Stability of CO_2 Atmospheres on Desiccated M Dwarf Exoplanets

Abstract

We investigate the chemical stability of CO_2-dominated atmospheres of desiccated M dwarf terrestrial exoplanets using a one-dimensional photochemical model. Around Sun-like stars, CO_2 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 H_2O_2 photolysis can maintain a CO_2 atmosphere. However, this cycle breaks down for atmospheric hydrogen mixing ratios <1 ppm, resulting in ~40% of the atmospheric CO_2 being converted to CO and O_2 on a timescale of 1 Myr. The increased O_2 abundance leads to high O_3 concentrations, the photolysis of which forms another CO_2-regenerating catalytic cycle. For atmospheres with <0.1 ppm hydrogen, CO_2 is produced directly from the recombination of CO and O. These catalytic cycles place an upper limit of ~50% on the amount of CO_2 that can be destroyed via photolysis, which is enough to generate Earth-like abundances of (abiotic) O_2 and O_3. 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 O_2 and O_3 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.

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