Astronomers have long theorized that the Moon was formed when an object the size of Mars slammed into Earth, sending a huge mass of rock into orbit around it. “According to the prevailing model prediction, approximately half of the Moon-forming material should come from the Mars-size impactor,” said Junjun Zhang, an isotope geochemist at the University of Chicago and the study’s first author.

But samples of isotopic titanium from the Moon and from Earth now suggest that material from Earth is predominant on the Moon, researchers report in the journal Nature Geoscience.