The terrestrial craddle of lifeF. Albarède and J. Blichert-Toft
École Normale Supérieure and University Claude Bernard-Lyon I, UMR CNRS, 5570 Lyon, France
Published online: 17 September 2009
The Miller and Urey experiment in which amino acids were produced by electrical discharges in an atmosphere of water, methane, and ammonia was dismissed as being inconsistent with the secondary origin and the oxidizing character of the atmosphere. Since then, it has been found that simple geochemical arguments favor a dry accretion of the Earth and late delivery of water from the outer Solar System forming the primordial hydrosphere. This ocean of water must have interacted with the underlying terrestrial magma ocean, the existence of which is now well documented by evidence from the extinct radioactivity of 146Sm (T1/2 = 103 Ma) in Archean rocks. Water-hot rock interaction, with its hallmark reaction of serpentinization, produced enormous amounts of H2, which interacted with nitrogen and carbon dioxide to form CH4 and NH3. From this follows that the Miller and Urey experiment should be reinstated as the main template of the origin of life.
Sustainable life needs nutrients that can be steadily resupplied, most notably phosphate for which mid-ocean ridges act as a sink. In the modern world, phosphate is brought to the ocean by the erosion of continents, which are the hallmark of plate tectonics. On Earth, the elevation contrast between continents and abyssal plains is a result of plate tectonics: granitic rocks form by dehydration and melting of wet basaltic rocks at subduction zones. Life does not seem to be sustainable without plate tectonics, and plate tectonics in turn seems to result from the injection of water into the mantle by the subduction process, which “softens” mantle material and enhances convection. The earliest continental crust of modern affinity identified so far is attested to by Hadean detrital granitic zircons (ZrSiO4) present in the Archean Jack Hills conglomerates (West Australia). This crust seems to have formed by remelting a primordial ~4.35 Ga old crust extremely enriched in incompatible elements and which may have been derived from the early differentiation of the planet.
The emergence of life therefore is conditioned by the interaction between magma and water, which excludes small planets such as the Moon, and possibly Mars, with a thick lithosphere of buoyant plagioclase. Life survival depends on a regime of plate tectonics, which again excludes Mars with its stagnant lithospheric lid, and Venus which probably lost its continents some 700 Ma ago after most of its surface water was engulfed into the mantle. It is also unlikely that a “water world” such as Europa may sustain Earth-like forms of life.
© EDP Sciences 2009