IAPETUS

The walnut

Mass: 1.8 x 1021kg (4.0 x 1021lbs) Diameter: 1,469km (913 miles) Parent planet: Saturn Discovered: 1671, Giovanni Cassini

Iapetus has two distinct claims to a place in any list of weird satellites. The first became obvious when it was discovered in 1671 – it is much dimmer when seen on one side of its orbit compared to the other. Its leading hemisphere – the half that faces ‘forwards’ as it orbits Saturn – is dark brown, while its trailing hemisphere is light grey. One early theory to explain the colour difference was that the leading side is covered in dust generated by tiny meteorite impacts on small outer moons, which spirals towards Saturn.

However, images from Cassini reveal a more complex story. Most of the dark material seems to come from within Iapetus, left behind as dark ‘lag’ when dust-laden ice from the moon’s surface sublimates – turns from solid to vapour. The process was likely started by dust from the outer moons accumulating on the leading hemisphere, but once it began, the tendency of the dark surface to absorb heat has caused a runaway sublimation effect.

Iapetus is also ringed by a mountainous equatorial ridge that is 13 kilometres (eight miles) high and 20 kilometres (12 miles) wide, giving the moon its distinctive walnut shape. The origins of this ridge are puzzling – some theories suggest it is a ‘fossil’ from a time when Iapetus span much faster and bulged out at the equator, while others think it could be debris from a ring system that once encircled the moon and collapsed onto its