THE TRAPPING OF Ar, Kr, AND Xe IN MARTIAN CLATHRATES AND THE POSSIBILITY OF DETECTING CLATHRATES ON MARS BY SEASONAL CHANGES IN THE Xe/Kr RATIO. T. D. Swin-

Content Provider	Semantic Scholar
Author	Thomas, Caroline Mousis, Olivier Lunine, Jonathan I. Picaud, Sylvain
Copyright Year	2009
Abstract	Introduction: The existence of hydrate clathrates on Mars could be a substantial factor in the distribution and total inventory of that planet’s volatiles. Clathrates have been discussed as a potential major reservoir for CO2 [1], methane [2-4], and the noble gases krypton and xenon [5, 6]. Various authors have proposed that clathrates could be crucial for understanding Martian geomorphology [7-11], the presence and abundance of methane in the Martian atmosphere [2, 3], Mars’ atmospheric history as deduced from noble gas outgassing [5, 12], and the abundance patterns of noble gases in Martian meteorites [6]. However, the thermodynamic stability field of clathrates is such that conditions on Mars are tantalizingly close to the stability boundary [13], so it is not known, and cannot be calculated a priori, whether clathrates are abundant, nonexistent, or something in between. Hence it is important both to determine what their effects might be, and to search for ways to test for their presence. Calculations: To calculate the relative abundance of the different species incorporated in clathrate hydrates on Mars, we have used an approach based on the statistical model developed by van der Waals and Platteuw [14]. However, it differs from this latter approach by the use of experimentally determined dissociation curves in our code instead of calculated dissociation pressures [4, 15, 16]. This allows us to determine the relative abundances in clathrate hydrates down to very low temperatures whereas existing codes such as the CSMHYD program [17] are restricted to higher temperatures. Indeed, the dissociation pressure of the multiple guest clathrate in the CSMHYD program is calculated in an iterative way that requires the equality between the chemical potential in the clathrate phase and that in the gas phase. The determination of this equilibrium requires knowledge of the thermodynamics of an empty hydrate, such as the chemical potential, enthalpy and volume difference between ice (chosen as a reference state) and the empty hydrate. The experimental data available at standard conditions (T = 273.15 K, P = 1 atm) allow the CSMHYD program to calculate chemical potentials, and hence dissociation curves, as long as the temperature and pressure is not too far from the reference point. This method fails to converge at temperatures below 140 K for the clathrates considered in this study. Our approach avoids this problem, because it uses experimentally determined dissociation curves, which are valid down to low temperatures. Our approach relies on four key assumptions: the host molecules contribution to the free energy is independent of the clathrate occupancy (this assumption implies in particular that the guest species do not distort the cages), the cages are singly occupied, there are no interactions between guest species in neighboring cages, and classical statistics is valid, i.e., quantum effects are negligible [17]. We considered a Martian atmosphere whose composition is dominated by CO2, N2, Ar, Kr, Xe, O2 and CO, which derives from the data published by [18]. Moreover, since methane has recently been detected in the Martian atmosphere [19-21] and because its abundance might have varied over the ages, we consider initial gas phase abundances of CH4 up to 1%, although it turned out the methane abundance had a very small effect on Kr and Xe. For clathrate formation at a total pressure of 7 mbar, typical of the current Martian atmosphere, the relative abundance of Xe in the clathrate would be 33 times that the atmospheric Xe abundance, while that of Kr would be 0.57 times its atmospheric abundance. Argon is poorly incorporated into clathrates – its relative abundance in the clathrate would be less than 2% of its atmospheric abundance.
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Language	English
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Resource Type	Article