The Strange Tailing Side of Saturn's Iapetus

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 kilometers just last
month. Pictured above, from about 75,000 kilometers 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 kilometers 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' colors 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.