Ancient Earth rock found on the moon.

Abstract

What may be the oldest-known Earth rock has turned up in a surprising place: the moon. A 2-centimeter chip, embedded in a larger rock collected by Apollo astronauts, is actually a 4-billion-year-old fragment of our own planet, scientists say. Sometime after the granitic rock formed, an asteroid impact blasted it from Earth. It found its way to the moon, where it was later engulfed in a lunar breccia, a motley type of rock. Finally, Apollo 14 astronauts returned it to Earth in 1971. Although geologists have found meteorites on Earth that came from the moon, Mars, and asteroids, this would be the first example of a terrestrial meteorite. The rock also holds clues to this ancient time on Earth, called the Hadean. It shows that the granitic rocks that make up the continents were already forming.