Searchlight of the Solar System

Saturn's moon Iapetus has a bright hemisphere and a dark hemisphere, quite unlike those of any other body in the solar system. Arthur C. Clarke used Iapetus in his book "2001: A Space Odyssey" as a cosmic signal to alert humanity to a superior intelligence's presence (the movie moved the setting to Jupiter, however, because Saturn is just so far away). New pictures from space probe Cassini are provoking renewed debate about what the light and dark sides are made of!