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A. H. Delsemme (Toldeo, OH)
The last 25 years have brought several lines of evidence that none of the volatile elements of the biosphere were within the primordial Earth, but were brought later by comets. Empirical facts establish that the primordial Earth was totally degassed; other observations prove that, during its first billion years, the Earth was hit by billions of cometary impacts. One of the best proofs of the cometary origin of seawater is its deuterium enrichment, which was mainly due to comets that accreted in Jupiter's zone. Most of these comets were ejected at hyperbolic velocities to outer space, and only a few were captured by the Oort Cloud. Most comets of the Oort Cloud originated in the zones of the outer giant planets. This explains why recent comets have a much higher deuterium enrichment than seawater.