Moons of our Solar System
Moons of our Solar System

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Moons of our Solar System

3.1 The Moon’s origin

What’s the most likely origin of the Moon? There are three classic explanations but all are flawed in some way.

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Just exactly how the moons of the Solar System were formed is still a subject of debate in the scientific community. There are three classic explanations for the origin of our own Moon that have been around for a number of years, as well as a more modern idea that has become the most widely accepted.
The first of the classic theories is called the fission hypothesis. The fission theory suggests that the Earth and the Moon were once part of the same planetary body, but that while this body was still hot, the Earth and Moon became separated.
This idea is supported by similarities in the rock composition between the Earth and the Moon. However, a necessary condition for this theory would seem to be that the two bodies also have to move in the same plane, whereas, in fact, the Moon’s orbital plane is tilted by about five degrees. Another complication is that Earth would have to have been spinning about eight times faster than it does now, before it could split apart. A controversial twist on this explanation that was developed in 2012, suggests that the fission process was aided by a natural nuclear explosion occurring deep inside the Earth, at the core-mantle boundary.
The second idea is known as the capture hypothesis. This explanation suggests that the Moon was formed somewhere else in the Solar System, and was later captured by the Earth’s gravitational field, then pulled into orbit as it passed nearby. Unfortunately, this hypothesis does not explain why the composition of the Moon and Earth are so similar, or how the Moon’s orbit was then stabilised after its capture.
The third longstanding theory is the accretion hypothesis. This suggests that the Moon and the Earth condensed separately from the material that formed the Solar System. The Moon was built up from the material left in orbit around the Earth, after the Earth formed. The problem with this explanation is that if both bodies were made out of the same material, then the Moon should have the same iron content as the Earth, but we know that this isn’t the case.
A newer concept called the giant impact hypothesis has become the most widely accepted theory to explain how the Moon was formed - though even this doesn’t fit in all aspects. This hypothesis proposes that an object the size of Mars struck the early Earth soon after the formation of the Solar System, around four and a half billion years ago. Because of this impact, the iron core of the impacting body became smeared onto the Earth’s core, while large volumes of rocky material were ejected from the outer layers of both bodies. The ejected rocky material eventually condensed in orbit around the Earth to form the Moon.
This theory can explain the Moon’s low density, its low iron content, and its impoverishment in volatile elements. This hypothesis is also supported by the evidence we can see in many places that collisions like this were common late in the formative stages of the Solar System. However, the isotopic compositions of the Moon and Earth are so similar, that many scientists are beginning to doubt this explanation.
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See here for another bonus video recorded by Professor David Rothery at the 2014 Lunar and Planetary Science Conference, this time with Bill Bottke about the Moon’s origin.

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If you wanted to look into this further, you might find the following links of interest:

  • David Rothery talks to Bill Bottke [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . David Rothery recorded this bonus video message at the 2014 Lunar and Planetary Science Conference, this time with Bill Bottke about the Moon’s origin.
  • When did the Moon form? In this short article, David Rothery discusses two pieces of research published in 2017, which suggest pushing the date slightly earlier and modifying the mechanism to a series of slightly smaller ‘giant impacts’ in place of a single Moon-forming giant impact.
  • Multiple impacts to make the Moon? A blog by an Open University PhD student examining alternative hypotheses.


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