"Asteroids, comets and terrestrial water": Conferencia de apertura de curso en la Academia Canaria de Ciencias

• Autores: Humberto Campins Camejo, Michael J. Drake
• Localización: Revista de la Academia Canaria de Ciencias: = Folia Canariensis Academiae Scientiarum, ISSN 1130-4723, Vol. 20, Nº. 1-2, 2008, págs. 137-168
• Idioma: inglés
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• Resumen

  • There is no agreement on the origin of water on Earth and Mars. A number of sources ha ve been proposed but the pieces ofthis puzzle do not currently fit into a coherent picture. We list various geochemical measurements that can serve as discriminators and we use them to examine the principal proposed mechanisms for delivery of terrestrial and Martian water. Important developments have occurred in our understanding of the presence of water during early and late stages of Earth's formation. lt has long been thought that the region ofthe solar nebula where Earth formed was too dry for hydrous mineral phases to be stable and form a "wet" Earth, and that water was delivered by impacts of asteroids and/or comets with Earth after it formed.

    However, severa! recent measurements support the existence of water oceans on Earth shortly after its formation, as early as 4.3 to 4.4 x 109 years ago. The source or sources of this early water are not obvious at this time. Two recent mechanisms proposed for the formation of a wet early Earth are adsorption of nebular gas onto fractal dust grains in Earth's formation region and migration of hydrated silicates from the outer asteroid belt region of the solar nebula. On the other hand, late stage delivery of significant quantities of water from asteroidal and cometary sources appears less likely that previously thought. Isotopic and molecular ratio considerations do not favor asteroids or carnets as the main contributors to what is commonly termed the "late veneer" in Earth's forrnation. There are three important caveats to this last statement. First, our measurement of the composition of carnets and asteroids may not be representative of their bulk atomic, isotopic and molecular composition. Second, carnets and asteroids currently sampled spectroscopically and by meteorites may be unlike those falling to Earth during its forrnation.

    Third, the processes involved in planetary accretion, degassing, and the evolution of a hydrosphere and atmosphere are complex and may have fractionated the chemical and isotopic signatures of the source(s) of water. lndependent of the role carnets and asteroids may have played in the delivery of water, they appear to have been the principal source of organic compounds once the Earth's crust had solidified.