Iapetus

• Iapetus is mostly composed of ice, with around 20% rocky material.
• It has a distinctive 2-tone appearance where one side (the leading side) is darker than the other. This is thought to be caused by sublimating water ice changing colour when exposed to sunlight.
• Another unique feature is the mountain range around 3/4 of the equator (see below).

The equatorial ridge was discovered by the Cassini spacecraft in 2004. It is 1,300 km long and 20 km wide. There are many competing theories as to how the ridge formed, with scientists leaning towards the idea that Iapetus once had a ring system that fell inwards.