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A.H. Delsemme (University of Toledo, Ohio)
The volatile materials present in the biosphere, including the atmosphere, the seawater and all volatile carbon compounds, were brought to the primitive Earth by a heavy bombardment of comets. These comets were carried into the inner Solar System by an orbital scattering due to the growth of the giant planets. The bulk of the Earth's bombardment came from comets that accreted in Jupiter's zone; in this zone, steam convecting from the hot inner part of the Solar System condensed into icy chunks before their accretion into larger cometary nuclei. This process diminished their deuterium enrichment. In contrast, interstellar enrichment was kept in comets that accreted in the zones of the outer giant planets; they are the major source of the long-period comets observed now. The short-period comets, which come from the Kuiper belt, should also have the same interstellar enrichments. The deuterium amount in seawater is the mixture of 80% of water with a 6-fold enrichment, from Jupiter's comets, with 20% of water with a 16-fold enrichment, from comets that accreted in the zones of the outer giant planets. The deuterium enrichment of seawater remains one of the best telltales of the cometary origin of seawater. This origin may have far-reaching cosmological consequences, in particular for the emergence of life elsewhere.