Astronomy Picture of the Day

Explanation: What has happened to Saturn's moon Iapetus? Vast sections of this strange world are dark as coal, while others are as bright as ice. The composition of the dark material is unknown, but infrared spectra indicate that it possibly contains some dark form of carbon. Iapetus also has an unusual equatorial ridge that makes it appear like a walnut. To help better understand this mysterious moon, NASA directed the robotic Cassini spacecraft orbiting Saturn to swoop within 2,000 kilometres just last month. Pictured above, from about 75,000 kilometres out, Cassini's trajectory allowed unprecedented imaging of the hemisphere of Iapetus that is always trailing. A huge impact crater seen in the south spans a tremendous 450 kilometres and appears superposed on an older crater of similar size. The dark material is seen increasingly coating the easternmost part of Iapetus, darkening craters and highlands alike. Close inspection indicates that the dark coating typically faces the moon's equator. Whether Iapetus' colours are the result of unusual episodes of internal volcanism or external splattering remains unknown. This and other images from Cassini's Iapetus flyby are being studied for even greater clues.