How meteoroids built up Iapetus' mountain ridge

Moons in the solar system come in many different forms. Some are boulder-sized, while one is larger than the planet Mercury. Some are mixtures of rock and iron, while others hide oceans and rocky cores under icy surfaces. Two even look a bit like walnuts, each hosting a bulge of material around its equator. Interestingly, both of these, Iapetus and Pan, orbit Saturn. And had I written this post a month ago, “two” would have been “one.” Scientists discovered March 8 that Saturn’s tiny satelli