Physical Science:

The most widely accepted theory for the formation of our Moon is that about 4.5 billion years ago our Earth was impacted by a Mars-sized object, Theia [Figure 12.2.5]. The impact would have melted both the Earth and Theia, and a large amount of debris would have been ejected into space. This debris eventually coalesced to form the Moon. This is why the composition of Moon is very similar to that of Earth’s mantle. Another piece of evidence for the giant impact theory is that the Moon has a lower density than Earth, which suggests that it is composed of lighter materials. The impact would have caused a lot of the heavier materials to sink into the Earth, leaving behind a lighter mixture of materials that eventually formed the Moon. However, there are other competing theories that propose different mechanisms for the Moon’s formation. These include the capture of a passing object by Earth’s gravity, or the fission of a rapidly spinning Earth.