Two faced
Name Iapetus
Planet Saturn
Size 8.7x smaller than Earth
Discovered 1671
Orbiting around 2.2 million miles from Saturn, Iapetus is an icy moon that bears two faces. Iapetus is almost entirely frozen, with a mean global density only slightly above that of ice. Just two per cent of its total mass is rock, and its surface temperature can plummet as low as -183 degrees Celsius.
What makes Iapetus special is its different hemispheres, which are noticeably split into sides where one is darker than the other. There have been many explanations as to why Iapetus has a permanent dark side, but the prevailing theory is that another of Saturn’s moons, Phoebe, is to blame. Phoebe kicks up cosmic dust into space by being bombarded with micrometeorites, forming a ring around Saturn. Half of Iapetus passes through the ring while it orbits in the opposite direction. As if moving through a cloud of soot, Iapetus collects dust on its surface as it goes.