2015ApJ...806..249G

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 H₂O₂photolysis can maintain a CO₂atmosphere. However, this cycle breaks down for atmospheric hydrogen mixing ratios <1 ppm, resulting in ∼40% of the atmospheric CO₂being converted to CO and O₂on a timescale of 1 Myr. The increased O₂abundance leads to high O₃concentrations, the photolysis of which forms another CO₂-regenerating catalytic cycle. For atmospheres with <0.1 ppm hydrogen, CO₂is 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) O₂and 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 O₂and O₃in 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.