Moon Formation

Our closest cosmic neighbour the Moon has long been the subject of scrutiny, the inspiration of literature, and a powerful mover of emotion. At the basis of all ancient questions revolving around the Moon has to be this—where did the Moon come from?

A Look at Moon Formation

A moon is a heavenly body that has attained orbit around a planet, kept in check by the twin forces of the planet’s gravity and that of the moon itself. Some planets have a moon and some do not, and many have multiple moons. In understanding moon formation in general, it is important to take a look at how the Earth’s Moon came to be.

There are many theories about where the Moon came from and how it formed, although not so many as other astronomical puzzles. The dominate theory is called the “giant impact theory”, which is also sometimes referred to as the “big whack” or the “big splash.” The nicknames come from the basic idea of the theory, which is that a large object struck the Earth millennia ago, causing part of the Earth’s crust to eject into space, some of which was captured into orbit and eventually came to form the Moon.

Under this hypothesis, the object in question is a protoplanet named Theia, or sometimes Orpheus, which formed on the same general orbit as Earth but some distance behind. Over the course of millions of years this protoplanet continued to grow until it reached roughly the size of Mars; at this point its size and mass caused its orbit to alter. Roughly 4.5 million years ago Theia struck Earth, breaking apart and carrying off part of the Earth’s mantle. Over time (estimates are between one and 100 years) parts of both Theia and the debris of Earth came together to form the Moon.

The Evidence

Much of this giant impact theory was first proposed in 1975 by Drs. William K. Hartmann and Donald R. Davis, and is based on geological evidence gathered by the Apollo moon landing. Oxygen isotopes in the Moon rocks are nearly identical to those of Earth, along with other factors that suggest the Moon is composed of, at least in part, the same material as the mantle of Earth.

Some indirect evidence includes KREEP (potassium, rare earth elements and phosphorus) rich elements and suggests that the Moon may have had a magma ocean; a giant impact such as the one suggested by this theory could have provided the huge amounts of energy needed to create such an ocean. However, no other evidence beyond the KREEP infused rocks support a magma ocean ever having been on the Moon.

Another problem with the giant impact theory is the lack of siderophile elements in the Moon, the high density transition metals that would make up the Theia protoplanent, should the Moon’s composition include parts of Theia.

Final Thoughts

The Moon has always been a source of inspiration, and landing on the Moon one of humanities greatest accomplishments. As the science of astronomy advances and Man travels further into space, Earth’s constant companion will become better understood, providing scientists more pieces to the puzzles left to solve.

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