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Sci Adv. 2018 Mar 28;4(3):eaao5928. doi: 10.1126/sciadv.aao5928. eCollection 2018 Mar.

Oxygen isotopic evidence for accretion of Earth's water before a high-energy Moon-forming giant impact.

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Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.
Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, Laboratoire Géosciences Océan (CNRS UMR 6538), Plouzané, France.
British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.
Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA.
Centre de recherche en économie et statistique, 5 avenue Henry Le Chatelier, 91120 Palaiseau, France.


The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in Δ17O. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the Δ17O values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of post-giant impact additions to Earth. On the basis of this assumption, our data indicate that post-giant impact additions to Earth could have contributed between 5 and 30% of Earth's water, depending on global water estimates. Consequently, our data indicate that the bulk of Earth's water was accreted before the giant impact and not later, as often proposed.

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