Iapetus

Iapetus is the third-largest moon of Saturn, first observed by Giovanni Domenico Cassini in October 1671 on the western side of Saturn. It is named after the Titan Iapetus from the Greek mythology. Iapetus is best known for its dramatic 'two-tone' colouration. Cassini correctly surmised that Iapetus has a bright a dark hemisphere; and furthermore that it is tidally locked, always keeping the same face towards Saturn, so that the bright hemisphere is visible from Earth when Iapetus is on the western side of the planet, and the dark hemisphere on the other side. NASA's Voyager 2 flew past Iapetus on 2 August 1981. Passing at a relatively distant 966,000 km, the spacecraft's cameras could make out few details in the area of dark material. The images revealed the bright side to be icy and heavily cratered. Recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.