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How did the Moon form?

Museum planetary science researcher Prof Sara Russell explains the origins of Earth's closest companion.

Analysis of samples brought back from the NASA Apollo missions suggest that the Earth and Moon are a result of a giant impact between an early proto-planet and an astronomical body called Theia.

A few theories

'There used to be a number of theories about how the Moon was made and it was one of the aims of the Apollo program to figure out how we got to have our Moon,' says Sara.

Prior to the Apollo mission research there were three theories about how the Moon formed.

Capture theory suggests that the Moon was a wandering body (like an asteroid) that formed elsewhere in the solar system and was captured by Earth's gravity as it passed nearby. In contrast, accretion theory suggested that the Moon was created along with Earth at its formation. Finally, according to the fission scenario, Earth had been spinning so fast that some material broke away and began to orbit the planet.

What is most widely accepted today is the giant-impact theory. It proposes that the Moon formed during a collision between the Earth and another small planet, about the size of Mars. The debris from this impact collected in an orbit around Earth to form the Moon.

A grey piece of rock from the Moon

Lunar meteorite Dar al Gani 400

Apollo mission evidence

The Apollo missions brought back over a third of a tonne of rock and soil from the Moon.

'When the Apollo rocks came back, they showed that the Earth and the Moon have some remarkable chemical and isotopic similarities, suggesting that they have a linked history,' says Sara.

'If the Moon had been created elsewhere and was captured by the Earth's gravity we would expect its composition to be very different from the Earth's.

'If the Moon was created at the same time, or broke off the Earth, then we would expect the type and proportion of minerals on the Moon to be the same as on Earth. But they are slightly different.'

The minerals on the Moon contain less water than similar terrestrial rocks. The Moon is rich in material that forms quickly at a high temperature. 

'In the seventies and eighties there was a lot of debate which led to an almost universal acceptance of the giant impact model.'

Lunar meteorites are also an important source of data for studying the origins of the Moon.

'In some ways meteorites can tell us more about the Moon than Apollo samples because meteorites come from all over the surface of the Moon,' adds Sara, 'while Apollo samples come from just one place near the equator on the near side of the Moon.'

Earth's greatest spinoff

Before Earth and the Moon, there were proto-Earth and Theia (a roughly Mars-sized planet).

The giant-impact model suggests that at some point in Earth's very early history, these two bodies collided.

During this massive collision, nearly all of Earth and Theia melted and reformed as one body, with a small part of the new mass spinning off to become the Moon as we know it.

Scientists have experimented with modelling the impact, changing the size of Theia to test what happens at different sizes and impact angles, trying to get the nearest possible match.

'People are now tending to gravitate towards the idea that early Earth and Theia were made of almost exactly the same materials to begin with, as they were within the same neighbourhood as the solar system was forming,' explains Sara.

'If the two bodies had come from the same place and were made of similar stuff to begin with, this would also explain how similar their composition is.'

lunar landscape of a crater near the Moon south pole

A lunar landscape showing the Antoniadi Crater near the Moon's south pole

Lunar landscapes

The mineralogy of Earth and the Moon are so close that it's possible to observe Moon-like landscapes without jetting off into space.

'If you look at the lunar surface, it looks pale grey with dark splodges,' Sara says. 'The pale grey is a rock called anorthosite. It forms as molten rock cools down and lighter materials float to the top, and the dark areas are another rock type called basalt.'

What are the dark spots on the Moon?

Similar anorthosite can be seen on the Isle of Rum in Scotland. What's more, most of the ocean floor is basalt - it's the most common surface on all the inner planets in our solar system.

'However, what is really special on the Moon, that we can't ever replicate on Earth, is that the Moon is geologically rather dead,' Sara says.

The Moon hasn't had volcanoes for billions of years, so its surface is remarkably unchanged. This is also why impact craters are so clear.

By looking at the Moon we can tell a lot about what the Earth was like four billion years ago.

Prof Sara Russell explains more about the Moon's formation:

A balancing influence

Having a moon as large as ours is something that's unique in our solar system.

'While other planets have tiny moons, the Earth's Moon is almost the size of Mars,' Sara says.

'If you look at other similar planets to ours, they wobble quite a lot in their orbit (the North Pole moves) and as a result the climate is much more unpredictable.'

A piece of moon rock in a glass prism

A piece of Anorthosite breccia moon rock displayed in a glass prism

The Moon has helped stabilise Earth's orbit and reduced polar motion. This has aided in producing our planet's relatively stable climate.

'It's a subject of quite a lot of scientific debate as to how important the Moon has been in making it possible for life to exist on Earth.'

Does Earth have more than one moon?

There may indeed be several objects in orbit around Earth. But to the best of our knowledge they are objects that the planet has drawn into its orbit - most likely captured asteroids. These natural satellites don't share the same important history as the Moon and they likely exist only temporarily in Earth's orbit.

See a piece of the Moon at the Museum

Explore gems and minerals, including a piece of Apollo Moon rock, in the Museum's Earth galleries.

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