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.
The rocks came from the asteroid belt, which lies between the orbits of Mars and Jupiter. They ranged in diameter from about 100 meters to tens of kilometers, assert Robert G. Strom of the University of Arizona in Tucson and his colleagues in the Sept. 16 Science.
From the sizes of the craters, Strom's team determined the diameters of the projectiles that created them. The researchers then analyzed a compilation of recent asteroid surveys that provide the