How did the Moon form?
Short answer: Probably from an ancient collision with a small planetary object.
Long answer: It's hard to say exactly how the Moon was made, and there have been many theories over the years. Currently, the most widely-accepted explanation is called the giant impact hypothesis. In this hypothesis, the young Earth was hit by a small planet about the size of Mars about 4.5 billion years ago. This hypothetical planet is called Theia, named after the mythical Greek Titan who was the mother of Selene, goddess of the Moon.
After the collision, the resulting debris formed a ring around Earth. Over a long period of time, the rocks in this debris field came together and formed the Moon.
There are a few problems with this hypothesis. For example, astronomers haven't yet come up with a completely satisfactory model that starts with an impact and ends with a single Moon. However, there is a reasonably large body of evidence that does support the hypothesis, for example:
- There is a lot of evidence that similar collisions were common in the early Solar System.
- There is evidence that similar collisions with resulting debris discs have happened in other planetary systems.
- The ratios of stable isotopes are the same on the Moon and Earth, strongly implying that they share a common origin.
- The Earth's rotation lines up well with the Moon's orbit.
- The Moon is less dense than Earth.
- The Moon has a relatively small iron core.
- Samples from the Moon suggest that its surface was once molten.
The origin of the Moon requires more research and a more complete explanation that addresses inconsistencies within the giant impact hypothesis. It is definitely an area of continued interest for astronomers and planetary scientists.
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