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.