Metal Rain

Asteroids that slammed into Earth and the moon more than 4 billion years ago were vaporized into a mist of iron. A cloud of iron mist will have wrapped around the globe after any such collision, falling to Earth as metal rain. The iron mist thrown up from the high-velocity impacts of these asteroids traveled fast enough to escape the moon’s gravity but stayed gravitationally stuck on more massive Earth. This may help explain why the chemistry of the Earth and the moon differ.