Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one piece of evidence or one step of the giant-impact model. Numbers come from NASA, LPI and the Natural History Museum; pictures are not to scale.
Tap the buttons and slide the sliders.
Tap a rock. Does its fingerprint match Earth’s?
Lunar samples share Earth’s oxygen-isotope mix; Martian and other meteorites don’t. Compare each with Earth.
Tap a rock. Is it like an Earth rock?
The fingerprints are drawn as a schematic, not real isotope data.
Pick an idea. Which clues does it pass?
Score each hypothesis against composition, volatiles, iron and angular momentum.
“–” means the clue is not a clear test of that idea.
Drag the slider. When do the iron cores merge? Where does the rock go?
Scrub through the oblique impact: core merger, then mantle ejecta into orbit.
Slide it slowly and watch the bump.
Sizes and speeds are simplified for the picture.
Switch between the usual models (months to years) and the 2022 simulation (hours).
Compare conventional disk accretion with the 2022 NASA Ames / Durham high-resolution result.
Tap a button and watch the bits clump into a Moon.
Set the angle, speed and size, then launch. Can you make a Moon?
Vary impact angle, speed and impactor mass. Only some combinations give a Moon-mass disk.
A simple toy with made-up rules that follow the simulations’ big trends, not a real simulation.
Slide through time. How big does the Moon look over the beach?
From ~15× closer and a ~6-hour day to 384,400 km and 24 hours today.
Slide time. Does the Moon get smaller?
Distances not to scale; the Moon’s size in the sky is exaggerated at the start.
Scientists think the Moon was made in a giant bump, long, long ago. It is their best idea, and they are still studying it.
Astronauts brought Moon rocks home. Scientists looked deep inside them. Their oxygen is almost the same as in Earth rocks! Rocks from Mars are different.
Long ago, Earth was very hot and covered in glowing lava. There were no oceans. Another young planet, called Theia, bumped into Earth from the side. Hot rock splashed up into space.
The hot rock made a glowing ring around Earth. Bits stuck together, like a snowball rolling in snow. One big clump became our Moon.
The new Moon was very close and looked huge. Since then, it has been slowly moving away, a tiny bit every year.
Want more? See why the Moon changes shape, how it makes an eclipse, or how gold is made.
The leading explanation is the giant-impact hypothesis. About 4.5 billion years ago, a young planet roughly the size of Mars, often called Theia, crashed into the young Earth.14 It hit in a glancing blow, not head on.5 The crash blasted rock from the outer layers of both worlds into space as vapour and molten rock. That debris spread into a ring around Earth, and the ring clumped together into the Moon.35
Most models say the clumping took months to years.2 A detailed 2022 computer simulation by NASA and Durham University suggests it might even have happened in just hours.2
In short: the Moon is probably made of pieces of the young Earth and Theia, thrown into orbit by one enormous crash. Scientists agree on this big picture but still debate the details.6
Nothing like the blue planet we know. Earth itself had formed only a little earlier, about 4.54 billion years ago (the full story is in how was Earth created?).9 The Moon-forming crash happened around 60 million years after the solar system began, though some estimates stretch that to about 175 million years.16
At that time the young Earth was hot, glowing red with rivers and seas of lava.3 There were no oceans and no life. Any water could only exist as vapour in the air.6 Both Earth and Theia had already sorted themselves into layers, with a heavy iron core in the middle and a rocky mantle around it.5 That detail turns out to matter a lot.
How do scientists test a crash nobody saw? They rerun it in computers, changing Theia's size, speed and impact angle, and check which versions produce a Moon like ours.45 Only the right starting conditions throw a Moon's worth of rock into orbit.5 The "aim Theia" toy in the player is a simplified version of that idea.
