Saturn's moon Iapetus was discovered by Giovanni Cassini in 1671. It's the 22nd closest moon to Saturn, and the most distant of the planet's large moons. It measures 1472 km across, and orbits above Saturn's center at an altitude of 3.6 million km. Iapetus has got to be one of the most bizarre objects in the entire Solar System. When Cassini first discovered Iapetus, over 300 years ago, he noticed that it was brighter on one side of Saturn than the other. He correctly realized that Iapetus is tidally locked to Saturn, so that it always shows one face to the planet. He also realized that one hemisphere is dark, while the other is bright. This observation was confirmed when NASA's Voyager spacecraft made their flybys through the system, and even more images have been captured by NASA's Cassini spacecraft. Cassini also turned up a bizarre equatorial ridge that runs about halfway around the moon. Since Iapetus is tidally locked, one hemisphere is always leading in orbit, and the other is trailing. The leading hemisphere is dark colored, while the trailing hemisphere is as bright as snow. Astronomers think that temperature variations on Iapetus release water vapor, which moves across to the leading side, and freezes again, darkening in sunlight. Here are some images from a recent flyby of Iapetus, and even more amazing images of Iapetus. Want more info on Iapetus? Check out NASA's page from the Cassini mission, and more images from the Planetary Photojournal. LINK: