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 ) , both of which are produced when atmospheric water molecules H _ { 2 } O and HDO dissociate .
The freed H and D atoms then escape to space at different rates due to their different masses , leaving an enhanced D/H ratio .
The rate of change of D/H 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 , we will be able to estimate water loss more accurately .
In this study , we use a 1D photochemical model of the Martian atmosphere to determine how f depends on assumed temperature and water vapor profiles .
We find that for most Martian atmospheric conditions , f varies between 10 ^ { -1 } and 10 ^ { -5 } ; for the standard Martian atmosphere , f = 0.002 for thermal escape processes , and f \approxeq 0.06 when both thermal and non-thermal escape are considered .
Using these results , we estimate that Mars has lost at minimum 66-123 m GEL of water .
Our results demonstrate that the value of f is almost completely controlled by the amount of non-thermal escape of D , and that photochemical modeling studies that include fractionation must thus model both neutral and ion processes throughout the atmosphere .