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

Abstract

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 H 2 is released together with CH 4, a good agreement is found between the present CH 4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈10 3 yr. If H 2 is released together with CH 4, as expected if CH 4 originates in serpentinization, the average level of CH 4 during the last 10 3 yr should have been at least ten times lower than the present one. The lack of present H 2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H 2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO 2 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.