Iapetus, one of the moons of Saturn, photographed by Voyager 2, 1981

Iapetus, one of the moons of Saturn, photographed by Voyager 2, 1981. Iapetus, the 17th and third largest of Saturn's moons is a mysterious body. Its trailing hemisphere, seen here, is reflective and therefore very bright whereas the leading hemisphere is extremely dark, and the dividing line between the two hemispheres is very sharp. Measurements of the density of Iapetus suggest that it must be composed almost entirely of water ice. The dark side appears to be coated with organic materials, possibly hydrogen cyanide polymers. Their origin is uncertain, and NASA's Cassini probe will attempt to investigate the mystery in 2004. The two Voyager spacecraft were launched by NASA in 1977 to explore the planets in the outer solar system. Voyager 2 passed Saturn in August 1981, before continuing on to Uranus and Neptune. ©SSPL/NASA