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Birth of the Moon

A collision made the Moon

About 4.5 billion years ago, a planet roughly the size of Mars slammed into the young Earth. The glowing debris gathered into the Moon. Scroll through it below, then see the evidence.

Illustration of the Mars-sized planet Theia approaching the young, glowing Earth

AI-generated illustration. AI-generated illustration for BotBuhdy.

4.5 billion years ago

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The young Earth is still hot and partly molten. A second young planet, which scientists call Theia, is on a crossing path.

Theia strikes Earth a glancing blow. In a few hours, both worlds' outer layers are blasted into molten rock and vapour.

Much of the debris falls back, but some is thrown into orbit, forming a hot, spinning disk around Earth.

Clumps in the disk collide and merge. Within years at most, and perhaps within hours according to some simulations, they build the Moon.

The newborn Moon starts very close, then slowly spirals outward. Today it is about 384,400 km away and still moving off by about 3.8 cm a year.

Animation by BotBuhdy. Sizes, distances and times are squeezed so it fits on one screen.

The giant impact idea

The leading explanation is called the giant impact hypothesis. It was proposed in the mid-1970s by two separate teams (William Hartmann and Donald Davis, and Alastair Cameron and William Ward) and has been tested with computer simulations ever since.

It explains several odd facts at once:

  • The Moon has only a small iron core. Its average density is about 3.34 g/cm³, much lower than Earth's 5.51. In the simulations, the iron cores of the colliding bodies mostly end up in Earth, and the Moon forms from lighter rocky material.
  • The Moon is very dry. Its rocks are poor in water and other easily boiled-off ingredients such as potassium and zinc, as you would expect if they formed from hot vapour and molten rock.
  • The Moon was once covered in a magma ocean. The pale, ancient highlands are made of a rock called anorthosite, which forms when light crystals float to the top of a huge body of molten rock. A violent, high-energy birth explains where that heat came from.
  • The spin of the Earth-Moon pair. The total spin (angular momentum) of Earth and the Moon fits a big, off-centre collision.
Illustration of the giant impact: a glowing plume of molten rock blasting out where Theia hits Earth
AI-generated illustrationThe giant impact as an artist might imagine it. Real simulations suggest the collision was a glancing blow, not head-on.AI-generated illustration for BotBuhdy.
Illustration of a glowing disk of molten debris circling Earth, with a young Moon gathering at its edge
AI-generated illustrationAfter the impact: a disk of molten rock and vapour circling Earth, from which the Moon gathered.AI-generated illustration for BotBuhdy.

The puzzle: the Moon is almost a twin of Earth

Here is the catch. In the classic simulations, most of the Moon is made from Theia. Planets that formed in different parts of the solar system usually carry different "fingerprints" in their isotopes (versions of the same element with different weights). Mars rocks and most meteorites clearly differ from Earth. Yet when scientists measured Apollo samples, the Moon's oxygen, titanium and other isotopes turned out to be almost identical to Earth's.

That has led to new versions of the idea:

Leading idea

Classic giant impact

A Mars-sized Theia hits at an angle. Explains the small core and the dry rocks, but struggles with the matching isotopes unless Theia formed near Earth from similar material.

Active research

Synestia

A more violent, high-energy impact turns Earth and Theia into a spinning, doughnut-shaped cloud of vapour (a "synestia") that mixes thoroughly. The Moon condenses inside it, inheriting Earth-like isotopes (Lock and Stewart, 2018).

Possible

Many smaller impacts

A series of smaller collisions each make a moonlet, and the moonlets merge into one Moon over millions of years (Rufu and colleagues, 2017).

Possible

Moon in hours

High-resolution simulations from NASA's Ames Research Center (2022) suggest a Moon-sized body could be flung straight into orbit within hours of the impact.

Ruled out

Fission

George Darwin suggested in 1879 that a fast-spinning Earth flung off the Moon. Earth would have needed to spin far faster than any process can explain.

Unlikely

Capture or twin birth

Capturing a passing Moon intact is extremely hard, and a twin formed beside Earth should have a similar iron core. Neither explains the Moon's chemistry.

Is any of Theia still here? A 2023 study in Nature (Yuan and colleagues) suggested that two huge blobs of unusual rock deep in Earth's mantle, under Africa and the Pacific, could be leftover pieces of Theia. It is an intriguing idea, not yet settled.

The rock evidence from Apollo

Apollo 15 sample 15415, the white anorthosite Genesis Rock, on a laboratory stand
Sample 15415, the "Genesis Rock", collected by Apollo 15 in 1971. It is almost pure anorthosite, a piece of the Moon’s ancient crust that crystallised from the magma ocean.NASA (S71-42955), 1971. Public domain.

