Iapetus is the third-largest moon of Saturn, and eleventh in the solar system, Iapetus is best known for its dramatic ‘two-tone’ coloration. Iapetus is mostly composed of ice, with only a small (~20%) amount of rocky materials.
Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge is so high that it visibly distorts the moon’s shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.
Iapetus is heavily cratered. The largest, Turgis, has a diameter of 360 miles; its rim is extremely steep and includes a scarp about 9 miles high.
The difference in coloring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5-0.6, almost as bright as Europa). Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9. The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it. The original dark material is came from outside Iapetus, but now it consists principally of lag from the sublimation of ice from the warmer areas of Iapetus’s surface. It contains organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets.
Both hemispheres are heavily cratered.
The dark material is lag (residue) from the sublimation (evaporation) of water ice on the surface of Iapetus, darkened upon exposure to sunlight. Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 129 K in the dark Cassini Regio compared to 113 K in the bright regions. The difference in temperature means that ice preferentially sublimates from Cassini, and precipitates in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini. It is estimated that, at current temperatures, over one billion years Cassini would lose about 20 meters of ice to sublimation, while the bright regions would lose only 10 centimeters, not considering the ice transferred from the dark regions. This model explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini. The redistribution of ice is facilitated by Iapetus’s weak gravity, which means that at ambient temperatures a water molecule can migrate from one hemisphere to the other in just a few hops.
However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus.
A further feature of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 800 miles long, 12 miles wide, 8 miles high. Parts of the ridge rise more than 12 miles above the surrounding plains. The ridge forms a complex system including isolated peaks, segments of more than 120 miles and sections with three near parallel ridges. Within the bright regions there is no ridge, but there are a series of isolated 6 mile peaks along the equator. The prominent equatorial bulge gives the moon a walnut-like appearance.
It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least three current hypotheses, but none of them explains why the ridge is confined to Cassini Regio.
Temperatures on the dark region’s surface reach 130 K (−143 °C; −226 °F) at the equator, as heating is made more effective by Iapetus’s slow rotation. The brighter surfaces absorb less sunlight so temperatures there only reach about 100 K (−173 °C; −280 °F).
The orbit of Iapetus is somewhat unusual. Although it is Saturn’s third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.
Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see. From Iapetus, Saturn would appear to be 1°56’ in diameter (four times that of the Moon viewed from Earth).