STABILITY OF CO₂ATMOSPHERES 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 2 O 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 2regenerating 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.

O2- and CO-rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets

“…The model has also been used to determine the photochemical oxygen buildup in abiotic atmospheres (Hu et al 2012;James & Hu 2018). For the latter purpose, the model has been compared with other photochemistry model in detail and we have found good agreement (Gao et al 2015;Harman et al 2018). The photochemistry model solves the one-dimensional chemical-transport equation for 111 O, H, C, N, and S species including sulfur and sulfuric acid aerosols.…”

“…Using the spectrum of the M dwarf GJ 876, atmospheric photochemistry models have predicted an O2 mixing ratio up to 5% in an N2-dominated atmosphere with 0.05 bar CO2 (Tian et al 2014;Domagal-Goldman et al 2014;Harman et al 2015). Atmospheric photochemistry models have also shown that massive O2 and CO would be produced from CO2-dominated atmospheres of desiccated -and hence uninhabitable -rocky planets of M dwarf stars (Gao et al 2015).…”

The Muscles Treasury Survey. I. Motivation and Overview*

“…O 3 photolysis is then driven by NUV and blue optical photons. Therefore, on planets orbiting stars with strong FUV and weak NUV flux, a substantial O 3 atmosphere may arise via photochemical processes alone (Segura et al 2010;Hu et al 2012;Gao et al 2015;Harman et al 2015).…”

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…High-energy stellar flux heats upper planetary atmospheres and initiates photochemistry (e.g., Lammer et al 2007;Miguel et al 2015;Rugheimer et al 2015;Arney et al 2017). UVdriven photochemistry can produce and destroy potential biosignatures (O 2 , O 3 , and CH 4 ) and habitability indicators (H 2 O and CO 2 ) in exoplanet atmospheres (Hu et al 2012;Domagal-Goldman et al 2014;Tian et al 2014;Wordsworth & Pierrehumbert 2014;Gao et al 2015;Harman et al 2015;Luger & Barnes 2015). In particular, the ratio of far-to near-UV flux determines which photochemical reactions will dominate and thus the resultant planetary atmosphere.…”