Nib asked
How did the Moon form after the giant impact?
The Moon is a souvenir of a collision: a Mars-sized world called Theia hit the young Earth about 4. 5 billion years ago, the debris ring was mostly rock from the two worlds' outer layers, and it clumped into the Moon within years to a…
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1a big moon
The Moon is unusually large for the satellite of a rocky planet: its diameter is about 27 percent of Earth's. Old ideas each stumble. A passing Moon would have been very hard to capture. Spinning off from Earth would need Earth to spin impossibly fast. And a Moon grown alongside Earth should have Earth's iron, which it lacks.
The Moon is big for a moon, and the old guesses of capture, spin-off and growing alongside all fail.
a common mix-upThe Moon was not captured as a passing stranger, and it was not simply ripped out of the Pacific Ocean.
2the clue
Earth's average density is about 5.5 grams per cubic centimetre, the Moon's about 3.3, because the Moon has only a tiny iron core. Yet the Apollo rocks match Earth's mantle in their oxygen isotopes. A body made mostly of Earth-like outer rock with the iron missing is the signature of a giant collision.
The Moon has little iron but Earth-like rock: it was built from outer layers, after iron sank to a core.
What does the Moon's low density tell us?
Show the answer
It has very little iron, so it was not built from whole-Earth material.
3the impact
The leading idea is the giant impact hypothesis. About 4.5 billion years ago a planet roughly the size of Mars, nicknamed Theia, hit the young Earth at an angle. Earth already had a heavy iron core under a rocky mantle.
A Mars-sized world, Theia, hit the young Earth about 4.5 billion years ago.
4ring to Moon
The impact sprayed molten and vaporised rock, mostly from the outer layers of both worlds, into orbit around Earth. The heavy iron of Theia's core sank and merged with Earth's core, which is why the Moon is short of iron. Simulations show clumps in the ring gathering into the Moon in anywhere from months to about a century.
Debris from the outer layers formed a ring that clumped into the Moon within years to a century; the iron stayed in Earth.
Where did the iron from Theia go?
Show the answer
It sank and merged into Earth's core, so the Moon came out short of iron.
5the whole story
The whole story in one picture: Theia hits the young Earth, a ring of hot rocky debris forms, the Moon gathers within years to a century and the iron settles into Earth. The Moon has been drifting away ever since, at about 3.8 centimetres a year, measured with mirrors left by the Apollo astronauts. Open question: the Moon's isotopes match Earth's more closely than a simple Theia-dominated model predicts, and scientists are still refining the details.
The Moon is Earth-and-Theia rock regathered after a giant impact, and it is still moving away.
Every moonrise is a glimpse of a four and a half billion year old crash, still turning slowly in the sky.