The Moon Guide
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U. S. N. O.
The phase of the Moon right now

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< Moonquakes

Before astronauts went to the Moon there were several main theories about how the Moon formed. One of the goals of the Apollo missions was to try and choose which theory fit best, but in the end, a completely new theory was born. Before we look at all the different theories, we need to look at some of the differences and similarities between the Earth and Moon:

  • The Earth has a large iron core, but the Moon does not.
  • Earth has an average density of 5.5 g/cm3 and the Moon has an average density of 3.3 g/cm3 (because it is depleted in iron).
  • The Moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks, and meteorites from other parts of the Solar System (e.g. the asteroid belt) have much different isotope compositions.
Sizes of Earth ad Moon
Earth is about four times larger than the Moon
The original Moon-forming theories, and why they can’t be what really happened, are as follows:

The capture theory says that the Earth and Moon formed in two very different parts of the Solar System but that at some point the Earth ‘captured’ the Moon with its gravity as it passed near to the Earth. This can’t have happened as the oxygen isotopes of both the Earth and the Moon showed they formed in pretty much exactly the same place in the Solar System.

This theory says that the Earth and Moon formed (accreted) in the same part of the Solar System from similar building blocks, and that the Moon is a sister planet to the Earth. This theory failed because it can’t explain why the Moon is depleted in iron compared to the Earth.

Fission describes the somewhat crazy idea that the young Earth was spinning so fast that it spun a lump of material off that went on to form the Moon. This theory fails because it defies the laws of physics!

Giant Impact
You’ve already heard about the Giant Impact theory, which is the current favoured formation mechanism for the Moon. This theory accounts for both the similarities and the differences between the Earth and Moon’s composition. It assumes that the Earth’s iron had already drained into the core by the time the impact happened, and that the impactor also contained an iron core. The majority of debris thrown out by the giant impact came from their rocky mantles, and the core of the impactor melted on impact and merged with the iron core of the Earth. 

There is one thing this theory doesn’t explain, however, and that is why only one Moon formed, or why we don’t see evidence for this happening elsewhere in the Solar System!

Geology of the Moon's features >

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Geologist on the Moon

Lunar geology

Moon rocks on Earth

Moon quakes

How the Moon was formed

Geology of the Moon's features

What is the Moon made of?

Lunar gardening

About impacts

Ray craters

Ageing wrinkles

Meandering channels

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