Where did the Moon come from?

Dark Edge of the MoonIt struck me this morning, when @KnikiDavies retweeted this question from @BeeBecF, that although I've written a post about where moons come from, I haven't actually done one on where Earth's Moon came from. Allow me to put that right.

Just to make it obvious, 'the Moon' in the title of this post refers to our somewhat uninterestingly named, but nevertheless beloved major satellite. This thing:

You can read a little bit about the main theories of moon formation in my relevant post here, but what about our Moon?

Earth's own Moon is a bit different to most of the other moons in our solar system: it's much bigger, relative to its parent planet than any other moon that we know about, which implies that it came to be through different circumstances. Most moons are thought to have formed either alongside their parent by way of accretion (regular satellites) or to have formed elsewhere and then been captured by their (adoptive?) parent (irregular satellites). The prevailing theories are that our Moon did not come to be by either of these mechanisms.

Planet smash
The most popular theory for the formation of Earth's Moon goes by the rather excitingly titled giant impact hypothesis. This theory puts forward the idea that about 4 1/2 billion years ago, a young proto-Earth was involved in a cosmic collision with another body, a small planet about the size of Mars that is sometimes known as Theia.

Computer simulations suggest that Theia hit Earth from behind (relative to the direction of its orbit around the Sun) at an angle of about 45°, travelling at a relative speed of something below 4 kilometres per second. The energy of this collision caused much of the Earth's crust and mantle along with most of Theia's mantle (about 2% of the body's total mass) to be ejected into orbit around the Earth, as Theia's iron core sank to become part of the Earth's. It is thought that about half of the mass ejected into orbit coalesced to form the Moon in a relatively short timescale (about a month, at the low end of estimates).

Evidence
For:
Various lines of enquiry suggest that the Moon has an unusually small iron core when compared with other moons and planets. This would be in keeping with the giant impact hypothesis, as most of the heavier elements, such as iron, would have sunk to the core of the Earth, while a greater proportion of lighter elements were thrown into orbit.
Rocks collected during the Apollo Moon Landings have shown the Moon's crust to have a similar composition to the Earth's, including identical oxygen isotope ratios, which implies that the two were formed at the same time. Other analyses of the Moon indicate that a large portion of it was molten at some point in the past, and the giant impact hypothesis would certainly allow for the energies required for this to happen.

Against:
There are a few issues with the giant impact hypothesis which are yet to be resolved:
  • There is no evidence that the Earth ever had a magma ocean, which is implied by the giant impact hypothesis.
  • The ratios of certain elements that make up the Moon are not explained by the giant impact hypothesis- if the hypothesis is true, these must be explained by some other mechanism.
  • The identical oxygen isotope ratios put forward as evidence above also work against the hypothesis in one way: whilst supporting the idea that the Moon was made from the same material as the Earth, it is unlikely that Theia would have sported the same ratio of oxygen isotopes. Where have Theia's isotopes gone?
Alternative ideas
While there are other plausible hypotheses for the formation of our Moon, they all fall down at the same hurdle*: they do not adequately explain the high angular momentum content of the Earth-Moon relationship. This can be explained perfectly by certain initial conditions in the giant impact hypothesis. Other ideas include:
  • The Earth and Moon formed at the same time from the same accretion disk.
  • The Moon formed elsewhere and was captured at a later date by Earth's gravitational field.
  • The Moon was ejected from the Earth's molten surface by centrifugal force.



* There are other issues with them, too, but this is a common theme.

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