STABILITY OF CO2 ATMOSPHERES ON DESICCATED M DWARF EXOPLANETS.

We investigate the chemical stability of CO₂-dominated atmospheres of desiccated M dwarf terrestrial exoplanets using a one-dimensional photochemical model. Around Sun-like stars, CO₂ photolysis by Far-UV (FUV) radiation is balanced by recombination reactions that depend on water abundance. Planets orbiting M dwarf stars experience more FUV radiation, and could be depleted in water due to M dwarfs’ prolonged, high-luminosity pre-main sequences. We show that, for water-depleted M dwarf terrestrial planets, a catalytic cycle relying on H₂O₂ photolysis can maintain a CO₂ atmosphere. However, this cycle breaks down for atmospheric hydrogen mixing ratios <1 ppm, resulting in ∼40% of the atmospheric CO₂ being converted to CO and O₂ on a timescale of 1 Myr. The increased O₂ abundance leads to high O₃ concentrations, the photolysis of which forms another CO₂-regenerating catalytic cycle. For atmospheres with <0.1 ppm hydrogen, CO₂ is produced directly from the recombination of CO and O. These catalytic cycles place an upper limit of ∼50% on the amount of CO₂ that can be destroyed via photolysis, which is enough to generate Earth-like abundances of (abiotic) O₂ and O₃. The conditions that lead to such high oxygen levels could be widespread on planets in the habitable zones of M dwarfs. Discrimination between biological and abiotic O₂ and O₃ in this case can perhaps be accomplished by noting the lack of water features in the reflectance and emission spectra of these planets, which necessitates observations at wavelengths longer than 0.95 μm. [ABSTRACT FROM AUTHOR]