Based on analysis of some of the world's oldest rocks, scientists have the first direct evidence that a cataclysmic meteor shower changed early Earth's chemical composition.

The find offers support for the theory that meteors delivered gold and other precious metals to infant Earth.

The presence of precious metals in Earth's mantle and crust poses a puzzle, because these elements are attracted to iron. (Get an overview of Earth's layers).

When Earth first formed roughly 4.5 billion years ago, the planet was basically a ball of magma. As the planet cooled, denser material sank toward the center, eventually producing a core made mostly of iron.

(See "Infant, Magma-Ball Earth Glimpsed Via Newfound Rocks.")

But that means any iron-loving—or siderophile—elements present in the