Re: Maybe the carbon crash landed there
I assume that the idea is that the Moon originated from near-surface bits of the pre-Earth combined with whatever hit it, which all both got very hot and spent time as very small chunks of material (either as vapour or as relatively tiny, very hot particles) before coalescing to be the Moon.
So small fragments have huge surface area, and they're very hot so the carbon boils away.
Compare that with the big chunk which formed the Earth: this was plausibly cooler (the lost surface layers of the pre-Earth object took most of the collision energy away) and very much larger (it didn't spend time as very hot dust or vapour). So the surface-to-volume is relatively tiny and the carbon boils off more slowly.
At least some argument like that seems plausible, but note I know nothing about this (I should read about it): I'm just trying to work out how it might have worked, on the assumption that the people who do study this have thought about it.
Biting the hand that feeds IT