The Apollo missions of the late 1960s and early 1970s brought back 382 kilograms (842 pounds) of Moon rock and soil.1 Those samples, plus meteorites that came from the Moon, hold the main clues:4
| Clue | What we found | Why it fits a giant impact |
|---|---|---|
| Oxygen "fingerprint" | Moon rocks have almost the same oxygen isotopes as Earth rocks. Rocks from Mars and other bodies don't.12 | The Moon is made largely of Earth-like material. |
| A tiny iron core | Earth's core is about 30% of its mass. The Moon's is only about 1.6 to 1.8%.1 | The heavy iron cores stayed in Earth; the Moon got mostly rocky mantle.3 |
| Dry rock | Moon rocks have very little water and few elements that boil away easily.13 | The heat of the crash drove them off. |
| A magma ocean | Pale rock called anorthosite, which floats to the top of molten rock, covers the Moon's bright highlands.14 | The Moon was once molten hundreds to thousands of kilometres deep: a sign of a very violent birth.1 |
| Lots of spin | The Earth-Moon pair has an unusually large amount of rotational energy.3 | A glancing blow adds spin. |
Before Apollo, there were three main ideas. One of Apollo's goals was to find out which was right. The answer turned out to be none of them.4
| Idea | What it says | Why it lost |
|---|---|---|
| Capture | The Moon formed elsewhere in the solar system and Earth's gravity caught it as it passed.4 | A captured Moon should be made of very different stuff from Earth. Braking a passing body into just the right orbit also seemed very unlikely.45 |
| Fission | A young Earth spun so fast that a chunk broke away and became the Moon.4 | Scientists could not find any combination of spin and properties that would throw off the right kind of Moon.5 |
| Co-formation (accretion) | The Moon and Earth were built side by side from the same cloud of dust and gas.45 | It can't explain the Moon's low iron or the system's large spin.5 |
There was one more problem for fission and co-formation. Both predict a Moon with the same minerals as Earth, but Moon rocks are slightly different: drier, and rich in material that forms at high temperature.4 After a lot of debate in the 1970s and 1980s, the giant-impact model emerged in the early 1980s and won over most lunar scientists.45 In fairness to the old ideas, each got something right. The Moon really is made of Earth-like material, as fission and co-formation said, and Earth's gravity does capture small asteroids, which can orbit for a while as temporary "mini-moons".4
Scientists aren't sure, and the estimates range widely. Most models say the debris ring clumped together over months or years.2 The Lunar and Planetary Institute says perhaps a hundred years or less.3 In 2022, researchers at NASA's Ames Research Center and Durham University ran some of the most detailed impact simulations ever made. They found that a Moon-like body might have been launched straight into orbit within hours of the crash.2 If that's right, more of the Moon would come from Earth, which could help explain the matching fingerprint.2 Future samples will help test it.
Just after it formed, the Moon was about 15 times closer than it is today, and a day on Earth lasted only about six hours.3 (The University of Chicago puts it at about 17 times closer.6) Imagine a Moon filling a huge part of the sky.
Since then, the pull of the tides has slowly pushed the Moon outward and slowed Earth's spin. The Moon now averages 384,400 kilometres away and is still moving off by about 3.8 centimetres (1.5 inches) a year.17 We know because scientists bounce lasers off mirrors that Apollo astronauts left on the surface.1 Days are still very slowly getting longer too.1 To see how the Moon raises tides today, read what causes tides?
The big picture is well supported, but plenty of questions are open:16
The Apollo samples all came from the side of the Moon facing us, mostly near its equator. In 2024 China's Chang'e 6 brought back the first rocks from the far side, and future samples from the poles, including from NASA's Artemis missions, should help settle these questions.12
The crash that made the Moon shaped our planet too. The Moon raises most of our ocean tides, and it helps steady the wobble of Earth's axis, which keeps our climate more stable than it would otherwise be. How important that was for life is still debated.48 The Moon's changing position also gives us its phases (why does the Moon have phases?) and eclipses (what causes a solar eclipse?). And because the Moon has had no active volcanoes for billions of years, its surface is a nearly unchanged record of the early solar system.4
Keep exploring: why the Moon changes shape every month, how it can block out the Sun in an eclipse, and how another kind of cosmic crash made the gold on Earth.
The leading explanation is a giant impact. About 4.5 billion years ago, a Mars-sized young planet, often called Theia, hit the young Earth in a glancing blow. Rock from both worlds was blasted into orbit, formed a ring around Earth, and clumped together into the Moon.
About 4.5 billion years ago. Moon rocks suggest it formed roughly 60 million years after the solar system began, and some estimates stretch that to about 175 million years.
No. Earth formed first, about 4.54 billion years ago. The Moon was made later, from a collision with the already-existing young Earth, so Earth is a little older.
Mostly rock from the outer layers of the young Earth and of Theia. The impact turned it into a ring of vapour, dust and molten rock circling Earth, which then clumped together into the Moon.
Very different from today. Young Earth was hot, with seas of lava and no liquid oceans. Its day was much shorter, and after the impact the new Moon hung about 15 times closer than it does now.
Apollo Moon rocks have almost the same oxygen isotopes as Earth rocks. The Moon has a tiny iron core, very little water and few easily vaporised elements, and it was once covered by a magma ocean. All of these fit a hot, violent birth from Earth-like rock.
Life would be different. The Moon raises most of our ocean tides and helps steady the wobble of Earth's axis, which keeps our climate more stable. How important the Moon was for life to begin is still debated by scientists.