The Role of Ionosphere/Thermosphere Coupling Processes in the Escape of Species from Mars

We discuss coupled ionosphere/thermosphere models of Mars and implications for the photochemical escape processes of the atomic species O, C and N. Escape also occurs by ion outflow, and the relative rates of ion loss are determined by ion-neutral chemistry. The escape flux of ions has been computed by a number of investigators, including Ma et al. and S. Brecht, and some measurements are available from the Phobos spacecraft. The relative escape rates of ions depend on the composition of the ionosphere, which is determined by ion-neutral chemistry. Photochemical escape of atoms often occurs by processes that involves ions, such as dissociative recombination of O2+, N2+, CO+, and NO+, which yields fragments of various energies, many of which exceed the escape energy for Mars. Ions other than N2+ are formed mostly or partially by ion-molecule reactions. Except for NO+, the ions may be destroyed by ion-molecule reactions also. The ratio of ion-molecule reactions to dissociative recombination depends on the presence or absence of neutral species with which the ions can react. At high altitudes, the densities of neutral species is smaller than at lower altitudes. Therefore, above the "exobase", dissociative recombination may be more important. Since many ions react with H2, its density profile is important in determining the photochemical escape of heavy ions. Earlier in the history of Mars, the atmosphere may have been more reducing. A larger abundance of H2 would decrease the densities of O+, N2+, CO+, and CO2+, which react with H2. We model the ionosphere and photochemical escape mechanisms for the higher solar fluxes and more reducing atmosphere of early Mars.