Object Analysis

Iapetus is the third-largest moon of Saturn and is famous for its "two-faced" appearance. One hemisphere (Cassini Regio) is as dark as coal, while the other is as bright as snow. This extreme contrast is unique in the Solar System and was first noticed by Giovanni Cassini in 1671.

The dark material is thought to be dust swept up from space (likely from the outer moon Phoebe) that landed on the leading side of Iapetus. Because dark surfaces absorb more heat, the ice on the dark side sublimates and migrates to the colder, brighter side, reinforcing the contrast in a runaway feedback loop.

Iapetus also features a massive equatorial ridge that runs three-quarters of the way around the moon. The ridge reaches heights of 20 km (12 miles), making it significantly taller than Mount Everest. It gives the moon a distinct "walnut" shape, and its origin—whether from an old ring collapse or internal cooling—remains a mystery.