How did the Moon form?

For millennia, the Moon has been a constant sight in the night sky. Even dinosaurs, that last roamed the Earth 66 million years ago would have grazed by the light of the Moon. However there was a time before the Moon and this raises a question. How did the Moon come to be here?

The Moon

The story starts with the early Solar System. This was the time when the Solar System was nothing more than a big disk of gas and dust. The best way to explain this would be to imagine some cake ingredients spilled out on a worktop. As you start to stir the mix small grains of flour start to stick together. Next these small balls of flour begin to stick to each other forming even bigger balls of flour. This process is called accretion and is how the planets formed. The disk formed small lumps of rock then formed small planets called protoplanets. It was a chaotic place with objects colliding all the time. Sometimes they stuck together and other times they destroyed each other.

The most widely accepted theory on how the Moon formed is called the Giant Impactor Theory. This theory states that during this chaotic time, the proto-Earth collided with another small planet which astronomers called Theia. This impact would have caused catastrophic damage to the Earth but happened long before life started. The impact would have created a ring around the Earth. Similar to how the Earth formed, the bits of rock and debris in this ring accreted together to form the Moon.

Technicians examine the largest lunar rock collected during the Apollo missions. Credit : NASA

There are a number of lines of evidence that point to this theory being correct. Some of the main evidence comes from the Apollo missions. During this time, large quantities of lunar rock was returned to Earth. This allowed scientists to analyse the composition of the rocks on the Moon. A measurement called the stable-isotope ratio is very similar in lunar rocks and those of Earth. This indicates that there is a single origin for the Earth and Moon and means it is very unlikely that the Moon formed somewhere else and was captured into Earth orbit.

Further evidence can be found in the rocks of the Moon. While they share a very similar composition to the Earth, they are slightly different. This means that it is unlikely that both the Earth an Moon formed at exactly the same time. If they did, one would expect both to have identical compositions. In addition to that, the composition of the Moon’s rocks shows that the quantity of volatiles like water are smaller on the Moon than Earth. This is consistent with the heat produced in a giant impact which would have vaporised these volatiles.

All of this makes for a convincing argument in favor of the Giant Impact Theory. What do we know about the impact and Theia, the object involved in the collision? The name for the hypothesised planetoid, Theia, comes from Greek mythology and was the mother of Selene, the goddess of the Moon. Much of what we know about the collision comes from computer generated simulations. To do this, scientists load what they know about about how the Solar System and planets work into a computer model. They make guesses about the things they don’t know, such as the angle of the impact or the size of the planet. Once they run the simulation they can then see if the result matches what we see today.

Simulations suggest that Theia was moving relatively slowly. The angle of the impact is still debated. Some models suggest it could have been a head on collision. Others suggest it could have been at an angle of about 45 degrees. The origin of Theia is also debated. Some studies suggest it was what is called an Earth trojan that formed in the inner Solar System. These are bodies that orbit the Sun in a similar orbit to the Earth but trail it. Some other recent studies also suggest that Theia could have formed in the outer Solar System and could have brought a lot of the water on the Earth. Unfortunately, we may never know for sure the answers to these questions.

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