The evolution of the Martian atmosphere, with regard to water, is influenced by non-thermal atmospheric escape processes of heavy constituents. Since Mars does not have an appreciable intrinsic magnetic field at present and a comparatively small gravitational acceleration, all known atmospheric loss processes work and several important atmospheric constituents, namely H, H2, N, O, C, CO, O2 and CO2 are lost from the atmosphere. Thermal ions in planetary ionospheres can be converted to fast neutral atoms with ballistic trajectories reaching very high altitudes via charge exchange interaction with neutral atomic hydrogen and oxygen.
When water evaporates from the Martian surface and subsurface ice reservoirs, the water vapor reaches the upper atmosphere where incoming solar UV radiation can break up the molecules into hydrogen and oxygen, which oxidizes the surface soil or can be ionized and swept away by the solar wind. In determining the evolution of the Martian atmosphere and searching for present and past hydrogen reservoirs, the understanding of the atmosphere-surface-interaction and gas-exchange between the Martian atmosphere and the surface material is essential.
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