Higher Martian Atmospheric Temperatures At All Altitudes Increase The D/H Fractionation Factor And Water Loss

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.