Collision Course

The ancient moon- forming planet Theia probably came from the inner solar system

- Jacek Krywko

Artist's impression of the planet Theia colliding with ancient Earth to create the moon

ROUGHLY FOUR AND A HALF billion years ago the planet Theia slammed into Earth, destroying itself, melting large portions of our planet's mantle and ejecting a huge debris disk that later pulled together to become the moon. Scientists have long wondered about both the composition and the origin of Theia. Now they have evidence that it formed very close to home.

The original giant-impact model of the moon's formation, proposed in the 1970s, predicted that the moon was made mostly of material from the colliding object. This scenario implied there should be differences between the chemical compositions of the moon and Earth, but research has found that the two are nearly identical—far more similar than two independent planetary bodies should be. A study published recently in Science took a close look at other things Theia gave us besides the moon: additional molybdenum and iron left behind on Earth from the collision.