Why is Earth's moon so much bigger than all the rest?

Question posed by Robin.

First of all, I'll clarify the title: There are plenty of moons in our solar system that are at least as big as Earth's moon ('The' Moon). Out of interest, I include an image I found that compares the largest moons in our solar system. It also includes, for the sake of comparison, Mercury and Pluto (which is, of course, no longer a planet):

As you can see, the Moon is (currently) the fifth biggest moon in our solar system, by diameter.

So what's the title all about?

Ganymede, Callisto, Io and Europa are moons of Jupiter, Titan is one of Saturn's, Triton belongs to Neptune, and Titania orbits Uranus. Of the eight largest moons in our solar system, seven of them orbit gas giants. The Moon, 5th largest, orbits Earth, which is tiny in comparison: Neptune, the smallest of the four gas giants, is still almost four times the size of Earth (by diameter). The rule is that big moons orbit big planets. The Earth - Moon system is the exception to that rule. Take a look at the image below and you'll get an idea of how large our Moon actually is in comparison to other moons in our solar system, particularly in relation to the size of their parent planets. Beneath that picture, I've included an image that compares the relative sizes of the Earth and Jupiter, so that you can hopefully see how closely matched the Moon and Earth are in terms of size when compared to Jupiter and its moons:

 Selected moons of the solar system, with Earth for scale, by Edward Yanquen, CC BY 2.0, via Wikimedia Commons

A size comparison of Earth & Jupiter

So why is the Moon so big compared to Earth?

Current theory says that the reason lies in the method of formation of the Earth. A short while ago, I wrote about how moons are formed. I'll refer to the methods stated in that post now.

Most moons are thought to have found themselves in orbit around their parent planets by way of two main routes. These are accretion (a moon formed from the same primordial matter as its parent at about the same time) and capture (a wandering moon is caught by its parent's gravitational field, and drawn into orbit). Neither of these possibilities fit the Earth-Moon system very well, for the following reasons:

• Accretion: Moons that form in this way are likely to have a similar composition to their parent planet. The compositions of Earth and the Moon are too different for this to be the likely method of formation.
• Capture: The Moon's orbit around the Earth is very close to being circular. The likelihood of this happening with such a relatively large satellite is very low.

I also mentioned two other less common possibilities for moon formation: fission, where matter is thrown off a body that is rotating very fast, and impact, where a large body hits the parent hard enough for matter to be ejected into space:

• Fission: The Moon orbits outside of the equatorial plane of the Earth, making this an unlikely scenario. Also, there is not sufficient angular momentum in the Earth-Moon system; there would need to be a reason for it slowing down.
• Impact: Current theories suggest that an object of comparable size to Mars collided with the Earth, which caused much of the Earth's mantle (and that of the colliding object) to be vaporised. This material, due to the collision forces as well as hot expanding gases, was then thrown into orbit around the Earth. This material then cooled and coalesced to form the Moon.

So far, this last theory does not contradict the evidence available and helps to explain things such as the differences in the composition of the Moon and the Earth.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.