Cassini solves Cassini's mystery

Saturn's moon, Iapetus, is dusted on one side, as if you took a baseball, sprinkled it with cocoa powder, and turned it on its side. Why? That's what Giovanni Cassini asked in 1671. And we finally know why, thanks to the Cassini probe that's currently exploring the Saturn system. As it turns out, the moon's incredibly slow rotation (it takes 80 days to rotate once on its axis) is to blame. (NY Times) Two factors. First, the leading edge of the planet collects dust as it flies through space. Second, as the moon's surface slowly warms, ice melts around the equator (exposing more dust) and freezes in other areas (covering other dust).