Simulations of the hydrogen and deuterium thermal and non-thermal escape at Mars at Spring Equinox

Abstract

We present simulations of the thermal and nonthermal escape processes for H and D, under atomic, molecular and ion forms at Mars during spring equinox. These processes include Jeans escape, several photochemical reactions and the escape associated to the solar wind interaction with Mars. While the hydrogen escape is dominated by the atomic Jeans escape, we find that the deuterium escape is dominated by the photochemical atomic escape. Ions escape represent only 10% of the total escape for both species and is mostly due to charge exchange between neutral and solar wind protons. Including all the processes, we find a D/H fractionation factor (D/H escape ratio divided by the D/H atmospheric ratio) f = 0.04, with a main uncertainty associated to the elastic collisional cross sections needed to accurately derive the photochemical escape rate. Using this fractionation factor and considering a 30 m exchangeable reservoir of water, the average hydrogen escape rate needed to fractionate the Martian water from its primordial value to its current D/H value during the last 4.5 Gyr is ~1.0 × 1028 s−1 which is larger than the current average escape rate (~ 2 × 1026 s−1).