Mars Atmospheric Evolution: What Can Dynamical Simulations Tell Us?

The history of the martian atmosphere and climate over time cannot be properly understood without knowing the role of loss of water and other volatiles to space. Furthermore, the martian climate system is an integrated one, from below the surface to above the exobase. Thus, volatile exchange and loss rates cannot be properly investigated without determining the role of the upper atmosphere and its coupling below (e.g. surface-atmosphere interactions, dynamics and energetics, dust storms) and influences above (e.g. solar wind interaction).Dynamical models (General Circulation Models) for the entire martian atmosphere (˜0-250 km) are beginning to be developed and exercised that address global energetics, chemistry, and dynamics. These models capture the key processes coupling the Mars lower and upper atmospheres, basic photochemistry, as well as solar wind interaction processes. Important volatile loss processes include : (a) solar wind stripping (i.e. pick-up ion loss), (b) photochemical loss (e.g. dissociative recombination of O2+), (c) thermal loss (i.e. Jeans escape of light species), and (d) impact ejection of the atmosphere (i.e. sputtering). Each of these processes depends on the intensity of solar EUV radiation, which affects thermospheric temperatures and densities, ionospheric properties, exosphere structure, and ultimately the fluxes of escaping atoms and ions. General Circulation Models (GCMs) provide the global context in which to understand present day escape processes and extrapolate these processes into the past for ancient solar and martian conditions. Here we consider the effects of higher solar EUV fluxes of the ancient sun upon the martian thermosphere-ionosphere structure. A reasonable characterization of this atmospheric structure, and an understanding of the underlying process that drive its variations, provide the foundation upon which escape rates can be estimated over Mars history. The combination of key spacecraft measurements (e.g. exobase temperatures and densities, ionization rates, hot atom distributions, pick-up ion production rates) and detailed models (e.g. GCMs, ionospheric models, sputtering codes, MHD codes) are needed to quantify present-day volatile escape rates. Model extrapolation of these escape rates into the past and integration of these rates over time yields an estimate of the volatile (e.g. water) loss over most of martian history.