Japetus (Natural Colors; credits: Lunexit)

Da "NASA - Picture of the Day", del giorno 9 Agosto 2009:"What has happened to Saturn's moon Japetus? 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.
Japetus also has an unusual equatorial ridge that makes it appear like a walnut. To help better understand this seemingly painted moon, NASA directed the robotic Cassini Spacecraft orbiting Saturn to swoop within 2000 Km in 2007.
Pictured above, from about 75.000 Km out, Cassini's trajectory allowed unprecedented imaging of the hemisphere of Japetus that is always trailing.
A huge impact crater seen in the South spans a tremendous 450 Km and appears superposed on an older crater of similar size. The dark material is seen increasingly coating the easternmost part of Japetus, darkening craters and highlands alike.

Close inspection indicates that the dark coating typically faces the moon's Equator and is less than a meter thick. A leading hypothesis is that the dark material is mostly dirt leftover when relatively warm but dirty ice sublimates. An initial coating of dark material may have been effectively painted on by the accretion of meteor-liberated debris from other moons.
This and other images from Cassini's Japetus flyby are being studied for even greater clues".