^1,2Timo Hopp,^1,3Nicolas Dauphas,³Maud Boyet,⁴Seth A. Jacobson,⁵Thorsten Kleine
Science 390, 819-823 Link to Article [DOI: 10.1126/science.ado062]
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA
Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
²Department of Earth and Planetary Sciences, The University of Hong Kong, Hong Kong, China
³Laboratoire Magmas et Volcans, Université Clermont Auvergne, Centre National de la Recherche Scientifique, Institut de Recherche pour le Développement, Observatoire de Physique du Globe de Clermont-Ferrand, Clermont-Ferrand, France
⁴Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI, USA
⁵Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
Reprinted with permission from AAAS

The Moon formed from a giant impact of a planetary body, called Theia, with proto-Earth. It is unknown whether Theia formed in the inner or outer Solar System. We measured iron isotopes in lunar samples, terrestrial rocks, and meteorites representing the isotopic reservoirs from which Theia and proto-Earth might have formed. Earth and the Moon have indistinguishable mass-independent iron isotopic compositions; both define one end of the range measured in meteorites. Combining our results with those for other elements, we performed mass balance calculations for Theia and proto-Earth. We found that all of Theia and most of Earth’s other constituent materials originated from the inner Solar System. Our calculations suggest that Theia might have formed closer to the Sun than Earth did.