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Scientists now know that the Moon formed about 4.5 billion years ago. What’s interesting is that this formation happened about 100 million years after the Earth and the rest of the planets in the Solar System formed. And this has been a bit of a mystery. How can we explain the formation of the Moon.
There have been a few theories for the formation of the Moon proposed.
- Fission hypothesis – As the early Earth was forming, it was spinning much more rapidly than it is today. A blog of material broke off from the Earth’s crust and drifted away, remaining in orbit to this day. For this to be true, however, the Moon would have to be orbiting the Earth exactly on our orbital plane, and this isn’t the case.
- Capture hypothesis – Perhaps the Moon formed in another location in the Solar System, and then was somehow gravitational bumped near the Earth so that it could be captured by our gravity. This hypothesis is difficult to explain because it would have required the early to Earth to have an incredibly dense atmosphere.
- Co-formation hypothesis – Perhaps the Earth and the Moon formed together in the solar nebula, and the Moon has remained orbiting the Earth ever since. There are a few problems with this. If they formed together, you would expect the Moon to have the same consistency as the Earth, but it’s actually deficient in iron. Another problem is that the Moon seems to have formed about 100 million years after the Earth.
- Giant impact hypothesis – A Mars-sized object crashed into the Earth early on in its history. The impact was so catastrophic that an enormous amount of debris was kicked into orbit around the Earth, and this material eventually collapsed down into the Moon. This helps explain why the Moon has less heavier elements than the Earth.
It’s the giant impact hypothesis that most astronomers now accept for the formation of the Moon. It explains the Moon’s lower density, and lack of iron. It also helps explain why the Moon is 100 million years younger than the Earth. Finally, it helps explain why the Moon isn’t on the same orbital plane as the Earth.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?