Iapetus was discovered in 1671 by Giovanni Domenico Cassini; it is the outermost midsized icy satellite of Saturn. Its distance to Saturn is 3,560,000 km (or 59 Saturnian radii), and its diameter is 1,440 km. Its density is 1.02 g/cm3, indicating a very low rock/ice ratio. Iapetus is unique in its albedo distribution, which is very low on Iapetus’ leading side in its orbit around Saturn and ten times higher on the opposite side. It has been proposed that the dark material could be due to the permanent accretion of matter coming from the neighboring satellite Phoebe, which is also very dark. Another possible explanation is that the low albedo of Iapetus could be a result of organic deposits formed from methane ice impacted by dust particles.