A new study by German cosmochemists has confirmed a long-standing mystery about the similarity in the isotopic composition of the Moon and Earth, challenging the popular hypothesis that the Moon formed due to a collision between Earth and the planet Theia.

The giant impact hypothesis suggests that the Moon is 70% composed of debris from Theia, but oxygen-17 isotope analysis of 14 lunar soil samples showed almost complete identity with terrestrial rocks. As noted in Proceedings of the National Academy of Sciences, the study used samples collected by the Apollo missions from various parts of the Moon and compared them with Earth's mantle rocks. The similarities explain why this problem has been called the "isotope crisis."
Of the three previously proposed explanations—similar isotopic composition of Theia to Earth, intense mixing of matter during the collision, or the formation of the Moon mostly from Earth's mantle—the researchers favored the latter. This suggests that Theia was a special celestial body, perhaps a "bare" core, that almost completely sank into Earth, ejecting only terrestrial matter into space.
The study also provided an important conclusion about the presence of water on Earth before the formation of the Moon. The isotopic similarity of the Moon and Earth suggests that the planet did not receive its water reserves as a result of a later cometary bombardment, as cometary water would have had a different isotopic composition, which would have affected the ratio of oxygen isotopes.
These results confirm that the origin of the Moon is more complex than the giant impact hypothesis suggests, and stimulate further research into the chemical composition of space objects.