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First-Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan



Robert Hellmann, Allan H. Harvey


Recent work used the kinetic theory of molecular gases, along with state-of-the-art intermolecular potentials, to calculate the diffusivity ratios necessary for modeling kinetic fractionation of water isotopes in air. Here, we extend that work to the Martian atmosphere, employing potential-energy surfaces for water with carbon dioxide and with nitrogen. We similarly derive diffusivity ratios for methane isotopes in the nitrogen atmosphere of Titan. Surprisingly, the simple hard-sphere theory that is inaccurate for Earth's atmosphere is in good agreement with the rigorous results for water isotopes in the Martian atmosphere. A modest disagreement with the hard-sphere result is observed for the diffusivity ratio of CH$_3$D in the atmosphere of Titan. We present temperature-dependent correlations, as well as estimates of uncertainty, for these diffusivity ratios, providing for the first time the necessary data to model kinetic isotope fractionation in these environments.
Journal of Geophysical Research-Planets


Diffusivity, Isotopes, Mars, Methane, Titan, Water


Hellmann, R. and Harvey, A. (2021), First-Principles Diffusivity Ratios for Atmospheric Isotope Fractionation on Mars and Titan, Journal of Geophysical Research-Planets, [online],, (Accessed April 24, 2024)
Created April 22, 2021, Updated November 29, 2022