There's a theory kicking about that has the early Earth colliding with a Mars sized object which contributed most of its iron to the Earth's core, with the remaining fragments forming our moon. It's one way of explaining the high iron content of Earth relative to Venus and Mars.

For more reading on this subject may I suggest "Worlds in Collision" by Immanuel Velikovsky. The only problem being that it is almost impossible to find a copy of it today since it is one of the most banned and derided books of all time. Personally, I found it a very scholarly work.

I used to have a paperback copy myself but lent it out and never saw it again. The penalty for wishing to share fascinating information with friends is that you end up with a library full of books that you don't want to read a second time.

Beav