Explaining the redox imbalance between the H and O escape fluxes at Mars by the oxidation of methane

From a comparison between the different observations of Martian methane existing today, including the new TES methane maps (Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H₂ is released together with CH₄, a good agreement is found between the present CH₄ flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈10³ yr. If H₂ is released together with CH₄, as expected if CH₄ originates in serpentinization, the average level of CH₄ during the last 10³ yr should have been at least ten times lower than the present one. The lack of present H₂ release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H₂ has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO₂ ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr^?1, consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr.