Iapetus is Saturn’s third-largest moon and perhaps its strangest. Its two hemispheres differ so dramatically in brightness that Giovanni Cassini, who discovered it in 1671, could only see the moon when it was on one side of Saturn. The leading hemisphere (Cassini Regio) is painted a dark reddish-brown by dust blown off the distant retrograde moon Phoebe; the trailing hemisphere is bright water ice. Iapetus also features a mysterious 20-km-tall equatorial ridge that wraps two-thirds of the way around the moon — the tallest relief in the Solar System.