It’s been known for some time now that the Moon formed out of a cataclysmic collision between the Earth and a Mars-sized planet called Theia. What remains unsolved is just how the Moon ended up with so much of Earth’s granite. Every planet has its own, unique mix of Oxygen isotopes – this allows scientists to determine whether a meteorite came from Mars, the asteroid belt, or something more exotic. However, if the Moon formed from the impact of another planet with the Earth, then it, too, should have a unique isotopic signature, except it doesn’t! Moon rocks look almost exactly like Earth rocks! So where did Theia go?
I’ve started a new long-term project to simulate this impact in the hopes of tracking where all the material from both objects goes. So far, all signs point to the Moon forming out of ejected Earth granite, and all of the granite and iron from Theia stayed on Earth. The planet that crashed into the Earth to form the Moon is literally right beneath us, trapped inside the Earth.