Methane spikes, background seasonality and non-detections on Mars: A geological perspective

The story of methane (CH4) on Mars continues to grow. A long series of CH4 detections (spikes, plumes, seasonal variations) and non-detections in the martian atmosphere suggest that CH4 is occasionally released from subsurface rocks, but also that an unknown process of rapid CH4 removal in the atmosphere must exist. The observations can be interpreted integrating concepts of subsurface geological gas source variability (CH4 seepage flux patterns), atmospheric chemistry (CH4 removal processes), and atmospheric circulation (CH4 transport and dilution). While the last two are widely studied, potential gas seepage patterns on Mars have never been discussed. In this work, we use the factors controlling gas seepage flux patterns observed on Earth to address key questions about the significance of the martian detections and non-detections. As on Earth, CH4 seepage on Mars must occur mainly through advection (non-diffusion) mechanisms, driven by pressure gradients and permeability, typically related to faults and fracture networks, which control release patterns, in terms of intensity, duration and variability. CH4 release may occur, therefore, whenever gas pressure and/or rock permeability increase (e.g., via destabilization of storage bodies such as clathrates or zeolites; breaching of seals such as permafrost; planetary stresses, impacts, or gas pressure build-up along faults). Rapid atmospheric pressure changes, due to storms and winds, may also play a role. All these forcings can lead to local, episodic, irregular or seasonal gas releases, which may explain the observations acquired so far if relatively rapid (month or day time-scales) mechanisms of CH4 removal exist in the lower atmosphere or near-surface. The release patterns cannot reveal whether CH4 is biologic or abiotic. If CH4 sensors operate discontinuously and only in one site or over limited regions, then, non-detections will be the rule and detections will be relatively rare. The chances of CH4 detection may increase if the observations are focused and repeated on specific, potential areas of gas release determined by geological analysis.