Numerical modelling of the transport of trace gases including methane in the subsurface of Mars

Adam H Stevens,Manish R Patel,Stephen R Lewis

doi:10.1016/j.icarus.2014.12.033

Abstract

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 2km. 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 10−3kgm−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.

Highlights

Processes active in the subsurface of Mars are currently inaccessible to conventional instruments
Many proposed processes would have products that might be observed in the martian atmosphere
The mutually supportive detection of methane in the martian atmosphere by a number of different teams (Formisano et al, 2004; Mumma et al, 2009; Fonti and Marzo, 2010; Geminale et al, 2011; Mellon and Jakosky, 1993) has remained controversial (Zahnle et al, 2011) and recent in situ measurements have set an upper limit of atmospheric methane abundance around ten times lower than these observations (Webster et al, 2013)

Summary

Introduction

Processes active in the subsurface of Mars are currently inaccessible to conventional instruments. Many proposed processes would have products that might be observed in the martian atmosphere. The putative discovery of methane has led to the proposal of a number of potential sources in the martian atmosphere, surface and subsurface (Atreya et al, 2011). Many of these proposed sources would occur deep in the martian crust, and the observed methane could be an indication of active geological processes, where ultramafic rocks react with water in serpentinization reactions (Oze and Sharma, 2005), or could suggest the presence of methanogenic organisms in colonies hundreds of metres to kilometres down (Mancinelli, 2000). The formation of MCH would still require a genetic source to provide methane from below

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