Methane: the Ideal Biosignature for the JWST Era?

The next phase of exoplanet science will focus on characterizing exoplanet atmospheres, including those of low-mass, terrestrial planets. A comprehensive understanding of possible biosignatures that may be detected with the next generation of ground and space telescopes is warranted. While some biosignature gases, such as oxygen and phosphine, have recently been reviewed in depth (Meadows et al. 2018 and Sousa-Silva et al. 2020), these will likely be extremely difficult to detect with JWST. In contrast, while it has not been thoroughly reviewed, methane at Earth-like biogenic fluxes is one of the only biosignatures that may be readily detectable with JWST (Krissansen-Totton et al. 2018a). In fact, an early Earth-like, methane-rich atmosphere would be easier to detect with JWST than modern Earth’s oxygen-rich atmosphere (ibid). Here we present our preliminary work on a comprehensive review of methane biosignatures and false positives. Biogenic methane production, or methanogenesis, is a simpler metabolism than oxygenic photosynthesis, that is carried out by anaerobic microbes (i.e., those not requiring oxygen for growth). Methanogens use either CO2 and H2 or acetate as substrates (Schwieterman et al. 2018). This process could be widespread due to the likely ubiquity of the CO2+H2 redox couple in terrestrial planet atmospheres, and the antiquity of methanogenesis on Earth (Wolfe and Fournier 2018). We briefly review the current understanding of the origin and evolution of methanogens, the organisms responsible for methanogenesis, and how this process relates to origins-of-life theories. When CH4 is invoked as a possible biosignature it is often included with a strongly oxidizing companion gas (e.g., CO2 or O2/O3). This is because it is difficult to explain abundant CH4 if a terrestrial planet atmosphere’s redox state is more oxidizing so that the thermodynamically stable form of carbon would not be CH4 (Schwieterman et al. 2018). However, even in atmospheres devoid of oxygen, CH4 has a short photochemical lifetime on habitable zone rocky planets, and the large fluxes required to sustain high CH4 abundances are likely much greater than could be supplied by abiotic processes (e.g., magmatic outgassing, serpentinization) (Krissansen-Totton et al. 2018b, Wogan et al. 2020). In addition, many abiotic, geological processes that produce CH4 are expected to also produce abundant CO, which life readily consumes, so the presence of CH4 and CO2 but absence of CO strengthens the case for biogenicity (Krissansen-Totton et al. 2018b). Although CH4+CO2 (minus CO) might coexist in thermodynamic equilibrium on planets without large surface oceans (Woitke et al. 2020), in practice, such atmospheres would be photochemically unstable and, in particular, the CH4 would have a short lifetime (less than ~1 Myrs). In addition to briefly discussing methane on Mars and Titan, we review the presence of methane in Jovian and sub-Neptune planet atmospheres. In many giant planets, methane is the most abundant carbon-bearing gas and can be replenished indefinitely because, although methane is photodissociated in the upper atmosphere, hydrogen is never depleted and carbon and hydrogen can recombine deeper in the atmosphere where temperatures and pressures are high enough for methane production to be thermodynamically favorable and kinetically viable (Moses et al. 2013). On the other hand, terrestrial planets with high mean molecular weight atmospheres do not have deep enough atmospheres to replenish methane without an additional source (abiotic or biotic). In terrestrial atmospheres without a replenishment source, methane is photodissociated and hydrogen is lost to space on short timescales (~10s of thousands of years for ~1 bar atmospheres). For planets in the sub-Neptune regime, we seek to determine how much atmosphere is necessary for a planet to sustain methane via thermodynamic recombination against photodissociation. In summary, for terrestrial planets to have methane-rich atmospheres, the methane must be constantly replenished. We explore to what extent abiotic CH4 replenishment is possible based on prior work on abiotic methane sources including water-rock reactions, volcanic outgassing, and impacts (e.g., Etiope & Lollar 2013, Wogan et al. 2020, Kress & McKay 2003). We review methane false positives on terrestrial planets and determine if they are likely to produce methane fluxes as large as those caused by known biogenic sources. Through this comprehensive review, we will develop a framework for identifying methane biosignatures and discuss detectability prospects with JWST.