The Earth formed by accretion of Moon- to Mars-size embryos coming from various heliocentric distances. The isotopic nature of these bodies is unknown. However, taking meteorites as a guide, most models assume that the Earth must have formed from a heterogeneous assortment of embryos with distinct isotopic compositions1,2,3. High-precision measurements, however, show that the Earth, the Moon and enstatite meteorites have almost indistinguishable isotopic compositions4,5,6,7,8,9,10. Models have been proposed that reconcile the Earth–Moon similarity with the inferred heterogeneous nature of Earth-forming material, but these models either require specific geometries for the Moon-forming impact11,12 or can explain only one aspect of the Earth–Moon similarity (that is, 17O)1,2,3. Here I show that elements with distinct affinities for metal can be used to decipher the isotopic nature of the Earth’s accreting material through time. I find that the mantle signatures of lithophile O, Ca, Ti and Nd, moderately siderophile Cr, Ni and Mo, and highly siderophile Ru record different stages of the Earth’s accretion; yet all those elements point to material that was isotopically most similar to enstatite meteorites. This isotopic similarity indicates that the material accreted by the Earth always comprised a large fraction of enstatite-type impactors (about half were E-type in the first 60 per cent of the accretion and all of the impactors were E-type after that). Accordingly, the giant impactor that formed the Moon probably had an isotopic composition similar to that of the Earth, hence relaxing the constraints on models of lunar formation. Enstatite meteorites and the Earth were formed from the same isotopic reservoir but they diverged in their chemical evolution owing to subsequent fractionation by nebular and planetary processes13.
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S. Jacobson provided the PDFs for the Rubie et al. model15 and J. Siebert and J. Badro provided assistance during development of the code to reproduce their model19. Discussions with A. Morbidelli, D. Rubie, A. Campbell, F. Ciesla, R. Yokochi, M. Roskosz, N. Greber and C. Burkhardt were greatly appreciated. J. Hu double-checked all the derivations and codes used in this contribution. This work was supported by NSF (CSEDI, grant EAR1502591; Petrology and Geochemistry, grant EAR1444951) and NASA (LARS, grant NNX14AK09G).
Extended data figures
This table contains a compilation of isotopic anomalies in planetary materials.
About this article
Space Science Reviews (2018)