Prospectors who lost out in the gold rush probably wouldn't agree, but Earth has more gold than it should. The shiny stuff is one of several metals that are puzzlingly common. Now it seems they might have arrived during a last-minute bombardment with giant rocks after Earth had formed. The planets took shape when tiny rocks collided, forming ever larger lumps. After Earth was born a planet crashed into it, blasting a huge cloud of material into orbit, where it coalesced to form the moon.
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