Photochemical Escape of Oxygen From Mars: A Comparison of the Exobase Approximation to a Monte Carlo Method

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The non-thermal escape of neutral O atoms from Mars at the current epoch is largely due to dissociative recombination of O2+:

O2+(v, J) + e → O+ + O+ + ΔE.

There are five energetically allowed channels of this reaction, with exothermicities, and thus O kinetic energies, that depend on the on electronic energies of the O atoms produced, on the vibrational and rotational state of the initial O2+ ions, and on the ion and electron velocities. We here construct high and low solar activity models of the martian thermosphere/ionosphere for a solar zenith angle of 60°. The background neutral atmosphere comprises 12 neutral species, whose density profiles range from 80 to 700 km. We calculate the density profiles for 14 ions from 80 to 400 km for both eroded and non-eroded ionospheric models. Using previously described methods, we model the vibrational distribution of the O2+(v) ions as a function of altitude and compute the nascent kinetic energy distribution of the hot O atoms. We then predict the photochemical escape rate of O atoms from Mars in two ways: first by employing the exobase approximation, and second, by using a spherical Monte Carlo code, which tracks the energetic O atoms as they collide with other species in their paths from their initial production altitudes, energies and angles. The O atoms are assumed to escape if they reach 700 km with energies greater than the escape energy. We carry out these Monte Carlo calculations using two different approximations for the distribution of scattering angles in the center-of-mass frame. We first assume that the scattering is isotropic, and then we do the same calculations for a more realistic forward-scattering distribution. We find that the calculated O escape rates for the isotropic model are comparable to those for the exobase approximation, but that the escape rates for the more realistic forward scattering model are an order of magnitude larger.