Recent work used the kinetic theory of molecular gases, along with state‐of‐the‐art intermolecular potentials, to calculate from first principles 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 the interaction of water with carbon dioxide and with nitrogen. We also derive diffusivity ratios for methane isotopes in the atmosphere of Titan by using a high‐quality potential for the methane‐nitrogen pair. The Mars calculations cover 100–400 K, while the Titan calculations cover 50–200 K. Surprisingly, the simple hard‐sphere theory that is inaccurate for Earth's atmosphere is in good agreement with the rigorous results for the diffusion of water isotopes in the Martian atmosphere. A modest disagreement with the hard‐sphere results is observed for the diffusivity ratio of CH₃D in the atmosphere of Titan. We present temperature‐dependent correlations, as well as estimates of uncertainty, for the diffusivity ratios involving HDO, H₂¹⁷O, and H₂¹⁸O in the Martian atmosphere, and for CH₃D and ¹³CH₄ in the atmosphere of Titan, providing for the first time the necessary data to be able to model kinetic isotope fractionation in these environments.

中文翻译：

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火星和土卫六大气同位素分馏的第一原理扩散率比


最近的工作使用分子气体动力学理论以及最先进的分子间势，根据第一原理计算模拟空气中水同位素动力学分馏所需的扩散率。在这里，我们将这项工作扩展到火星大气，利用势能表面来实现水与二氧化碳和氮气的相互作用。我们还通过使用甲烷-氮对的高质量势来推导土卫六大气中甲烷同位素的扩散率。火星的计算范围为 100-400 K，而土卫六的计算范围为 50-200 K。令人惊讶的是，对于地球大气层来说不准确的简单硬球理论与火星中水同位素扩散的严格结果非常一致。气氛。土卫六大气中 CH ₃ D 的扩散率与硬球结果略有不同。我们提出了火星大气中涉及 HDO、H ₂ ¹⁷ O 和 H ₂ ¹⁸ O 的扩散率比以及土卫六大气中 CH ₃ D 和¹³ CH ₄的与温度相关的相关性以及不确定性估计。 ，首次提供了能够在这些环境中模拟动力学同位素分馏的必要数据。