Higher Martian atmospheric temperatures at all altitudes lead to
enhanced D/H fractionation and water loss
Abstract
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
$f\approxeq0.06$ 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.