How Was The Moon Formed

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How was the moon formed? Like the Universe, there is more than one theory as to how the Moon was formed. Let’s first have a look at four that have garnered substantial following some time in the past, including one that’s still most widely accepted today.

Fission Hypothesis

There are those who believed the Earth spun so fast that a portion of its crust broke off to form the Moon. This theory is consistent with the composition of the Earth’s crust and the Moon, which both have a low density of iron. Earth has a large iron core but has a lesser concentration of iron in its crust.

Hence, if a chunk would be taken off the Earth’s crust, we would have something like the Moon. This theory however fails considerably in total angular momentum and energy analyses in connection to the Moon’s formation.

Capture Hypothesis

Others once believed the Moon came from another part of the Universe with little iron, strayed near the Earth’s gravitational field, and was drawn into orbit. This, too, failed for three main reasons.

One was due to the fact that the isotope composition of Moon rocks is very similar to that of rocks from our planet. Second is the necessity for a very small (thus, highly unlikely) encounter velocity. And third, is the absence of a very thick atmosphere that should have been present to provide a so-called gas drag that would have helped in the capture process.

Co-Formation Hypothesis

Still others believed that both the Moon and the Earth were formed at practically the same time and in the same region in the solar nebula, drawing materials to each other from the dust around them. While this hypothesis is consistent with the proximity of the two bodies, it is difficult to explain why they the Moon doesn’t have as much iron as the Earth.

Giant Impact Hypothesis

Of all four hypotheses that strive to explain the Moon’s origin, it is the Giant Impact that has received the widest acceptance. In this hypothesis, the Moon is believed to have been originally a part of the Earth’s crust, whacked out by a collision between the Earth and another body bearing the size of Mars.

Since the whacked-out piece may have certainly come from the outer layer of the Earth (the crust), then this explains the lack of iron in the Moon. Furthermore, computer simulations also show how this theory is also consistent with angular momentum measurements.

We have written many articles about the Moon for Universe Today. Here’s an article about the giant impactor theory, and here’s an article about how scientists link the formation of the Earth and the Moon.

Here’s an article that talks about how the age of the Moon was determined. And some history from Stony Book labs, the people who analyzed the first moon rocks.

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?

Tagged as: how was the moon formed, Moon, moon origin