Where do the oceans come from?

Abstract

The chronology of the Earth's formation and the clues of origin given by stable isotopes for the Earth itself and the different water sources in the solar nebula and solar system, indicate that the present Earth's water budget comes from three main sources: hydrogen trapped in the Earth's mantle, with a very low D/H ratio, and water and hydrogen coming with a late veneer of water-rich bodies, mainly between 25 and 100 Myr after the birth of the solar system. These water-rich bodies are now represented by some carbonaceous meteorites (CI and CM) with a bulk D/H ratio roughly terrestrial (∼ 1.5 x 10 -4 ) and comets with a D/H ratio about double (∼ 3 x 10 -4 ) of the terrestrial ratio. The CI and CM D/H's result from a mixture of deuterium-poor, lower than terrestrial and deuterium-rich, roughly cometary hydrogen. Directly or indirectly, cometary water has participated (from 17 to ∼ 50%) to the Earth's water budget. The data presently available, in particular those concerning the labelling of the late veneer by osmium isotopes, and the atmospheric distribution of rare gases favour the idea of a direct participation of comets (50%) mixed with mantle-outgassed water (50%), very poor in deuterium. This model can also be applied successfully to Venus, but with a much lower cometary contribution. The strong initial isotope heterogeneity of terrestrial water calls for a very thorough initial mixing by outgassing and mantle recycling, made possible by the very high initial convection rates of the hot primary mantle, which can isotopically homogenize the two sources in the first 100 Myr of the Earth's history.