Dynamics and Escape of H2 and OH Molecules Induced by Hot Oxygen Atoms in the Upper Atmosphere of Mars

Abstract:

Collisions of hot oxygen atoms with background atmospheric gases constitute an important mechanism of escape of light atomic and molecular species to space, as well as contribute to deviation from thermal equilibrium in the upper martian atmosphere. In this study, we used newly available quantum-mechanical cross sections¹ to carry out a state-to-state analysis of one such process: a reactive collision of hot oxygen atoms with thermal molecular hydrogen gas. Dissociative recombination of O₂⁺ molecular ions and secondary collisions of energetic neutral atoms (ENAs) produced in charge-exchange between solar wind ions and atmospheric gases² were considered as the sources of hot oxygen atoms. The resulting kinetic energy distributions of the product OH and H₂ molecules were constructed and corresponding escape rates for several solar wind conditions were calculated. The produced molecules favor higher excited rotational and vibrational quantum states that significantly differ from thermal distributions, offering a chance for detection. In the final part of this study, we analyzed the influence of increased production of energetic oxygen atoms on the molecular production rates and escape due to the encounter of Mars with Comet C/2013 A1 (Siding Spring). The results of this study are directly applicable to ongoing efforts to better understand the evolution of Mars' atmosphere and the MAVEN mission.

¹ M. Gacesa and V. Kharchenko, Geophys. Res. Lett., 39, L10203 (2012); M. Gacesa and V. Kharchenko, arXiv:1407.3325 (2014).

² N. Lewkow and V. Kharchenko, Astroph. J., 790, 98 (2014).