Fully coupled photochemistry of the deuterated ionosphere of Mars and
its effects on escape of H and D
Abstract
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-2s-1 dominates atomic D loss under all solar conditions. The total
fractionation factor, f, is 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.