Carbon dioxide, argon, nitrogen and methane clathrate hydrates: Thermodynamic modelling, investigation of their stability in Martian atmospheric conditions and variability of methane trapping

Abstract

This paper deals with the stability of clahtrate hydrates at low temperature and low pressure in the conditions prevailing in the atmosphere and at the surface of Mars. We fit the classical van der Waals model to compare deviation from experimental results published in the literature. It appears to be acceptable and allows to simulating a Martian gas, CO2 dominated (95.3%) plus nitrogen (2.7%) and argon (2%). The hydrate is a CO2-based hydrate which is unstable during Mars’ summer and stable during Mars’ winter. The proportion of methane in the hydrate is estimated and is found to be from one tenth to one third of the composition of the gas phase. The proportion depends on the crystallographic structure which is assumed to be formed. In fact, both the structure I and II appear to be stable in the conditions of Mars’ winter. The consequences of these results on our understanding of the atmospheric cycle of Martian methane are drawn and analyzed. We propose upper limits on (i) the seasonal variation of methane due to a hypothesized alternate formation of CO2/N2/Ar/CH4 hydrates on the seasonal polar caps, and (ii) on the lifetime of atmospheric methane with respect to an hypothesized continuous trapping under hydrate form on the south polar cap. We show that these mechanisms have only small effects, and cannot play a significant role in the dynamics of methane in present Mars’ atmosphere. Hypothesized clathrate hydrates trapped in the permanent south polar cap could include methane in relative proportions between 0.1 and 0.4 times the average global atmospheric ratio. Searching for the spectral signatures of clathrate hydrates on the caps and, if detectable, of inhomogeneities of the CH4 mixing ratio in possible local atmospheric plumes forming during the sublimation of polar hydrate pockets in spectroscopic data from existing (Mars Express, MRO) and future (TGO-Exomars) missions is an interesting challenge of Mars science and astrobiology.