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Saturn's moon Iapetus, composite Cassini image. There is an intriguing change in colouration between the water ice of the poles and a darker material around the equator. Iapetus is the third largest moon of Saturn, but orbits further out than most of the larger moons, at a distance of 3.5 million kilometres. It was originally discovered by Giovanni Cassini, the 17th century astronomer after whom the Cassini spacecraft was named. It is primarily composed of water ice and has a mean diameter of 1436 kilometres. The heavily cratered region which dominates this image is known as Cassini Regio. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA) === , ===