Daniel Herwartz^1,2, Andreas Pack¹, Bjarne Friedrichs¹ and Addi Bischoff³

¹Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Goldschmidtstraße 1, 37073 Göttingen, Germany.
²Universität zu Köln, Institut für Geologie und Mineralogie, Zülpicher Straße 49a, 50674 Köln, Germany.
³Westfälische Wilhelms-Universität Münster, Institut für Planetologie, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany.

The Moon was probably formed by a catastrophic collision of the proto-Earth with a planetesimal named Theia. Most numerical models of this collision imply a higher portion of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar system bodies, the isotopic composition of Earth and the Moon should thus be distinct. So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have failed. Our triple oxygen isotope data reveal a 12 ± 3 parts per million difference in Δ¹⁷O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ¹⁷O values, and we speculate on an enstatite chondrite–like composition of Theia. The observed small compositional difference could alternatively be explained by a carbonaceous chondrite–dominated late veneer.

Reference
Herwartz D, Pack A, Friedrichs B and Bischoff A (in press) Identification of the giant impactor Theia in lunar rocks. Science 344:1146.
[doi:10.1126/science.1251117]
Reprinted with permission from AAAS

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