Monte Carlo calculations of the escape of atomic nitrogen from the Martian atmosphere

Monte Carlo simulations were carried out to compute the escape flux of atomic nitrogen for the low and high solar activity Martian thermospheres. This is the first time that such calculations have been employed to study the escape of atomic nitrogen from Mars. The total escape flux of atomic nitrogen at low and high solar activities was found to be 3.03 x 105 and 1.32 x 106 cm-2 s-1, respectively. The bulk of this escape derives from photodissociation of N 2, comprising ∼90% and 75% of the total escape flux of atomic nitrogen at low and high solar activities, respectively, the remainder of the contribution is from dissociative recombination of N2+.{09}This finding is surprising since conventionally dissociative recombination was thought to be the dominant escape mechanism. We find that our total predicted escape fluxes are a factor of ∼3 to 5 times greater than those previously published in the field owing to the enhanced escape from photodissociation of N2. A detailed description of the Monte Carlo algorithm and methodology used to derive these escape fluxes is provided here, along with a thorough review of previous exobase approximation research. In-depth analyses of the predicted escape probabilities for atomic nitrogen are carried out, and the relative contributions to the total escape flux from the thermal motion of the background gas, photoionization and solar wind pickup, and winds and co-rotation are computed here. We find that the inclusion of thermal motion in our models results in total escape flux enhancements of ∼20 and 26% at low and high solar activities, respectively; previously, many investigators assumed that this process was negligible. Conversely, photoionization and subsequent solar wind pickup was found to be a minor process only impacting those source atoms produced with energies close to the escape energy; the contributions to the total escape fluxes were found to be ∼8 and 13% at low and high solar activities, respectively. As for winds and co-rotation, Monte Carlo computations and back-of-the-envelope calculations were carried out and indicate that these processes are negligible for the escape of atomic nitrogen on Mars.