Photo: Two global images of Iapetus showing the extreme differences in brightness on its surface | NASA/JPL-Caltech/Space Science Institute
Iapetus is the 3rd largest of Saturn’s 82 moons, coming in behind Titan (1,599.9 mi) and Rhea (474.6 mi) with a radius of 456.4 miles. Because one of its hemispheres is dark in color and the other is light, this unusual natural satellite is often referred to as the “yin and yang” of Saturn’s moons. Astronomers theorize that this unique two-tone coloration is a result of one side of Iapetus’ surface gathering particles from Phoebe, one of Saturn’s dark moons. Another popular theory is that the dark material is residue left over from water and ice evaporating from the surface, as Iapetus’ dark side receives more exposure to sunlight than the other.
The light side of Iapetus | NASA
Iapetus orbits at a staggering distance of 2,213,000 miles away from Saturn’s surface, much further than any of the ringed planet’s other moons. It remains in orbit due to the strength of Saturn’s gravitational pull, the sheer force of which creates interesting effects on the moons that orbit closer to the planet’s surface. For example, Saturnian moon Enceladus is home to a ...