Science Puzzle
Where Did the Moon Come From?
The Apollo astronauts brought back 382 kilograms of Moon rock. In the lab, it turned out to be remarkably like the rock of Earth's mantle, the thick layer beneath the crust, right down to its mix of different kinds of oxygen atom. Yet the Moon has only a small iron core, while Earth's is huge, and the Moon rocks show signs of having once been melted.
Where did the Moon come from?
The Answer
From a giant collision. The best explanation is that, about 4.5 billion years ago, a planet about the size of Mars smashed into the young Earth. Molten rock thrown out by the impact formed a ring round the Earth, which gathered together into the Moon.
The impactor is usually called Theia. It probably struck a glancing blow rather than head-on. The heat of the crash melted much of both bodies and flung a huge cloud of rock, mostly from their outer layers, into orbit. Their heavy iron cores sank and merged into the Earth. The orbiting debris then clumped together, quite quickly by the standards of planets, into the Moon.
That one event explains all three clues. The Moon is made mostly of mantle-like rock, because that's what was thrown out. It has little iron, because the iron stayed in the Earth. And its rocks were once melted, because the crash released enough energy to melt them.
Other ideas, such as the Moon being a passing body captured by Earth's gravity, or spinning off from a fast-turning Earth, don't fit the evidence nearly as well. Scientists still argue about the details: exactly how big Theia was, how it hit, and why the Moon and Earth are quite so alike. The newly formed Moon was much closer than it is today and would have filled a far bigger part of the sky.
The principle: Evidence from composition. The Moon's Earth-like rock, its small iron core and its melted past fit best with it forming from debris thrown out when a Mars-sized body hit the young Earth.