Planetary science: Earth's building blocks.

Abstract

Earth grew by the accretion of meteoritic material. High-precision isotopic data reveal how the composition of this material changed over time, forcing revision of models of our planet's formation. See Letters p.521 & p.525 The bodies that formed the Earth have isotopic natures that have so far remained unclear. Here Nicolas Dauphas shows that elements with differing affinities for metal can be used to decipher the isotopic nature of the Earth's accreting material through time. He finds that the mantle signatures of lithophile, moderately siderophile and highly siderophile elements record different stages of the Earth's accretion, yet all the examined elements point to material that is isotopically most similar to enstatite meteorites. The author concludes that enstatite meteorites and the Earth were formed from the same isotopic reservoir but diverged in their chemical evolution as a result of subsequent fractionation by nebular and planetary processes. Mario Fischer-Godde and Thorsten Kleine show that all chondrites, including carbonaceous chondrites, have ruthenium isotopic compositions that are distinct from that of the Earth's mantle. The ruthenium isotope anomalies increase in the order of enstatite to ordinary to carbonaceous chondrites, demonstrating that material formed at greater heliocentric distance has larger ruthenium isotope anomalies. The authors conclude that the 'late veneer' of material accreted to the Earth following the Moon-forming impact must not have originated in the outer Solar System, and that the late veneer was not the primary source of volatiles and water on the Earth.