5.7 Methane clumped signatures in astrobiology
Our experiments suggest methane produced by abiogenesis on other worlds may yield apparent Δ12CH2D2 deficits. This is because the difference between equation 6 and 9 remains insensitive to the absolute value of the D/H ratio; only D/Hdifferences between various hydrogen pools matter in causing combinatorial effects (Rockmann et al., 2016, Taenzer et al., 2020), not the absolute values of the source pools. Since D/H differences inevitably follow from lower-T hydrogen additions to carbon, we expect Δ12CH2D2 deficits to be common anywhere methane is synthesized from the reduction of CO or CO2. This means that on other worlds, where the D/H ratio of molecular hydrogen is unknown and could be extremely different from Earth, abiotic methane should still exhibit Δ12CH2D2 deficits comparable to the values observed here (Fig. 3). As on Earth, if this methane is entrained in a hydrothermal system with ambient temperatures approaching 300 °C, this signature may be replaced by isotopologue equilibrium, a situation that may be uncommon however given the low temperatures conditions prevailing on most other solar system bodies.
While biosignatures have been a long-term focus of investigation, both field and experimental consideration of potential abiosignatures has been flagged as a critical gap in the search for life beyond Earth (NASEM, 2022). As such, bond reordering of abiotic methane may be a useful tool. The in-situ discovery of methane gas in Mars’s atmosphere by Curiosity (Webster et al., 2015) indicates that methane is produced on the planet, likely from an on-going source. Methane is also detected in the plumes of Saturn’s moon Enceladus, together with molecular hydrogen (Waite et al., 2017). There, the origin of CH4is thought to be abiotic, derived from hydrothermal reactions occurring at depth in the oceans of Enceladus, but at unconstrained temperatures (Waite et al., 2017). A Bayesian analysis of the escape rates of Enceladus has recently challenged a hydrothermal origin of methane, and suggested it might potentially be biological (Affholder et al., 2021). It is not clear that conventional isotopic tools can yield unambiguous results as to the origin of methane on Mars and Enceladus, because sound interpretations of bulk isotope ratios by conventional means require contextual information: what is the meaning of methane D/H ratios with no constraints on the D/H of the co-existing molecular hydrogen? What is the overall 13C/12C of the carbon reservoirs from which methane might evolve? This is underscored by the complex interpretations of 13C/12C ratios measured on Mars rocks by Curiosity (House et al., 2022). On Mars and on Enceladus, if methane is synthesized abiotically at high temperature, we see no physical reason against the appearance of equilibrium Δ13CH3D and Δ12CH2D2 signatures resulting from reordering, as on Earth. Should Δ13CH3D and Δ12CH2D2 measurements ever be done for Enceladus methane, any equilibrium temperatures ≥ 250°C would rule out a microbial origin of methane.
A complication in developing methane isotopologues as tools for identifying biogenicity to extraterrestrial methane is that combinatorial effects are also associated with microbial methane and under the right conditions, with thermogenic methane (i.e., produced from thermal decomposition of organic matter). All of these processes can yield large Δ12CH2D2 deficits in the absence of bond reordering because all of them involve multiple steps of hydrogen addition and draw upon pools of hydrogen with distinct D/H. Experiments verify this effect for hydrogens with variable D/H causing Δ12CH2D2deficits in microbial methane (Taenzer et al., 2020). Current evidence suggests that microbial methane is unique in that it shows non-equilibrium Δ13CH3D values, together with markedly negative Δ12CH2D2 values, probably as the result of classical kinetics involving carbon. The release of methane during cracking of organic matter in the laboratory can result in large Δ12CH2D2 deficits with near equilibrium Δ13CH3D (Dong et al., 2021), and could thus be confused with purely abiotic methane. In any case, mass-18 methane isotopologues significantly aid us in differentiating microbial, thermogenic, and abiotic methane on other planets or icy moons independent of the unknown bulk13C/12C and D/H of sources.