Saturn's moon Iapetus is equally enigmatic. Scientists surmise that the bright side is water ice and the dark side is an organic material of some kind. But how the dark material got there is a mystery. The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp. But by far the most intriguing natural satellite of Saturn is its largest.
Titan lies hidden beneath an opaque atmosphere more than fifty percent denser than Earth's.
If Titan received more sunlight, its atmosphere might more nearly resemble that of a primitive Earth. What fascinates scientists about Titan's atmosphere is that it is filled with a brownish orange haze made of complex organic molecules, falling from the sky to the surface. Thus in many ways it may be a chemical factory like the primordial Earth. Because of the very dim sunlight at Saturn's orbit, solar arrays are not feasible and plans call for power to be supplied by a set of radioisotope thermoelectric generators, which use heat from the natural decay of plutonium to generate electricity to run Cassini. These power generators are of the same design as those used on the Galileo and Ulysses missions.