Pockets of methane in the shallow Martian subsurface. Implications for rapid and seasonal changes of the atmospheric methane on Mars.

Martian atmospheric methane remains a mystery. Recent discoveries of the abrupt and seasonal changes in the atmospheric methane (Webster et al., 2015,2018) require presence of some local methane sources which have not been identified. Such sources should be able to release significant amounts of methane on a short timescale. We propose that the pockets of methane can form just several cm below Martian surface. The mechanism of gas pockets formation involve migration of salts in the Martian soil due to sublimation of shallow subsurface ice or evaporation of briny water during RSL events. As water evaporates and ice sublimates from the top of the soil, salts remains in the top layer of soil causing soil cementation and formation of the gas diffusion barrier in the soil.

We conducted laboratory studies of the Mars analogues soils mixed with various amounts of salts and water exposed to a broad range of temperatures and pressures in the Mars simulation chamber. We injected neon as a proxy of methane at the bottom of the soil sample and monitored neon gas permeability through the soil sample by measuring gas pressure differential above and below the soil sample. We also monitored the neon flux through the soil using a RGA mass spectrometer. We found that under Martian-like atmospheric pressures, a mixture of JSC-Mars-1A and 5-10% of Mg perchlorate or sodium chloride produce gas impermeable soil caps capable to withstand an excess of 5 mbars of neon under the cap at the soil temperatures of up to +9 C. The layer of cemented soil is only 2 cm thick. Caps remain gas impermeable after subsequent cooling of the sample soil sample to the subzero temperatures. Gas permeability of the soil caps under various temperatures and atmospheric pressures will be reported at the conference.

Our results suggest that regardless of the nature of the methane source on Mars (e.g. biology, serpentinization, methane clathrates etc.), it is possible to accumulate methane in the gas pockets in the areas of shallow permafrost and abundant salts or on the RSL slopes. Gas pockets formed so close to the surface can be activated by either motion of the MSL rover itself, by impacts of small meteorites, or annual climate oscillations and therefore cause abrupt changes in the atmospheric methane abundance detected by MSL's Curiosity rover.