The Moon: The Story Behind Its Origin and Formation

By Robert Hazen, Ph.D.George Mason University

In addition to the planets, there are many other objects that are bound to the Sun. For example, the moon. Most planets have one or more moons. The Earth has a single moon, a very large moon. Mars has two very tiny moons, and then Jupiter and Saturn have more than a dozen moons each. There is an amazing story of the Earth’s Moon’s formation. Read on to find more.

An illustration of the Earth's Moon with the Earth and the Sun in background
The Moon is a part of the Earth’s mantle. (Image: Scott Norris Photography/Shutterstock)

The Moon and The Earth: Origins and Relationship

The current models of the evolution of the Moon and the Earth illustrate planetary-forming processes very well, because the Moon is a real oddity, in a sense. It is one of the solar system’s most enigmatic objects. First of all, it is the only large moon that is part of any terrestrial planet.

The fact is that Mercury, Venus, Mars don’t have large moons—but the planet Earth does. Also, the age of the Moon seems to be very similar to the Earth; it was formed at about the same time, and it’s not clear how that might have happened. The Moon might be very likely the result of one of the epic late-stage collisions.

Curiously, the composition of the Moon is very similar to the Earth’s mantle. That is not its core or its crust, not the average composition of the Earth, but just one layer of the Earth, an outer, fairly thick layer.

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The Moon’s Composition

Now, this has significance. How can it be that you have a moon that appears to have what’s called a differentiated composition?

An image of the Earth seen from the surface of the Moon.
The Moon and the Earth are the same age. (Image: Romolo Tavani/Shutterstock)

When the Earth formed, it had one large average composition, but then the iron metal was sucked to the core because of gravity. 

What was left was the mantle, of a different composition. The Moon has that different composition. It suggests a relationship between the origin of the Earth and the origin of the Moon.

Here is what scientists think happened: There was a nebula. The planet Earth formed in its space around the Sun; but a competing mass, a mass perhaps the size of Mars, was also sweeping up debris in a very similar orbit. Towards the end of the formation of the planets, the two objects were competing for the same space in the Earth orbit, and those two objects had to collide.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

The Late-Stage Collision

The Mars-like object impacted the Earth in an epic collision, an unimaginable collision, and in a matter of seconds, a huge chunk of the Earth’s mantle was ripped out and thrown into space. That molten mass of mantle then became, it coalesced into, the sphere that is now the Moon in orbit around the Earth. A late-stage collision; an epic process forming our own Moon. 

According to current models, that’s what happened 4.5 billion years ago. The surface of both the Earth and the Moon had to be molten at that point. Not only were radiometric clocks set then, 4.5 billion years ago, but many of the other characteristics of the Earth arose from that point. This is an amazing story, but the solar system is a truly amazing place.

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Common Questions about the Story Behind the Moon’s Origin and Formation

Q: What’s the relationship between the origin of the Moon and the origin of the Earth? 

Examination of the Moon shows that the Moon and the Earth have the same age. On the other hand, the composition of the Moon is the same as the composition of the Earth’s mantle. This means that there must be a relationship between the origin of the Moon and the Earth.

Q: How does the Moon’s composition suggest a relationship between the origin of the Earth and the origin of the Moon?

When the Earth formed, it had one large average composition, but then the iron metal was sucked to the core because of gravity. What was left was the mantle, of a different composition. The Moon has that different composition. It suggests a relationship between the origin of the Earth and the origin of the Moon.

Q: What’s the story of the Moon’s formation?

According to recent models, researchers believe that the Moon’s formation took place about 4.5 billion years ago at the same time as the formation of the Earth. After the collision of the Earth with another object, a great chunk of the Earth was ripped out. That chunk took the shape of the current Moon after coalescing.

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The Moon: The Story Behind Its Origin and Formation