The Cassini spacecraft, which has been orbiting Saturn and observing the planet and its moons since July of 2004 (Cassini’s partner, the Huygens probe, landed on and is exploring the moon Titan), sent back photos in September of one of the most unusual objects in the solar system. Iapetus is Saturn’s third-largest moon, and its unusual shape and bizarre colouring make it stand out so much that it was featured in Arthur C. Clarke’s famous novel, 2001: A Space Odyssey.
Discovered in 1671 by a gentleman named Giovanni Domenico Cassini, Iapetus is distinguished by a dramatic two-tone colouring that make it look like a spherical yin-yang symbol. One pole is bright white and the other, very dark. The two colours can be distinguished even at small scales, as Cassini (the spacecraft) discovered, with craters composed of both light and dark patches but no grey areas at all. The bright part is ice, and scientists believe that the dark material— which constitutes a very thin layer on the surface of the ice— is what gets left behind by sublimated ice (also called “lag’).
The moon orbits Saturn in such a way that the same side always faces the planet. From Earth, when Iapetus is on Saturn’s western side, we see the bright part, and when it’s on the eastern side, the moon looks dark.
Another interesting feature of Iapetus is its equatorial ridge— so pronounced that it can be seen even from far away. Unlike most moons, it has neither a spherical nor an ellipsoid shape. Iapetus looks more like a walnut.
