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This image shows the limb of Saturn's moon Iapetus, captured by #Cassini on Sept. 10, 2007. A ridge of 10km-high (6.2 mile) mountains encircles the equator, possibly the remains of a former satellite that broke apart as its orbit degraded. Iapetus is 1,470 km (913 miles) wide. 
Here's a wide-angle view of Iapetus from the same observation set on 9/10/07. Color comp made from images acquired in IR, G, and UV.
Iapetus is an icy moon stained dark. This "telephoto" view of the eastern limb region from the previous image shows where dark material has coated its leading forward-facing side. This dark material originates from even more distant moon Phoebe, which is likely a captured comet.
