Iapetus is the third largest of Saturn's moons, with a diameter of 900 miles. Like most of the moons of Saturn it has a low density which implies that it is made primarily of ice. Its trailing edge is relatively bright (with an albedo of 50%), but its trailing edge is one of the darkest surfaces in the Solar System, with an albedo of only 3 to 6% (as dark as asphalt). The boundary between the darker and lighter areas is surprisingly sharp, and the contrast in their appearance is quite striking.
      The reason for the difference in appearance of the two sides is unknown. One suggestion is that Iapetus has had episodes of ice vulcanism which has released methane-rich materials from the interior, and subsequent darkening of the methane ice by the absorption of ultraviolet radiation from the Sun has made the material quite dark. This would be a method of formation similar to the lava flows which created the lunar maria, although the cause of darkness would, if due to darkening of methane, be totally different from our own Moon. An alternative suggestion (apparently now confirmed; see note about Phoebe noted above) is that dark material is ejected from Phoebe, an outer moon which is just as dark as the dark areas on Iapetus, then swept up by the leading edge of Iapetus. However, the color of the dark material on Iapetus is slightly different from that of Phoebe, and sweeping up debris should lead to a fuzzier boundary between the light and dark areas. In particular, the 60-mile-wide dark ring surrounding a lighter central area, near the bottom of the large illustration, appears far more likely to be some kind of internal eruption than swept-up material.

As Voyager swept past Iapetus the dark leading side moved to the left and out of view, while the lighter trailing side moved into view. The dark boundary on the right side of the photos is the moon's terminator (the dividing line between day and night).

The leading edge of Iapetus is covered with dark material, probably sifting through the satellite system as a result of impacts with the dark outer moon, Phoebe. This material overlies old craters on the leading side of the moon, but not at the poles or the trailing side. Note the odd equatorial ridge, several miles high, making it look like the moon consisted of two halves, crudely stuck together. The origins of this ridge are unknown, and will probably remain so for some time to come. Also note the subsidence area on the right side of the image. A closer view of the subsidence area is shown below. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod050201)

A close-up of the subsidence area, including a large landslide.The appearance and cratering is the same in the subsidence area as in the surrounding area. Only the subsidence itself differentiates the two areas.

A view of Iapetus showing the dark/light contrast in more detail. Some kind of very dark material (albedo less than 5%) coats the leading "edge". Enhanced in color, this image is one of four used to create the mosaic of the entire moon, second above.

Another view, looking at the dark/light contrast from a different angle (off to the right relative to the viewing direction in the previous image). (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060103)