Other
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..842t&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Other
Scientific paper
Introduction Recent observations have evidenced traces of methane (CH4) heterogeneously distributed in the Martian atmosphere [1]. However, because the lifetime of CH4 in the atmosphere of Mars is estimated to be around 250-430 years on the basis of gas-phase chemistry [1], its actual sources on Mars remain controversial. Among other assumptions, it has been proposed [2] that clathrate hydrates located in the subsurface of Mars could be at the origin of the small quantities of CH4 detected. In the present work, we have calculated the relative abundance of CH4 in clathrate hydrates on Mars, using a statistical model based on the theory of van der Waals and Platteeuw [3]. Thermodynamic approach To calculate the relative abundance of CH4 incorporated in clathrates formed from the martian atmosphere, we use the same approach as in our previous studies devoted to the trapping of gases by clathrates on Titan [4][5]. This approach is based on the statistical model proposed by van der Waals and Platteuw [3] and it is only briefly summarized here. In this formalism, the relative abundance fG of a guest species G in a clathrate (of structure I or II) is defined as the ratio of the average number of guest molecules of species G in the clathrate over the average total number of enclathrated molecules, as : fG = bLyG;L + bSyG;S bL P J yJ;L + bS P J yJ;S ; (1) where the sums in the denominator run over all species present in the system, and bS and bL are the number of small and large cages per unit cell, respectively. The occupancy fractions yG of the guest species G for a given type of cage and for a given type of clathrate are determined from the Langmuir constants which are related to the strength of the interaction between each guest species and each type of cage. Thus, this statistical approach relies on the accurate determination of the interactions between the guest species G and the water molecules forming the surrounding cage. In a first approximation, this cage is assumed to be spherical and the corresponding interactions are represented by a spherically averaged Kihara potential, the integration of which within the cage giving the Langmuir constants. Results Figure 1 shows that irrespective of the initial gas phase abundances, the relative abundances of Ar, N2 and CH4 slightly increase in clathrates when the formation temperature increases, whereas that of CO2 slightly decreases. However, irrespective of their initial gas phase abundances, Ar and N2 are poorly trapped. Similarly, at low initial gas phase abundances (less than a few percent), methane is poorly trapped in clathrates (see Figs. 1.a to 1.d). On the other hand, at higher initial gas phase abundances (greater than 10%), the trapping efficiency of methane in clathrates significantly increases. We note that the most abundant guest in clathrates is CO2, except in the cases where the gas is dominated by methane. 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. References [1] Krasnopolsky, V. et al. (2004) ICARUS, 172, 537-547. [2] Chastain, B. K. & Chevrier, V. (2007) Planet. Space Sci., 55, 1246-1256. [3] van der Waals, J.H. & Platteeuw, J.C. (1959) Adv. Chem. Phys., 2, 1-57. [4] Thomas, C. et al. (2007) A&A, 474, L17-L20. [5] Thomas, C. et al. (2008), Planet. Space Sci., doi:10.1016/j.pss.2008.04.009
Ballenegger Vincent
Mousis Oliver
Picaud Sébastien
Schmitt Bernard
Thomas Caroline
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