Astronony Picture of the Day has been great viewing this past week.

This is Iapetus, from 13/1/12.

Iapetus is one of Saturn’s moons. It is famous for having a “bright” side and a “dark” side – in technical terms, its albedo varies. Its discoverer, Cassini, for whom both the large obvious space between Saturn’s rings (the Cassini Division) and the spacecraft sent to Saturn to photograph the system are named, correctly deduced this when he couldn’t see it on one side of Saturn a few months after it being clearly visible on the other, then it reappeared on the other side: an improved telescope showed it as two magnitudes dimmer when its “dark” side faced the Sun (and Earth.)

In this view you can see some of the dark material on the right as well as a huge impact crater, superimposed on another seemingly as large.

Iapetus also has an equatorial ridge – see this picture from 1/2/2005 – which makes the moon look like a walnut!