The special position of helium among the elements has been recognized since the early days of radium. Like all the other noble gases (except the radiogenic Am), the maximum estimated abundance of helium in the atmosphere and the rocks of the earth's crust falls far below any estimate of its cosmic abundance. The theories of earth formation, which become increasingly credible in their modern versions, agree in ascribing this loss of helium' t o its inability to form solid or liquid phases or compounds at any but the lowest temperature, as well as to its very low atomic m a s s , allowing escape from such weak gravitational f ields as those of the prototype planet. It s e e m s likely, on such grounds, that the helium generally found today is not a residue of preplanetary time; and one of the points of the present paper is a demonstration of this plausible hypothesis. The present helium of the atmosphere must derive from the gradual emission of helium from the crust in such outgassing processes asvolcanic effluent, or by the slow processes of erosion and weathering of the solid rocks. The helium itself comes from the steady radioactive decay of the natural radioactive elements of the uranium and thorium ser ies in the rocks. Now the layer of igneous rocks weathered away since earliest geologic t ime has a thickness estimated at least at 150 kg./cm.2, and allowing for the newly-discovered thick undersea sediment, the true value is probably several times as great. The helium released in those rocks by U and Th decay amounts to more than 100 t i m e s the present content of the atmosphere. Even though it cannot be assumed that all the radiogenic helium enters the atmosphere during the process of rock weathering, the discrepancy is too large t o explain away by trapping in sediment. Helium is lost from the accessible atmosphere. Where has i t gone? For a long time it has been regarded as lost t o space, and this opinion now may be accepted as certain. There is increasing evidence for high temperatures in the upper atmosphere, largely from oionospheric studies, that imply temperatures between 1000 and 3000 K above 200 km. altitude.' Such temperatures are adequate to cause projection of helium atoms into hyperbolic orbits with respect to the earth and t o imply rates of loss fully capable of attaining equilibrium with the rates of release into the atmosphere from the rocks.

Radiogenic origin of the helium isotopes in rock

The geochemistry of thorium and uranium

Study of cosmic-ray intensity from isotopes produced in iron meteorites by cosmic-ray bombardment

G.R. Martin

Papers

Recent studies on iron meteorites. IV The origin of meteoritic helium and the age of meteorites

Recent studies on iron meteorites. III Determination of the uranium and thorium contents