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

Tre’Shunda James1,2 and Renyu Hu1,3
Astrophysical Journal 867, 17 Link to Article [DOI: 10.3847/1538-4357/aae2bb]
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
2Occidental College, Los Angeles, CA 90041, USA
3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Atmospheric chemistry models have shown that molecular oxygen can build up in CO2-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 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 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 CO2-dominated atmospheres. The mixing ratio of O2 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 O2 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 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.

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