Non-thermal escape of molecular hydrogen from Mars

We present a detailed theoretical analysis of non-thermal escape of molecular hydrogen from Mars induced by collisions with hot atomic oxygen from the Martian corona. To accurately describe the energy transfer in O + H-2(v, j) collisions, we performed extensive quantum-mechanical calculations of state-to-state elastic, inelastic, and reactive cross sections. The escape flux of H-2 molecules was evaluated using a simplified 1D column model of the Martian atmosphere with realistic densities of atmospheric gases and hot oxygen production rates for low solar activity conditions. An average intensity of the non-thermal escape flux of H-2 of 1.9 x 10(5) cm(-2)s(-1) was obtained considering energetic O atoms produced in dissociative recombinations of O-2(+) ions. Predicted ro-vibrational distribution of the escaping H-2 was found to contain a significant fraction of higher rotational states. While the non-thermal escape rate was found to be lower than Jeans rate for H-2 molecules, the non-thermal escape rates of HD and D-2 are significantly higher than their respective Jeans rates. The accurate evaluation of the collisional escape flux of H-2 and its isotopes is important for understanding non-thermal escape of molecules from Mars, as well as for the formation of hot H-2 Martian corona. The described molecular ejection mechanism is general and expected to contribute to atmospheric escape of H-2 and other light molecules from planets, satellites, and exoplanetary bodies. Citation: Gacesa, M., P. Zhang, and V. Kharchenko (2012), Non-thermal escape of molecular hydrogen from Mars, Geophys. Res. Lett., 39, L10203, doi: 10.1029/2012GL050904.