  Although deuterium (D) on Mars has received substantial attention, the
deuterated ionosphere remains relatively unstudied. This means that we also
know very little about non-thermal D escape from Mars, since it is primarily
driven by excess energy imparted to atoms produced in ion-neutral reactions.
Most D escape from Mars is expected to be non-thermal, highlighting a gap in
our understanding of water loss from Mars. In this work, we set out to fill
this knowledge gap. To accomplish our goals, we use an upgraded 1D
photochemical model that fully couples ions and neutrals and does not assume
photochemical equilibrium. To our knowledge, such a model has not been applied
to Mars previously. We model the atmosphere during solar minimum, mean, and
maximum, and find that the deuterated ionosphere behaves similarly to the
H-bearing ionosphere, but that non-thermal escape on the order of 8000-9000
cm$^{-2}$s$^{-1}$ dominates atomic D loss under all solar conditions. The total
fractionation factor, $f$, is $f=0.04$--0.07, and integrated water loss is
147--158 m GEL. This is still less than geomorphological estimates. Deuterated
ions at Mars are likely difficult to measure with current techniques due to low
densities and mass degeneracies with more abundant H ions. Future missions
wishing to measure the deuterated ionosphere in situ will need to develop
innovative techniques to do so.

Earth and Planetary Astrophysics