Evolution of the Martian atmosphere

Evidence has become overwhelming that the Mars atmosphere has undergone considerable loss of volatiles to space. Mechanisms by which atmospheric species have been lost, include Jeans' escape (for H), photochemically driven loss (N and O), and sputtering by solar-wind pick-up ions (all species). Measurements of stable isotopes in the atmosphere, obtained by direct measurement and from the SNC meteorites, support this view. In particular, the measured ratios of D/H, super(1) super(5) N/ super(1) super(4) N, super(3) super(8) Ar/ super(3) super(6) Ar, and super(1) super(3) C/ super(1) super(2) C all require loss of substantial volatiles; in each case, more than half and, more likely approaching 90% of the available gas must have been lost. The only isotope ratio not showing substantial loss is super(1) super(8) O/ super(1) super(6) O. As it is unlikely that O has not escaped, the lack of fractionation indicates the presence of a large reservoir of oxygen; this reservoir most likely is the crust, with exchange of atmospheric oxygen with oxygen in silicate minerals plausibly occurring as a result of the circulation of hydrothermal water. This geochemical view of Martian volatiles is consistent with the geological view--that the early Mars climate was substantially warmer, presumably due to the presence of a thicker atmosphere capable of some greenhouse warming, and that the atmosphere has evolved substantially through time to get to the cold and thin present-day atmosphere. Both the nature of the early climate and the presence of hydrothermal waters have substantial implications for the presence of extant or extinct life.