Diffusive and advective transport of methane in the martian subsurface

During the whole geological history of Mars, methane formation mechanisms could have taken place in the deep subsurface and could still be active nowadays. After its generation, methane would migrate upwards and be either directly released at the surface or trapped in subsurface reservoirs (clathrates, zeolites or sealed traps) where it could eventually accumulate over long time periods before to be episodically liberated during destabilizing events. In this work, methane transport through the martian subsurface is studied using a one-dimensional numerical model considering adsorption onto, advection and diffusion through the regolith. The total CH4 flux is given by the sum of the advective flux calculated via Darcy’s law and the flux resulting from molecular and Knudsen diffusion. The latter is determined using the mean transport pore model. Surface diffusion is generally not significant and is thus not taken into account in this study. Pressure diffusion and thermal diffusion are also neglected as their contribution is rather small compared to molecular diffusion (concentration gradients) and Knudsen diffusion, which are normally considered to be the most important diffusive processes in soils. Finally, adsorption is modeled similarly to Meslin et al. (2011). Experimental methane fluxes from clathrate dissociation are imposed at the base of the model to describe the lower boundary condition. The CH4 subsurface reservoirs are assumed to be located several meters to several tens of meters below the surface following the clathrate stability zone