Numerical modelling of the transport of trace gases including methane in the subsurface of Mars
We model the transport of gas through the Martian subsurface in order to quantify the timescales of release of a trace gas with a source at depth using a Fickian model of diffusion through a putative Martian regolith column. The model is then applied to the case of methane to determine if diffusive transport of gas can explain previous observations of methane in the Martian atmosphere. We investigate which parameters in the model have the greatest effect on transport timescales and show that the calculated diffusivity is very sensitive to the pressure profile of the subsurface, but relatively insensitive to the temperature profile, though diffusive transport may be affected by other temperature dependent properties of the subsurface such as the local vapour pressure. Uncertainties in the structure and physical conditions of the Martian subsurface also introduce uncertainties in the timescales calculated. It was found that methane may take several hundred thousand Mars -years to diffuse from a source at depth. Purely diffusive transport cannot explain transient release that varies on timescales of less than one Martian year from sources such as serpentinization or methanogenic organisms at depths of more than 2 km. However, diffusion of gas released by the destabilisation of methane clathrate hydrates close to the surface, for example caused by transient mass wasting events or erosion, could produce a rapidly varying flux of methane into the atmosphere of more than 10-3 kg m-2s-1 over a duration of less than half a Martian year, consistent with observations of Martian methane variability. Seismic events, magmatic intrusions or impacts could also potentially produce similar patterns of release, but are far more complex to simulate.