars.

Files

Additional details