The Origin of the Moon

describe the likely origin of the Moon (the giant impact hypothesis)
describe the evidence that allowed astronomers to develop the giant impact hypothesis.

Where did the Moon come from? Has it always been there?

The Giant Impact Hypothesis

The theory goes like this. About 4.3 billion ears ago, just 50 million years after the formation of the solar system, a large object the size of Mars collided with the Earth. This object has been named Theia after the mother of the Moon goddess Selene.

A lot of the object was absorbed to become part of Earth and a lot of debris was ejected into a disc surrounding the Earth. The rocks in this disc came together, due to gravity, to form the Moon.

What evidence is there?

The Apollo mission that landed on the Moon brought back many samples of rock. Some of these rocks contained exactly the same amount of a certain isotope of Oxygen as rocks found on Earth suggesting that they had a common origin.

The Moon's surface was once molten. What could have provided the energy to melt it? Perhaps a giant impact.

Belts of warm dust have been detected around other stars. Belts similar to the one from which the Moon may have formed after an impact.