Small chemical differences between Earth and the Moon support the idea that the Moon formed from remnants of a large early planet, or protoplanet, that smashed into Earth 4.5 billion years ago.
Previous studies did not find differences in isotopic chemistry between Earth and Moon rocks, puzzling many who think that the protoplanet must have left a chemical mark on the Moon. Daniel Herwartz at the University of Cologne in Germany and his team measured levels of the isotope oxygen-17 in a range of Earth rocks, lunar meteorites that fell to Earth (pictured) and three lunar samples collected by astronauts. The authors found 12 parts per million more oxygen-17 in the Moon rocks than in Earth samples.
The impacting protoplanet, called Theia, could have been chemically similar enough to Earth, at least in terms of oxygen, that any imprint from Theia in the Moon's chemistry would have been difficult to detect, the authors say.
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Early planet helped make Moon. Nature 510, 190 (2014). https://doi.org/10.1038/510190c
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DOI: https://doi.org/10.1038/510190c
