Why is the Earth's moon so big?

3,168

Most moons are captured bits of rock left over from the formation of the planet as it swept its bit of orbit clear.

Our moon is (probably) the result of a very big bit of rock hitting the early Earth and splitting it into two. The smaller bit ended up as the moon.

This also explains the relatively high density of the Earth (it got left with all the heavy core parts) and the low density of the moon (it got all the light surface rocks)

Share:
3,168
Bryson S.
Author by

Bryson S.

Updated on June 12, 2020

Comments

  • Bryson S.
    Bryson S. over 3 years

    It could just be me, but it seems like our Moon is WAY bigger than it should be for a planet of our size. If you look at satellite-to-planet mass ratios for the largest moons of Jupiter, Saturn, Uranus, Neptune and then the Earth, the values are on the order of $\mathbf{10^{-5}}$ for Jupiter, $\mathbf{10^{-4}}$ for Saturn, $\mathbf{10^{-5}}$ for Uranus, $\mathbf{10^{-4}}$ for Neptune, but a whopping $\mathbf{10^{-2}}$ for the Earth-Moon system. What gives? Why is the relative size of our Moon two orders of magnitude larger than the average for the solar system?

    • CuriousOne
      CuriousOne about 9 years
      That's because technically the Earth-Moon system is a(lmost) a binary planet. The formation mechanism for the Moon is very different from that of the small moons of the large gas planets. The best current hypothesis for the formation is an impact of a Mars sized object into early Earth, while the smaller moons of the gas planets are formed either by condensation of material from the proto-planetary cloud or they are captured by the gas-giants at a later stage.
    • b_jonas
      b_jonas about 9 years
      Is there something about this in the essay "The tragedy of the Moon" by Isaac Asimov? As far as I can remember, it's only about the consequences of a large moon, not about the causes.
    • Kyle Oman
      Kyle Oman about 9 years
    • Bryson S.
      Bryson S. about 9 years
      @Kyle Definitely related, but I am inquiring about the asymmetry found in our solar system with respect to satellite size, not hypothesizing the effect of a permutation to our current arrangement.
    • asmaier
      asmaier over 6 years
      The mass ratio for the Pluto-Charon system is even $10^{-1} $.