The Martian Thermosphere/Ionosphere: Ancient and Modern

We review our current understanding of the thermosphere and ionosphere of Mars at high and low solar activities, including density profiles of ions and minor neutral species, their variation with solar activity, and current and escape rates. We then discuss models of the ancient Martian thermosphere, focusing on the era about 2.5 Gyr before present, when the solar EUV flux was predicted to be about 3 times the present value. The model ancient thermosphere is derived using the Mars Thermospheric General Circulation Model (MTGCM), and the ionosphere is modeled using predicted fluxes from the ancient sun from Ayres [1997]. We discuss changes in the chemistry, thermal structure and escape rates