LAUSR

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 H$_2$O 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 non-thermal 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 + non-thermal escape. We estimate that Mars has lost at minimum 66-122 m GEL of water. Importantly, our results demonstrate that the value of $f$ depends critically on non-thermal escape of D, and that modeling studies that include D/H fractionation must model both neutral and ion processes throughout the atmosphere.