Iapetus!
(image from NASA)
Iapetus is the third-largest moon of Saturn, and orbits much further out than the planet's other large moons--and on a much more inclined orbit. It is also called "Saturn VIII" (it was originally "V" but then three more moons were discovered :p; this must have been back when the Roman numerals were supposed to represent orbit order, rather than discovery order).
Iapetus is composed primarily of water ice, which means that its 735 km-radius mass is relatively low density.
(image from NASA)
Iapetus has a two-tone color system, in a pattern sort of resembling a tennis ball! One part is dark, the other light, and the difference is so great that the moon's discoverer, Cassini (and the best images of the moon have been taken by the craft named after him), when he found it in 1671, could only see it when it was on one particular side of Saturn, where it was showing its light side; he was finally able to see it on the other side, with its dark side forward, in 1705, using an improved telescope.
(image from NASA)
Iapetus is a crazy-ass moon! It bulges at the equator, squishes at the poles, and has an ancient equatorial ridge running around the dark part, rising up to 20 km above the rest of the surface; the moon has been described as "walnut-shaped." As is not surprising around a planet with such a huge ring system, Iapetus' surface is very heavily impact-scarred. The large crater with the diagonal rise in the middle that you see in many of the images here is the crater Engelier, 504 km in diameter (the largest found so far is Turgis, 580 km in diameter).
(image from NASA)
The dark stuff is a foot-thick layer of what is thought to be mostly residue (called "lag," apparently) from the evaporation of water ice from the surface. Wikipedia says (bolding mine ;): "It contains
organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets..." The current theory is that stuff from nearby and opposite-orbiting moon Phoebe--whose large ring of dust was just announced last month, detected by the
Spitzer Space Telescope--hit Iapetus, covering half of the moon's surface--this could have happened because Iapetus rotates very slowly: a day on Iapetus is over 79 Earth days. Dark surfaces absorb more heat than light ones, and the color difference caused ice in the dark areas to evaporate more rapidly than in the uncovered, light areas, and that increased evaporation created more dark residue, perpetuating the dark/light surface area division; the dark areas are 15 degrees (C) warmer than the light areas in daylight (128 K vs 113 K)! The estimate is that in 1 billion years, 20 meters of dark surface evaporates away, vs only 10 cm of light surface.
In the upper middle of this shot of some 10-km-high mountains in the dark-side ridge, you can see a bright spot where an impact knocked off the dark residue, exposing the brightly reflecting ice below:
(image from NASA)
Scientists haven't yet agreed on what caused the ridge, but it may be due to effects of heating and cooling when the planet was forming. Its oblate shape suggest that an Iapetian day may have lasted only 10 hours at the early point where it cooled enough for a thick ice crust to form, freezing its shape.
