Origin of the Moon

The origin of the Moon is one of the great mysteries of space science. Most scientists believe the Moon was formed when a large body, about the size of the planet Mars, called Theia, crashed into the early Earth. This powerful impact sent bits of Earth and Theia flying into orbit, where they eventually came together to form the Moon. This idea is known as the giant-impact hypothesis.

There are several versions of this hypothesis, and scientists are still studying the Moon’s beginnings. They look at clues like the Moon’s composition to understand its history. One important clue is that the Moon’s oxygen looks almost exactly like Earth’s oxygen. This suggests that the Moon is made mostly from Earth’s material, with very little from Theia. By studying these clues, scientists continue to learn more about how our Moon came to be and why it looks the way it does today.

Formation

Main article: Giant impact hypothesis

One popular theory about how the Moon formed is called the giant-impact hypothesis. It suggests that a body about the size of Mars, named Theia, crashed into the early Earth. This collision threw a lot of material into space, which eventually came together to form the Moon.

This idea explains some special features of the Moon, like its small core and lack of certain elements. The Moon also used to be much closer to Earth and has moved away over time. Scientists continue to study this theory to learn more about how our Moon came to be.

Derivatives of the hypothesis

The giant-impact hypothesis helps explain how the Moon formed, but it leaves some questions unanswered. For example, the Moon contains certain elements that are not as scarce as we would expect after a huge impact.

One surprise came in 2001 when scientists measured the composition of Moon rocks brought back by the Apollo missions. They found that the Moon’s ingredients matched Earth’s closely, unlike other bodies in the Solar System. This was puzzling because most of the material was thought to come from Theia, a planet that should have had a different composition. Further studies, such as those looking at titanium in Moon rocks, have reinforced this match with Earth, challenging older ideas about the Moon’s origin.

Other hypotheses

There are several ideas about how the Moon might have come to be. One idea is that the Moon was captured by Earth's gravity. This was a popular thought until the 1980s because it could explain some of the Moon's features, but it has problems, like how the Moon could have been caught without crashing into Earth.

Another old idea is that Earth spun so fast it threw off a piece, which became the Moon. This thought was suggested a long time ago but we now know it doesn’t fit with what we see, like the age of the Moon and the materials it’s made of.

Some think the Earth and Moon formed together from the cloud of dust and gas that became our Solar System. This idea also has some difficulties explaining certain features of both bodies.

A very unusual idea suggests the Moon might have been formed from a natural nuclear explosion on early Earth, caused by certain heavy elements coming together. While this is an interesting thought, it remains just one of many possibilities.

Additional theories and studies

Scientists have studied the Moon's origin through many ideas and discoveries. In 2011, they suggested that the Moon might have collided with another smaller moon very early in its history. In 2017, careful measurements of rocks brought back by astronauts showed the Moon is about 4.53 billion years old. Recent studies continue to explore the Moon's composition and history, revealing new details about its past.