Mars: an evolving atmosphere.

Photochemical reactions in the martian exosphere produce fast atoms of oxygen, carbon, and nitrogen and provide large escape fluxes of these elements. They appear to play a crucial role in the evolution of the martian atmosphere. The relative outgassing rates of H(2)O and CO(2) on Mars are comparable with terrestrial values, although absolute rates for Mars are lower by a factor of 10(3). Nitrogen is a trace constituent, less than 1 percent, of the present martian atmosphere.