The Case and Context for Atmospheric Methane as an Exoplanet Biosignature (2204.04257v1)

Abstract: 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 CH$_4$ and CO$_2$ with limited CO due to the strong redox disequilibrium between CH$_4$ and CO$_2$. 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 CH$_4$ that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO$_2$ with comparatively little CO.

• The paper establishes that methane’s short photochemical lifetime implies active replenishment, supporting its potential biogenic origin.
• The paper employs photochemical and geochemical models to distinguish biogenic methane from abiotic sources using the CH4/CO2 ratio.
• The paper underscores the need for multi-spectral observations, particularly with JWST, to accurately interpret methane detections in exoplanet atmospheres.

Methane as an Exoplanet Biosignature: Implications and Detection Strategies

The paper of atmospheric methane as a potential biosignature for exoplanets presents both an intriguing opportunity and a significant challenge for exoplanetary science. The paper "The Case and Context for Atmospheric Methane as an Exoplanet Biosignature," by Thompson et al., provides an in-depth analysis of the viability of methane as a biosignature and sets forth a framework to differentiate between biogenic and abiotic sources. As forthcoming observations, primarily with the James Webb Space Telescope (JWST), aim to detect these biosignatures, this research becomes particularly timely.

Atmospheric Methane as a Biosignature

Methane (CH4) has long been considered a potential biosignature due to its significant presence in Earth's biogenic activities. The paper rigorously argues that methane’s short photochemical lifetime in planetary atmospheres renders its sustained presence indicative of active replenishment processes, potentially of biological origin. On Earth, these processes are predominantly driven by methanogenesis accomplished by anaerobic microorganisms. The research establishes criteria under which methane might serve as a compelling biosignature in exoplanetary atmospheres:

• High atmospheric methane concentrations with implied surface fluxes exceeding known abiotic sources.
• Presence of a terrestrial bulk density, suggesting limited surface volatile reservoirs.
• Anoxic atmospheres that accompany high CH4-to-CO2 ratios, highlighting a state of redox disequilibrium.

Differentiating Biogenic from Abiotic Methane

Thompson et al. present a comprehensive analysis of abiotic processes capable of generating methane, including volcanic outgassing, low-temperature water-rock reactions, and impacts from comets and asteroids. The authors utilize photochemical models to demonstrate that abiotic methane production is unlikely without concurrent significant production of CO or other byproducts such as hydrogen.

A crucial finding of the paper is that for most terrestrial planets, abiotic methane production would also lead to either rapid depletion of available reductants like H2 or significant simultaneous outgassing of CO2, which would shift the atmospheric redox equilibrium away from the state observed when methane is biogenically sustained. This premise supports the use of the CH4/CO2 ratio as a diagnostic measure to distinguish between abiotic and potentially biogenic methane on exoplanetary atmospheres.

Implications for Exoplanetary Observations

With the capability of JWST to detect methane in exoplanetary atmospheres, the paper underscores the importance of contextualizing methane detections within a framework that considers planetary, astrophysical, and geochemical contexts. The paper suggests that beyond detecting methane, constraining the atmospheric CO and O2 levels is crucial for eliminating false positives that might arise from non-biological processes.

Looking forward, the paper implies the necessity of using multiple observational tools and methods. For instance, large infrared/optical/ultraviolet telescopes can complement methane detections by providing data on atmospheric composition and potential biosignature pairs, such as CH4 and CO2, paired with low CO to distinguish false positives.

Conclusions and Future Directions

The paper by Thompson et al. opens pathways for further research and technological development geared towards the nuanced analysis of biosignatures in exoplanetary atmospheres. It advocates for comprehensive models that integrate photochemistry, geochemistry, and atmospheric sciences to better understand and interpret potential biosignatures. This research lays down a robust framework essential for the upcoming era of exoplanetary exploration, where detecting life beyond Earth is becoming increasingly feasible. The insights provided by this paper will be crucial for interpreting future observations, nudging the scientific community closer to answering whether life exists beyond our planet.