The giant impact that made the Moon
A planet-sized collision that probably gave Earth its Moon.
Key facts
- When
- about 4.5 billion years ago
- Where
- Earth's orbit
- What hit
- a young planet perhaps the size of Mars, often called Theia
- Era
- Deep time
Far beyond anything else on this list: enough to melt much of Earth's outer layers and fling a disc of molten and vaporised rock into orbit.
Billions of times the Chicxulub impact. No weapon or natural event since comes close.
What happened
Within the first hundred million years or so of the Solar System, while the planets were still growing, the young Earth seems to have been struck a glancing blow by another planet. The idea, called the giant impact hypothesis, is the leading explanation for where the Moon came from.
In the usual picture, the impactor, nicknamed Theia after the mother of the Moon goddess in Greek myth, hit at an angle. Its iron core sank into Earth's, while a huge cloud of molten and vaporised rock from the outer layers of both bodies was thrown into orbit. Within a few years to a few centuries, much of that debris gathered into the Moon, which at first orbited far closer than it does today.
Scientists still argue over the details. Some models give Theia a size closer to Earth's own, or describe a spinning, doughnut-shaped cloud of vapour, called a synestia, from which the Moon condensed. What almost all agree on is that a collision on this scale happened.
Effects
The collision would have melted much of Earth's surface into a global ocean of magma, stripped away any early atmosphere, and set the length of Earth's day and the tilt of its axis. The Moon itself began covered in its own magma ocean, whose light crystals floated up to form the pale highlands we still see.
The evidence
The Apollo samples are the key. Moon rocks are almost identical to Earth rocks in their oxygen isotopes and many other chemical fingerprints, yet the Moon has only a tiny iron core and is poor in water and other easily vaporised substances, just as a body made from hot, rocky debris should be. The way the Earth and Moon spin and orbit together also fits a single large collision far better than older ideas, such as the Moon being captured or spinning off from a fast-rotating Earth. The puzzle still being worked on is why Earth and Moon are so alike when Theia should have left a different chemical mark.
Did you know?
Right after it formed, the Moon may have been only about a fifteenth to a twentieth of its present distance from Earth, and has been slowly drifting away ever since: it still moves about 3.8 cm further every year.
Some scientists think pieces of Theia may survive deep inside Earth, in two continent-sized regions of unusual rock near the core.
Related pages
How is impact energy measured? See energy explained, or try the impact calculator.