High levels of xenon-129 in the Martian atmosphere resulted from the decay of now extinct iodine-129 and indicate that the planet underwent a major differentiation and crust formation within the first 100 million years of its history. An early atmosphere probably formed around the same time but was modified by loss processes brought about by hydrodynamic escape of hydrogen, and ablation during the early heavy bombardment epoch, 4 billion years ago. Photogeological evidence indicates that fluvial processes were most active at that time. The low levels of argon-40 in the atmosphere combined with a relatively low level of volcanic activity, indicate that additions to the atmosphere, by volcanic outgassing since the early bombardment, have been small. Photochemical loss process, probably lowered the nitrogen content significantly in the last 4 billion years and caused the observed 60% depletion of nitrogen-14 relative to nitrogen-15. Theoretical estimates of the stability of water and ice, in and below the surface, combined with the likely porosity of a 10km deep megaregolith, indicate that Mars could hold subsurface water equivalent to a 500m deep ocean. Furthermore a crust with 1 wt% carbonate would trap the equivalent of several bars of atmospheric carbon dioxide. It is thus quite possible that major volatile reservoirs relevant to life, water and
carbon dioxide, existed within the crust, and may still exist, largely independent of the now tenuous atmosphere.