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Identification of the giant impactor Theia in lunar rocks

Science  06 Jun 2014:
Vol. 344, Issue 6188, pp. 1146-1150
DOI: 10.1126/science.1251117

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An analysis of motes of the Moon maker

How did the Moon form? According to the prevailing hypothesis, a Mars-sized body known as Theia smashed into Earth. Herwartz et al. analyzed fresh basalt samples from three Apollo landing sites and compared them with several samples of Earth's mantle. The oxygen isotope values measured in these lunar rocks differ significantly from the terrestrial material, supporting the giant-impact hypothesis.

Science, this issue p. 1146

Abstract

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.

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