A Theoretical Investigation of the Influence of Clathrate Hydrates on the Atmosphere of Mars

Introduction: Recently, a small quantity of methane (≈ 10 ppbv) has been detected in the atmosphere of Mars by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft [1]. The photochemical mean lifetime of the martian atmospheric methane is ≈ 300-600 years [1,2], and so it should not still exist today. To explain its presence, several scenarios have been invoked, like the release of methane from a subsurface reservoir, or the existence of an active biological (organisms living in the near subsurface of the planet [1,2,3]) or geological (e.g. olivine hydratation in the martian regolith or crust [4]) primary source of methane. The martian atmospheric methane could also come from the decomposition of possible methane clathrate hydrates in the near-subsurface [5,6]. Indeed, because they can trap methane over large timescales, clathrate hydrates could be a secondary reservoir, filled either by ancient or by current methane sources [5,6]. Such a mechanism has recently been studied by Chastain and Chevrier [6] with the program CSMHYD developed by Sloan [7], and for a model of an atmosphere containing only CO2 and CH4. We reinvestigate here this work, by using a statistical thermodynamic model based on experimental data and on the original work of van der Waals and Platteeuw [8]. This model enables calculations at lower temperatures than the CSMHYD, and for an initial gas phase containing more species. It is thus possible to study the composition of clathrate hydrates formed from the martian atmosphere, at temperatures even as lower as the extreme ones measured in the polar caps (≈ 130 K [9]).