The origin of methane on Mars
Résumé
The recent detection of CH4 in Mars atmosphere at an average ~10-20 ppb level [1, 2, 3 ,4] suggests that, either a subsurface hydrothermal activity, or some biogenic sources could still be active or have been active in a recent past. Assuming that CH4 is produced at depth and transported up to the surface, it must necessarily experience a transition from gas to solid phase (CH4 clathrate) in the zone from 3 km up to a few meters depth [5, 6]. Assuming that the lifetime of methane is of the order of 100 days [7], the present redox imbalance between the escape fluxes of H and O might be explained by the oxidation of CH4 [8]. If so, the release rate of H2, expected to be produced in the same time as CH4 if the production mechanism is serpentinization, is not significant. The reason why no H2 is released could be that CH4 has been stored in subsurface clathrates before being released, whereas H2 has been lost early after the CH4-producing hydrothermal episode [8]. Another possible reason is that CH4 is formed through homogeneous equilibration of carbon in deep hot fluids [9, 10]. A third reason could be that H2 is consumed by bacterial activity in the crust. Interestingly, from a model of the hydrogen isotopic fractionation generated by the escape of serpentinization-derived hydrogen, only 20% of the presently released CH4 could be due to serpentinization, suggesting other sources (like biology) [10]. The possibility of a biological origin for the Martian CH4 will be discussed.