This false-color mosaic shows the entire hemisphere of Iapetus visible from Cassini on the outbound leg of its encounter with the two-toned moon in September 2007.

Iapetus [pronounced eye-APP-eh-tuss; adjective form: Iapetian] has been called the yin and yang of the Saturn moons because its leading hemisphere has a reflectivity (or albedo) as dark as coal (albedo 0.03-0.05 with a slight reddish tinge) and its trailing hemisphere is much brighter at 0.5-0.6.

Giovanni Cassini observed the dark-light difference when he discovered Iapetus in 1671. He noted that he could only see Iapetus on the west side of Saturn. He correctly concluded that Iapetus had one side much darker than the other side, and that Iapetus was tidally locked with Saturn.

Scientists have long wondered why one hemisphere of Iapetus is so dark in comparison to its other hemisphere, and in comparison to other surfaces in the Saturn system. Iapetus may be sweeping up particles from the more-distant dark moon, Phoebe. If that is the darkening mechanism, it should be steadily renewing the dark surface because very few fresh bright craters are detected within the dark terrain. An alternate theory is that there might be ice volcanism distributing darker material to the surface. Volcano-like eruptions of hydrocarbons might form the dark surfaces, particularly after chemical reactions caused by solar radiation.

The September 2007 Cassini flyby of Iapetus showed that a third process, thermal segregation, is probably the most responsible for Iapetus’ dark hemisphere. Iapetus has a very slow rotation, longer than 79 days. Such a slow rotation means that the daily temperature cycle is very long, so long that the dark material can absorb heat from the sun and warm up. (The dark material absorbs more heat than the bright icy material.) This heating will cause any volatile, or icy, species within the dark material to sublime out, and retreat to colder regions on Iapetus. This sublimation of volatiles causes the dark material to become even darker — and causes neighboring bright, cold regions to become even brighter. Iapetus may have experienced a (possibly small) influx of dark material from an external source, which could have warmed up and triggered this thermal segregation process.

The second most notable feature of Iapetus is its “equatorial ridge,” a chain of 10-km (6-mile) high mountains girdling the moon’s equator. On the anti-Saturnian side of Iapetus, the ridge appears to break up and distinct, partially bright mountains are observed. The Voyager I and Voyager II encounters provided the first knowledge of these mountains, and they are informally referred to as the Voyager Mountains.

Voyager 2 photographed Iapetus in 1981.
Voyager 2 photographed Iapetus in 1981.

There are two theories on how the ridge formed. Some scientists think the ridge was formed at an earlier time when Iapetus rotated much faster than it does today; others think the ridge is made of material left from the collapse of a ring.

Iapetus has a diameter of 1,471 km (914 miles) and a density only 1.2 times that of liquid water. It has been suggested that Iapetus (like Rhea) is three quarters ice and one quarter rock.

Iapetus orbits at 3,561,300 km (2,213,000 miles) from Saturn. The great distance from Saturn’s tidal forces and from most of the other moons and ring particles has probably allowed the Iapetus surface to be largely unaffected by any melting episodes that could have caused some smoothing or “resurfacing” as on some of the moons closer to Saturn.

However, despite the great distance, Saturn has tidally locked Iapetus. The moon always presents the same face toward Saturn.

As with some other Saturnian moons, Iapetus is in resonance with Saturn’s largest moon, Titan, which orbits at 1,221,850 km (759,200 miles). That means that the two objects speed up and slow down as they pass each other in a complex set of variations. However, Iapetus has a diameter less than a third of Titan’s diameter, so Titan’s rotation and orbit are affected much less than those of Iapetus.

Giovanni Cassini discovered Iapetus on 25 October 1671. However, Iapetus appeared only as a bright, dark dot to astronomers until the Voyager I and Voyager II encounters in 1980 and 1981.

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“Saturn’s outermost main moon is unusual because its leading hemisphere – the half of the moon that permanently faces in the direction it orbits around Saturn – is much darker than its trailing hemisphere. It is thought the moon may have been contaminated with a dark material originating from another of Saturn’s moons, though it is uncertain exactly where Iapetus gets its dark hemisphere from.”



“The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process,” said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo.

Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the bright and dark faces of Iapetus. But yet to be fully understood is where the dark material is coming from. Is it native or from outside the moon?


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