How did the Moon form, and what is the giant-impact theory?

The leading explanation is that a planet about the size of Mars crashed into the young Earth some 4.5 billion years ago, and the debris from the crash clumped together to form the Moon. It is called the giant-impact hypothesis, and astronomers currently favor it. The planet that did the hitting has a name: Theia, after the Greek Titan who was the mother of Selene, goddess of the Moon.

The idea was first proposed in 1946 by the geologist Reginald Daly, and it explains several odd facts at once. The Moon has only a small iron core, which is why it is so much less dense than Earth: in the hypothesis, the impactor's iron core sank into Earth's core, while mantle material from both bodies was flung into orbit and became the Moon. The Moon is also short of volatile substances, which vaporize at low temperatures and could be lost in such a violent event. And rock from the Moon and from Earth has identical stable isotope ratios, a sign of shared origins.

Here is the catch, and it is a good one: that family likeness is also the theory's biggest puzzle. If Theia had been a separate body, it probably would have had a different oxygen isotope signature than Earth, yet the Moon's matches. In 2007, Caltech researchers put the chance of Theia having an identical signature at less than 1 percent. Proposed fixes include a 2018 model with a fast-spinning young Earth, a so-called synestia, and a 2019 model in which about 80 percent of the debris came from a magma-covered proto-Earth. And there is still no self-consistent model that starts with the giant impact and follows the debris into a single moon.

It is a reminder that "most astronomers favor it" is not the same as "settled." The Moon's birth is a reconstruction from clues, and the clues are still being argued over.