What whacked the inner solar system?
By Ron Cowen
For the first 700 million years of their existence, the moon and Earth and the other rocky planets took a beating. Space debris hammered these bodies so fiercely that their surfaces were stripped away. Moon samples brought back by the Apollo missions in the early 1970s confirmed that this violent era ended about 3.85 billion years ago. But researchers haven’t known the form of that early debris.
A team of planetary scientists has now determined that the culprits were ancient asteroids rather than comets.