Upper Limits to the Outflow of Ions at Mars: Implications for Atmospheric Evolution

Escape of ions is potentially important for the evolution of volatiles on Mars, but the mechanisms and rates of ion escape processes are not fully understood. Instruments on the Russian Phobos 2 orbiter have, however, measured fluxes of heavy ions apparently of ionospheric origin in the optical shadow of Mars. These ions are assumed to arise from escape processes induced by the interaction of the solar wind with the ionosphere. We determine here upper limits to the ion loss rates by imposing upward flux boundary conditions on models of the low and high solar activity Mars ionosphere. The maximum fluxes obtained for high solar activity are about a factor of 4 larger than the fluxes derived from the Phobos ASPERA measurements, and the major ion is predicted to be O2+ rather than O+. If ions are scavenged at or near their maximum rates, the resulting escape fluxes are significant when compared to other non-thermal escape mechanisms for heavy atoms.