Japetus

Caption NASA:"Images taken with Cassini's Ultraviolet Imaging Spectrograph shed some light on the dark side of Saturn's moon Iapetus. Scientists are trying to figure out what painted Iapetus' Dark Side. This is one of the biggest mysteries scientists are trying to answer during the upcoming Sept. 10, 2007, flyby.
The ultraviolet image on the left indicates water ice abundance across the surface: the bright North Polar Terrain (shown in red) is the iciest region in this view. Away from the Pole, as the color shifts to blue, less water ice is present in the surface.
The darkest terrain, which includes very little water ice, is shown in light blue.
The dark sky background viewed during the observation is shown as purple in this color scheme.

The ultraviolet-light image was taken during a flyby in December 2004. A visible light image taken on the same date is shown on the right for reference".