Coated Craters

Dark material has coated the low-elevation terrain and the interiors of craters in the southern portions of the quadrant on Iapetus that faces away from Saturn. This is part of the boundary region separating the dark leading and bright trailing hemispheres. The dark coating is thought to be no more than a few tens of centimeters thick and, as seen here, predominately appears on the northern-facing walls of craters in the south.
Farther south, the dark splotches are less numerous and appear almost absent at the highest latitudes (near the bottom of the frame). This is a strong indicator that thermal effects play a role in the darkening process of parts of Iapetus' surface: the colder the surface, the less common is the dark terrain. As on Earth, the higher latitudes on Iapetus receive less heating by sunlight.

At left, below center, the eastern rim of a great and ancient impact basin can be seen. With a diameter of almost 500 kilometres, it is one of the largest impact structures on Iapetus, 1468 kilometres across, and in the entire Saturn system.

The mosaic consists of three narrow-angle camera footprints across the surface of Iapetus. This view is centered on terrain near 35.1° south and 218.5° west. Image scale is approximately 231 meters per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft on 10 September 2007, at a distance of approximately 40 000 kilometres from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 31°.