What Is The Origin Of The Moon?

Robert Hazen:

The formation of the moon is one of the enduring scientific mysteries. It has been around as long as people have and it has generated a host of folk stories and myths. Fifty years ago, there were three popular explanations for the formation of the moon. George Darwin, Charles Darwin's son, thought that the young Earth was once a molten orb, and that it rotated so fast that it sort of threw off a blob of magma that became the moon. There was also the capture theory, the idea that the moon was a small planet with a similar orbit to Earth, and that it was captured by the Earth's gravity billions of years ago. Others have argued that the moon formed from the same cloud of dust, gas and debris that formed Earth and the other planets that orbit the sun.
Ultimately it was the Apollo moon landing, and the rock samples that came out of it, that were critical to understanding how the satellite formed. Those rock samples were inconsistent with all three of the theories I just described, and they forced scientists to come up with other hypotheses. They did and, fortunately, it seems to fit all the observations and is really quite robust now, in terms of scientific consensus. It seems that there was, indeed, a second planet-sized object (probably about the size of Mars) in more or less the same orbit as Earth. We have a rule in astrophysics that no two planets can occupy the same orbit, and so inevitably the smaller-sized object would have collided with Earth. In collisions of that sort, the larger body always wins.
In other words, Earth essentially swallowed the smaller planet, which has come to be called Theia (for the Greek goddess, who was the mother of the Moon). But because it was a glancing blow—ie, the collision between Earth and the theoretical planet was slightly off center—a huge amount of molten and incandescent silicone vapor was blasted into space, and into orbit around Earth. That is the material that ultimately formed the moon. The collision hypothesis explains some of the interesting compositional features of the moon, which were revealed by the Apollo astronauts.

I didn't know that.