Biosignatures & Paleobiology

The Case and Context for Atmospheric Methane as an Exoplanet Biosignature

By Keith Cowing
April 11, 2022
Filed under
The Case and Context for Atmospheric Methane as an Exoplanet Biosignature
Atmospheric CO to CH4 ratio may help distinguish biogenic and abiotic methane. Shown is ratio of atmospheric CO to CH4 for abiotic worlds and those with biospheres as a function of volcanic H2 flux. The curves show the calculated atmospheric CO/CH4 as a function of volcanic H2 flux for abiotic worlds (blue circles), H2-based biospheres (includes H2-consuming anoxygenic photosynthesis, CO-consuming acetogenesis, organic matter fermentation, and acetotrophic methanogenesis) (pink diamonds), H2-based and Fe-based photosynthesis biospheres (i.e., “hybrid,” orange triangles) from ref. 37, and the methanogen–acetogen ecosystem and anoxygenic phototroph– acetogen ecosystem from ref. 35 (i.e., their cases 2 and 3) (red squares). The horizontal shaded regions correspond to the distributions of atmospheric CO/CH4 for abiotic worlds (blue) and those with methanogenic biospheres (pink, yellow, and orange) as a function of volcanic H2 flux calculated by ref. 38. The atmospheric CO/CH4 for abiotic worlds is predicted to be several orders of magnitude greater than that for inhabited worlds. Refs. 35, 37, and 38 found that low CO/CH4 atmospheric ratios (∼0.1) are a strong sign of methanecycling biospheres for reducing planets orbiting Sun-like stars like Archean Earth, suggesting that atmospheric CO/CH4 is a good observable diagnostic tool to distinguish abiotic planets from those with anoxic biospheres. The light pink “+”-hatched region corresponds to an ecosystem with CO-based autotrophic acetogens (AG) and methanogenic acetotrophs (AT); the light orange “X”-hatched region corresponds to an ecosystem with H2-based methanogens (MG), AG, and AT; the orange “.”-hatched region corresponds to the most complex ecosystem consisting of MG, AG, AT, and anaerobic methanotrophy (MT) (38). All calculations assume a CO2-CH4-N2 bulk atmosphere.

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature.

Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that, in contrast to an Earth-like biosphere, known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH4 that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO2 with comparatively little CO.

Maggie A. Thompson, Joshua Krissansen-Totton, Nicholas Wogan, Myriam Telus, Jonathan J. Fortney

Comments: 10 pages, 5 figures, 15 pages Supplementary Information, 3 Supplementary Figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2204.04257 [astro-ph.EP] (or arXiv:2204.04257v1 [astro-ph.EP] for this version)
Submission history
From: Maggie Thompson
[v1] Fri, 8 Apr 2022 19:14:48 UTC (11,361 KB)

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