The proto-Earth as a significant source of lunar material

A giant impact between the proto-Earth and a Mars-sized impactor named Theia is the favoured scenario for the formation of the Moon(1-3). Oxygen isotopic compositions have been found to be identical between terrestrial and lunar samples(4), which is inconsistent with numerical models estimating that more than 40% of the Moon-forming disk material was derived from Theia(2,3). However, it remains uncertain whether more refractory elements, such as titanium, show the same degree of isotope homogeneity as oxygen in the Earth-Moon system. Here we present Ti-50/Ti-47 ratios in lunar samples measured by mass spectrometry. After correcting for secondary effects associated with cosmic-ray exposure at the lunar surface using samarium and gadolinium isotope systematics, we find that the Ti-50/Ti-47 ratio of the Moon is identical to that of the Earth within about four parts per million, which is only 1/150 of the isotopic range documented in meteorites. The isotopic homogeneity of this highly refractory element suggests that lunar material was derived from the proto-Earth mantle, an origin that could be explained by efficient impact ejection, by an exchange of material between the Earth's magma ocean and the protolunar disk, or by fission from a rapidly rotating post-impact Earth.