Where did the Moon come from?

• Collected itself at same time as Earth?
  • Then it should have same density as Earth.
  • Density is too low.
• Popped out of the Earth's mantle?
  • Density could be right.
  • But detailed composition of rocks don't match.
    • Moon rocks relatively lacking in volatile elements.
  • "Just popped out" doesn't sound very likely.
• Created in a collision?
  
  
  • Density could be right.
  • Composition could work: some of volatile elements lacking in Moon rocks could have been boiled away.
  • On the other hand, it seems to make the Earth rather special.
  • But why not? Venus and Mars don't have big moons.

My conclusion: we just don't know. But the collision idea seems promising.

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