The escape of O from Mars: Sensitivity to the elastic cross sections

Abstract

We have predicted the escape fluxes of energetic O from Mars for high and low solar activity models of the martian thermosphere, using a Monte Carlo code to determine the escape probabilities as a function of altitude and energy. Among the most important inputs to this code are the integral and differential elastic cross sections for hot O with various target species in the background atmosphere. In previous studies, we assumed that the integrated elastic cross section for each target species was 3×10−15cm2. Here we adopt more realistic elastic cross sections for O with each target species. We have identified calculations or measurements of such cross sections as a function of energy for O with seven of the twelve background species in our models. We adopt as constant a value that is appropriate to the 2–3eV energy range, which is just above the escape energy of O. For the five background species for which there are no data, we estimate the elastic cross sections as similar to those of a species of approximately the same size. The most important species for which there are no reported cross sections for elastic interactions with O is CO2, which is the major species in the martian thermosphere below about 200km. For our nominal model we adopt an elastic cross section that is slightly larger than that for O with N2. We then test the sensitivity of the model to the O–CO2 cross section by adopting values that are smaller or larger than this value. We report O escape probabilities for four cases, and the resulting escape fluxes and rates. There are small but not insignificant differences between the escape probabilities and fluxes for these cases. For low solar activity there is a factor of 2–3 range in the computed escape fluxes; for high solar activity, the factors are smaller, of the order of 1.5 or less. We compare these results with those of our previous calculations for a common constant collision cross section. We find that the O escape fluxes for the latter case, which are of the order of (2–4)×108cm−2s−1, are larger by factors of 9–28 than those of the four test cases here, which are in the range ∼(1–6)×107cm−2s−1.