Plain language summary
Methane is observed in various environmental settings on Earth, including but not limited to hydrothermal fluids and sedimentary systems. It is also found on other worlds, as it was reported to occur in Mars’s rocks, in Enceladus’s geysers, and as of recently, in an exoplanet’s atmosphere. Methane may be formed by life, by the degradation of organic matter, or by abiotic chemistry. However, identifying synthesis pathways has proven challenging because of the lack of unambiguous signatures of the provenance of methane molecules. We performed synthesis of abiotic methane in the laboratory and determined its composition. We observe isotopic compositions that are consistent with carbon isotope equilibrium associated with hydrogen isotope disequilibrium. These isotope signatures are non-unique, in contrast with mass-18 isotopologues data,13CH3D and12CH2D2. While Δ13CH3D values approximately track experimental temperature, Δ12CH2D2 values are extremely depleted, far below equilibrium. The data are explained by kinetically fractionated D/H pools contributing to methane molecules, i.e., a combinatorial effect. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically on Earth and on other worlds.