IAPETUS

exterior moon in the Saturn system, extraordinary because of its surface markings. Discovered by Gian Domenico Cassini in 1671, Iapetus orbits Saturn once every 79.33 Earth days at a distance of 3,561,000 km (2,212,800 miles). The satellite's radius and mass are, respectively, 730 10 km (454 6 miles) and 1.88 0.12 1024 grams, implying a bulk density of 1.16 0.09 grams per cubic centimetre. A solid object with so low a density must be composed mainly of ices. The period of rotation of Iapetus is observed to equal its period of revolution, as is the case with the Earth's Moon, and for the same reasonnamely, tides exerted on the solid body of the satellite by the gravity of the larger nearby planet. As a result, one face of Iapetus is always leading in its orbital motion and the other always trailing. Remarkably, the leading hemisphere is extremely dark, reflecting only a small percentage of the sunlight incident upon it, while the trailing hemisphere is as much as 10 times a better reflector. This corresponds to the greatest variation in brightness across any object known in the solar system. Impact craters were discovered during the early 1980s by the Voyager spacecraft in the trailing hemisphere but not in the leading one. The surface material in the bright trailing hemisphere is almost certainly water ice, possibly with an admixture of methane and other ices. The dark material at the surface of the leading hemisphere is thought to be complex organic molecules, perhaps supplied by Saturn's outermost satellite, Phoebe, or produced in situ by solar ultraviolet irradiation of the methane component of the ice. In the latter case, the rapid evaporation of methane ice and ballistic transport of material from the leading to the trailing hemisphere by micrometeorite impact are invoked to explain the brightness asymmetry between the hemispheres.