A satellite of Saturn, discovered by Giovanni Cassini in 1671. The Voyager probes confirmed a hypothesis, proposed by Cassini after he noticed variations in Iapetus's brightness, that one hemisphere is very much darker than the other. The satellite always keeps the same face towards Saturn and, as it orbits the planet, its dark and light hemispheres alternately face the Earth. The bright surface is cratered and probably ice-covered; the dark part is blanketed with a material ten times darker whose nature and origin are unknown. The density of 1.2 times that of water suggests that Iapetus contains a high proportion of ice, possibly including frozen methane and ammonia.
