When the sky rained rocks
Something violent happened to Earth and the Moon when they were in their infancy. Scientists have a name - and a tentative explanation - for the event, but unraveling the reality will likely require samples from the South Pole-Aitken basin.
About half a billion years after the solar system was born, some unknown event sent a shower of giant asteroids through the vicinity of Earth and the Moon. While planetary scientists who have studied the shower's effects on the Moon don't know for certain what caused it, they have named this epochal event the Late Heavy Bombardment.