The dominant model for the origin of our Moon is the impact model. In this model, about 4.5 billion years ago proto-Earth was hit by a Mars-sized body known as Theia. It’s generally been thought that the collision was somewhat off-center, causing the remnants of Theia and some outer layer material from Earth to form the Moon.  But new evidence suggests the collision was more head-on. 

If the Earth-Moon system was caused by an off-center collision with Theia, then we would expect to see close similarities in the chemical compositions the Earth and Moon, but still some differences. Earlier research found some differences in the amount of isotopes, such as the ratio of oxygen-17 to oxygen-16 differing by about 12 parts per million between the Earth and Moon. But new work analyzing oxygen isotopes in lunar rocks and volcanic rocks on Earth found their oxygen isotopes to be indistinguishable. Since oxygen is common in both rock samples, the fact that they are indistinguishable suggests that the material forming the Earth and Moon were mixed together before they formed. This could be achieved by a more head-on collision between proto-Earth and Theia.

If that’s the case, then much of Theia became a part of Earth’s core, and our planet is actually the product of two worlds.  It’s an interesting twist on the impact origin of the Moon.

Paper: Edward D. Young, et al. Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact. Science Vol. 351, Issue 6272, pp. 493-496 (2016)

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