Isotopic fractionation of methane in the martian atmosphere

The existence of methane in the martian atmosphere may be an indicator of subsurface life. Biological processes are known to fractionate the common isotopologues of methane, and hence measuring these isotopic ratios may yield constraints on the nature of the methane source. Any measurement of the isotopic ratios of atmospheric methane must consider the additional fractionation due to photochemistry in order to quantify the isotopic ratios of the source. Using a one-dimensional photochemical model, we find that photochemistry has a small (4.5‰) contribution to δ13C(CH4) but has a large (114‰) contribution to δD(CH4). Confirmation of these fractionation values will require additional laboratory data on key model inputs, particularly the ultraviolet absorption cross sections of 13CH4 and kinetic rate coefficients for the reactions of 13CH4 and CH3D with OH and O(1D) at pressures and temperatures relevant to the martian atmosphere.