Modelled isotopic fractionation and transient diffusive release of methane from potential subsurface sources on Mars

Stevens, Adam H.; Patel, Manish R. and Lewis, Stephen R. (2017). Modelled isotopic fractionation and transient diffusive release of methane from potential subsurface sources on Mars. Icarus, 281 pp. 240–247.

DOI: https://doi.org/10.1016/j.icarus.2016.08.023

Abstract

We calculate transport timescales of martian methane and investigate the effect of potential release mechanisms into the atmosphere using a numerical model that includes both Fickian and Knudsen diffusion. The incorporation of Knudsen diffusion, which improves on a Fickian description of transport given the low permeability of the martian regolith, means that transport timescales from sources collocated with a putative martian water table are very long, up to several million martian years. Transport timescales also mean that any temporally varying source process, even in the shallow subsurface, would not result in a significant, observable variation in atmospheric methane concentration since changes resulting from small variations in flux would be rapidly obscured by atmospheric transport. This means that a short-lived 'plume' of methane, as detected by Mumma et al. (2009) and Webster et al. (2014), cannot be reconciled with diffusive transport from any reasonable depth and instead must invoke alternative processes such as fracturing or convective plumes.

It is shown that transport through the martian regolith will cause a significant change in the isotopic composition of the gas, meaning that methane release from depth will produce an isotopic signature in the atmosphere that could be significantly different than the source composition. The deeper the source, the greater the change, and the change in methane composition in both δ¹³C and δ_D approaches -1000 ‰ for sources at a depth greater than around 1 km. This means that signatures of specific sources, in particular the methane produced by biogenesis that is generally depleted in ¹³CH₄ and CH₃D, could be obscured. We find that an abiogenic source of methane could therefore display an isotopic fractionation consistent with that expected for biogenic source processes if the source was at sufficient depth. The only unambiguous inference that can be made from measurements of methane isotopes alone is a measured δ¹³C or δ_D close to zero or positive implies a shallow, abiogenic source. The effect of transport processes must therefore be carefully considered when attempting to identify the source of any methane observed by future missions, and the severe depletion in heavier isotopologues will have implications for the sensitivity requirements for future missions that aim to measure the isotopic fractionation of methane in the martian atmosphere.

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