Much of the water that once flowed on the surface of Mars was lost to space long ago, and the total amount lost remains unknown. Clues to the amount lost can be found by studying hydrogen (H) and its isotope deuterium (D), which are produced when atmospheric water molecules H₂O and HDO dissociate. The difference in escape efficiencies of H and D (which leads to an enhanced D/H ratio) is referred to as the fractionation factor f. Both the D/H ratio and f are necessary to estimate water loss; thus, if we can constrain the range of f and understand what controls it, we will be able to estimate water loss more accurately. In this study, we use a 1D photochemical model of the neutral Martian atmosphere to determine how f depends on assumed temperature and water vapor profiles. We find that the exobase temperature most strongly controls the value of f for thermal escape processes. When we include estimates of nonthermal escape from other studies, we find that the tropopause temperature is also important. Overall, for the standard Martian atmosphere, f = 0.002 for thermal escape, and f = 0.06 for thermal + nonthermal escape. We estimate that Mars has lost at minimum 66–122 m global equivalent layer of water. Importantly, our results demonstrate that the value of f depends critically on nonthermal escape of D, and that modeling studies that include D/H fractionation must model both neutral and ion processes throughout the atmosphere.

中文翻译：

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在所有高度都较高的火星大气温度会增加D / H分馏因子和水分流失

很久以前，曾经流过火星表面的大部分水都流失到太空，而损失的总量仍然未知。可以通过研究氢（H）及其同位素氘（D）来发现损失量很大的线索，这些氢是由大气水分子H ₂ O和HDO解离而产生的。H和D的逸出效率之差（导致D / H比提高）被称为分馏因子f。D / H比和f都是估算失水量所必需的；因此，如果我们可以限制f的范围并了解它的控制因素，我们将能够更准确地估计水的损失。在这项研究中，我们使用了中性火星大气中的光化学一维模型，以确定如何˚F取决于假定的温度和水蒸气分布。我们发现外基质温度最强烈地控制着热逃逸过程的f值。当我们从其他研究中得出非热逸出的估计值时，我们发现对流层顶温度也很重要。总体而言，对于标准火星大气层， 热逃逸的f = 0.002  ，热+非热逃逸的f = 0.06。我们估计火星至少损失了66-​​122 m全球等效水层。重要的是，我们的结果表明f的值主要取决于D的非热逸散，并且包括D / H分级的建模研究必须模拟整个大气层的中性和离子过程。