[Paper Review] Stability of CO2 Atmospheres on Desiccated M Dwarf Exoplanets

Peter Gao, Renyu Hu+3 more|arXiv (Cornell University)|Jan 27, 2015

Atmospheric Ozone and Climate1 references4 citations

TL;DR

This study investigates the chemical stability of CO2-rich atmospheres on dry, terrestrial exoplanets orbiting M dwarf stars using a 1D photochemical model. It identifies catalytic cycles involving H2O2 and O3 photolysis that preserve CO2, but shows that with H2 < 1 ppm, up to 40% of CO2 is converted to CO and O2 within 1 Myr, leading to abiotic O2 and O3 levels comparable to Earth's, detectable via long-wavelength spectroscopy.

ABSTRACT

We investigate the chemical stability of CO2-dominated atmospheres of desiccated M dwarf terrestrial exoplanets using a 1-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 (Luger &amp; Barnes 2015). 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 &lt;1 ppm, resulting in ~40% of the atmospheric CO2 being converted to CO and O2 on a time scale 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 &lt;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 microns.

Motivation & Objective

• To assess the chemical stability of CO2-dominated atmospheres on desiccated terrestrial exoplanets around M dwarf stars.
• To understand how high levels of far-ultraviolet (FUV) radiation from M dwarfs affect CO2 photolysis and atmospheric evolution.
• To investigate the role of residual water and hydrogen in sustaining catalytic cycles that preserve CO2.
• To determine the conditions under which abiotic O2 and O3 can accumulate in such atmospheres.
• To evaluate the detectability of these abiotic O2/O3 signatures through spectroscopy at wavelengths >0.95 microns.

Proposed method

• A 1-dimensional photochemical model is used to simulate atmospheric chemistry on desiccated M dwarf exoplanets.
• The model incorporates photolysis of CO2, H2O2, and O3 by far-ultraviolet (FUV) radiation, with reaction rates dependent on stellar FUV flux.
• Catalytic cycles involving H2O2 and O3 are evaluated for their ability to regenerate CO2 from photolyzed products.
• The impact of low hydrogen mixing ratios (<1 ppm) on the breakdown of these cycles is analyzed to determine CO2 loss rates.
• Recombination reactions of CO and O to form CO2 are considered as a direct regeneration mechanism in hydrogen-poor atmospheres.
• Spectral signatures of O2 and O3 are modeled to assess detectability in reflectance and emission spectra beyond 0.95 microns.

Experimental results

Research questions

• RQ1 How does high FUV flux from M dwarf stars affect the photolytic destruction of CO2 in dry exoplanet atmospheres?
• RQ2 What catalytic cycles maintain CO2 stability in the absence of abundant water vapor?
• RQ3 At what hydrogen mixing ratio does the H2O2-based catalytic cycle fail, leading to significant CO2 loss?
• RQ4 What are the resulting abundances of CO and O2 after CO2 photolysis under low-H2 conditions?
• RQ5 Can abiotic O2 and O3 levels be distinguished from biologically produced ones using long-wavelength spectroscopy?

Key findings

• For hydrogen mixing ratios below 1 ppm, up to 40% of the atmospheric CO2 is converted to CO and O2 within approximately 1 Myr due to the breakdown of the H2O2 catalytic cycle.
• The photolysis of O3 produces O atoms that regenerate CO2 through reaction with CO, forming a secondary catalytic cycle that stabilizes CO2 in low-hydrogen environments.
• In atmospheres with hydrogen below 0.1 ppm, direct recombination of CO and O produces CO2, providing a third CO2-regenerating pathway.
• These catalytic mechanisms limit CO2 photolytic destruction to no more than ~50%, preserving significant CO2 levels.
• The resulting O2 and O3 abundances can reach Earth-like levels through abiotic processes, even in the absence of life.
• The absence of water features in reflectance and emission spectra at wavelengths longer than 0.95 microns can help distinguish abiotic O2/O3 from biological sources.