  Recent observations have evidenced traces of methane CH4 heterogeneously
distributed in the martian atmosphere. However, because the lifetime of CH4 in
the atmosphere of Mars is estimated to be around 300-600 years on the basis of
photochemistry, its release from a subsurface reservoir or an active primary
source of methane have been invoked in the recent literature. Among the
existing scenarios, it has been proposed that clathrate hydrates located in the
near subsurface of Mars could be at the origin of the small quantities of the
detected CH4. Here, we accurately determine the composition of these clathrate
hydrates, as a function of temperature and gas phase composition, by using a
hybrid statistical thermodynamic model based on experimental data. Compared to
other recent works, our model allows us to calculate the composition of
clathrate hydrates formed from a more plausible composition of the martian
atmosphere by considering its main compounds, i.e. carbon dioxyde, nitrogen and
argon, together with methane. Besides, because there is no low temperature
restriction in our model, we are able to determine the composition of clathrate
hydrates formed at temperatures corresponding to the extreme ones measured in
the polar caps. Our results show that methane enriched clathrate hydrates could
be stable in the subsurface of Mars only if a primitive CH4-rich atmosphere has
existed or if a subsurface source of CH4 has been (or is still) present.