Between 1969 and 1972, six Apollo landings brought back 382 kg (842 pounds) of lunar rocks, pebbles, sand and dust. Three robotic Soviet Luna missions returned about 300 grams more, and China's Chang'e 5 (2020) and Chang'e 6 (2024, the first samples from the far side) have added fresh material.

Those samples are why we know the Moon's age and story in such detail. Radiometric dating puts the oldest lunar crust at about 4.4 to 4.5 billion years. Tiny crystals of the mineral zircon in Apollo 14 samples point to the Moon forming within the first 60 million years or so of the solar system (Barboni and colleagues, 2017).

NASA still lends samples to researchers around the world, and keeps some sealed for future tools: in 2022 it opened Apollo 17 tubes that had been kept unopened for 50 years.

A huge young Moon

Illustration of a huge, glowing young Moon hanging over a steaming ancient ocean
AI-generated illustrationThe young Moon would have looked many times bigger than today, glowing faintly while its surface was still cooling.AI-generated illustration for BotBuhdy.
Illustration of the young Moon covered in a magma ocean with pale rafts of crust
AI-generated illustrationThe Moon’s magma ocean: pale crystals floated up to form the first crust, the bright highlands we still see.AI-generated illustration for BotBuhdy.

Drifting away 3.8 cm a year

The Moon formed much closer than it is now, perhaps only a few tens of thousands of kilometres away, and has been spiralling outward ever since. We can measure the drift today: about 3.8 cm a year, roughly as fast as your fingernails grow.

How? Apollo 11, 14 and 15 astronauts left laser reflectors on the surface (two Soviet Lunokhod rovers carry them too). Observatories fire laser pulses at them and time the echo, which comes back about 2.5 seconds later. Those measurements pin down the distance to within millimetres.

The cause is the tides. Earth's spin drags the ocean bulges slightly ahead of the Moon, and their gravity tugs the Moon forward, so it climbs into a higher orbit while Earth's spin slowly brakes. The rate has not been steady through history; it depends on the shapes of the ocean basins, and today's rate is thought to be unusually fast. See Tides & Earth for the interactive version.

Buzz Aldrin on the Moon beside the seismometer of the Apollo 11 experiments package, with the lunar module behind
Buzz Aldrin beside the Apollo 11 experiments: a seismometer, and a laser reflector that scientists still aim at today.NASA, Apollo 11 (AS11-40-5948), July 1969. Public domain.

Quick answers

How did the Moon form?

Most scientists think it formed from debris thrown into orbit when a Mars-sized planet, nicknamed Theia, collided with the young Earth about 4.5 billion years ago.

How old is the Moon?

About 4.5 billion years. Studies of lunar rocks and crystals give ages between roughly 4.35 and 4.51 billion years, so it formed within the first 60 to 200 million years of the solar system.

What was Theia?

A hypothetical young planet roughly the size of Mars. We have no direct sample of it, which is why scientists still debate how much of the Moon is made from Theia and how much from Earth.

Why are Moon rocks so similar to Earth rocks?

That is the big open question. Ideas include a very energetic impact that mixed everything together (a synestia), or Theia having formed near Earth from similar material.

Is the Moon getting farther away?

Yes, by about 3.8 cm a year, measured with lasers bounced off reflectors left by Apollo astronauts.

Sources and credits

  1. NASA Science, How did the Moon form?
  2. Hartmann, W. K. and Davis, D. R. (1975), Satellite-sized planetesimals and lunar origin, Icarus 24; Cameron, A. G. W. and Ward, W. R. (1976), The origin of the Moon, Lunar Science VII.
  3. Lock, S. J. et al. (2018), The origin of the Moon within a terrestrial synestia, Journal of Geophysical Research: Planets.
  4. Rufu, R., Aharonson, O. and Perets, H. B. (2017), A multiple-impact origin for the Moon, Nature Geoscience.
  5. Kegerreis, J. A. et al. (2022), Immediate origin of the Moon as a post-impact satellite, The Astrophysical Journal Letters; NASA Ames news release.
  6. Yuan, Q. et al. (2023), Moon-forming impactor as a source of Earth's basal mantle anomalies, Nature.
  7. Barboni, M. et al. (2017), Early formation of the Moon 4.51 billion years ago, Science Advances.
  8. Young, E. D. et al. (2016), Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact, Science.
  9. NASA Astromaterials (JSC), Lunar sample collection (382 kg from Apollo); NASA, opening of sealed Apollo 17 samples (2022).
  10. NASA, Lunar laser ranging and the 3.8 cm per year recession.

Photos credited to NASA, NOAA and other public-domain sources are used with thanks; BotBuhdy adds a small botbuhdy.com mark. Pictures labelled AI-generated illustration are artist-style illustrations made for this page, not photographs.