Photochemistry of water in the martian thermosphere and its effect on hydrogen escape

Abstract

Recent SPICAM IR solar occultations reveal significant abundances of water vapor up to ≈80 km near Mars perihelion. These abundances are stimulated by dust that heats the atmosphere and precludes condensation of water. This phenomenon correlates with detections of high escape of hydrogen near perihelion using the H Lyman-alpha in the SPICAM UV and HST observations. Here we present a self-consistent photochemical model of the martian neutral and ion composition at 80–300 km that accounts for variations of the atmospheric composition with solar activity and water abundance at 80 km. The model involves vertical transport by eddy, molecular, and ambipolar diffusion and both thermal and nonthermal escape of light species. The model predicts rather stable hydrogen escape of ≈1.9 × 108 cm−2 s−1 at 250 km during the most of the martian year beyond the perihelion period at LS = 200–330°. The reaction between H2 and CO2+ remains here the key process that determines the hydrogen escape. Therefore the HST observation of D and the FUSE observation of H2 at LS = 68 and 160°, respectively, do not need revision. Appearance of water in the thermosphere during the perihelion period results in significant increase in production of hydrogen by photolysis of water and reactions of its ions. Photolysis of H2O is the most effective at 160–180 nm and weakly depends on solar activity. Reactions of water ions start chiefly by charge exchange between CO2+ and H2O and end by recombination of H3O+. Both H2O photolysis and its ion reactions proceed near 100 km, and just a small part of the hydrogen production can escape. The calculated hydrogen escape at 250 km may be approximated by ΦH (cm−2 s−1) = 1.6 × 108 + 1.4 × 107fH2O (ppm). Here fH2O is the H2O mixing ratio at 80 km that is reduced to 150 km by a factor of 5 by the photolysis and ion reactions. Therefore the observed high hydrogen escape up to 109 cm−2 s−1 requires the dayside-mean water abundances up to 60 ppm at 80 km that are within those observed by the SPICAM IR solar occultations. The significant H2O abundances do not deplete densities of HCO+ and are compatible with the MAVEN/NGIMS ion composition. The H Lyman-alpha observations reflect mostly the dayside-mean photochemistry, and our one-dimensional model may adequately respond to the problem.