Production and escape of hydrogen on Mars

Spectroscopic observations of water vapor on Mars are shown to be consistent with the temperature profiles of Gierasch and Goody. The corresponding column photodissociation rates are about 1010 cm−2 sec−1 near the subsolar point, and perhaps 109 cm−2 sec−1 for a planetary average. The subsequent chemistry and transport of the fragments is discussed. Among the forms of odd hydrogen, HO2 may dominate over H and OH near the surface, but H is most important at great heights. The mixing ratio of H2 is between 10−6 and 10−4, depending on the assumptions made. H2 is converted into odd hydrogen by O(¹D) and CO2+, and very little penetrates to the exposphere. The escape flux of H is estimated to be around 108 cm−2 sec−1, and the concentration at 200 km around 104 cm−3. This flux, continued for 2×109 years, would remove 2.5 meters of solid ice and leave behind a corresponding large amount of free oxygen.