A collision eons ago between the earth and another celestial body throw vaporized rock into the atmosphere, some of which eventually became the moon. If this is true, you'd think that the earth and the moon would share the same basic materials, but there is more iron on the moon and more magnesium on earth.
Now researchers have an answer, and it's completely awesome. Magma rain would resolve the mystery, as rising rock vapor would see its magnesium oxide start to condense into droplets and fall back onto the planet's surface. The iron oxide inside the rock vapor wouldn't have condensed as easily, meaning far more of it got mixed into the disc that became the Moon.
Which evokes some interesting mental pictures resembling heavy metal album covers. Link -via Geekosystem
(Image credit: Fahad Sulehria)