Narrator: This is Science Today. The best way to learn about how early solar system activity affected the Earth is to study the moon. Paul Renne, a geologist at the University of California, Berkeley, says that's what he and his colleagues decided to do when looking into the impact history of meteoroids on Earth.
Renne: The Earth is a terrible recorder of ancient events. We have an atmosphere, we have oceans, we have plate tectonics. I mean, basically, the Earth cleans itself up very, very quickly. The moon by comparison, preserves its record really well.
Narrator: Ancient meteoroid bombardment is evident in the lunar soil samples taken by the Apollo missions. The molten rock and soil that formed from these impacts were churned up with all the detritus and debris on the surface of the moon.
Renne: That's a process that's called lunar gardening, which when you first hear about it, invokes some little man coming out and raking the moon everyday. But no, lunar gardening is just this constant churning that happens with impacts of meteoroids and comets and so forth.
Science Today, I'm Larissa Branin.