Iapetus is one of Saturn’s most mysterious moons: this icy moon with a diameter of 1468 kilometres presents the observer with two hemispheres that have developed in completely different ways.
The hemisphere facing the opposite direction of the moon’s orbit around Saturn, the so-called 'trailing side', as well as the polar areas, consist of white ice and reflect the light of the Sun almost 100 percent; on the surface of the hemisphere facing the direction of orbit, the so-called 'leading side', however, carbon compounds cover the ice – cyanide and other carbon compounds make the surface as black as tar.
This false-colour image shows the first mosaic of high-resolution image data from bright side of Iapetus; it consists of 60 separate images, which were recorded in September 2007 from a distance of 73 000 kilometres. This flyby had been planned in great detail by staff members of the Freie Universität Berlin and DLR. The transition zone between both extremes is especially interesting.
Decisive explanatory information about the cause of Iapetus’ strange characteristics has been provided not only by the image data from Cassini, but also through observations performed with NASA’s Hubble Space Telescope. Beyond the orbit of Iapetus, Saturn is surrounded by a giant torus of tiny, dark dust particles, inclined at 27 degrees to the equator and the main ring plane. The particles originate from impacts on the small moons of Saturn that orbit further out than Iapetus, such as Phoebe, whose orbit lies in the middle of the torus. The density of the particles is extremely low, yet high enough for some of the small particles that migrate towards the interior of the Saturn system are attracted by the leading side of Iapetus and, over a long periodof time, have been collected and compacted into a thin, black layer.

NASA/JPL/Space Science Institute