Some chemical aspects of the evolution of the terrestrial atmosphere

At the time of formation of the solar system an almost quantative separation of gases from solids must have taken place. An upper limit for the amount of gas that could have followed the solids when they accumulated into the planet Earth can be derived from the amount of neon present in the terrestrial atmosphere. This upper limit shows immediately that most of the nitrogen and other volatiles have been derived from the solid matter during, or after, its accumulation. The relative amounts of rare gases show that most, if not all, of the rare gases have undergone a separation process prior to the formation of the Earth. The probable nature of this process is interesting from the point of view of a theory of the origin of the solar system.

The free oxygen in the terrestrial atmosphere cannot have been present at the time when the Earth formed, but must have evolved during geologic time. The escape from the Earth of hydrogen, produced by photolysis of water vapor, was important for the formation of an atmosphere containing free oxygen. It has been shown experimentally that photolysis of water vapor yields hydrogen even in the presence of oxygen. Charged particle reactions in the ionosphere are probably also important for the maintenance of ~ 0.5 ppm of the H2 present in the atmosphere. The amount of oxygen produced by photolysis was presumably increased by oxygen produced by plant photosynthesis combined with incomplete re-oxidation of the organic plant material formed by this process. The amount of oxygen produced over geologic time, and used up by the oxidation of surface material of the Earth, presents a fascinating geochemical problem that cannot yet be solved in an unambiguous way.