Space Topics: Saturn
Iapetus
Two-Faced Moon
Click to enlarge > Iapetus has extreme topography and one of the most ancient surfaces in the solar system. It also has a tall mountain range running exactly around its equator. Credit: NASA / JPL / Space Science Institute
Size: 1,460 km - 3rd largest moon of Saturn
Orbital radius: 3,561,000 kilometers - 59.1 Saturn radii - far outside Saturn's ring system
Orbital period: 79.33 days - 5 times Titan's
Discovery: 1671 by Giovanni Domenico Cassini
Update November 15, 2005: added a page of data from Cassini's rev 17 flyby of Iapetus
Iapetus orbits much farther away from Saturn than any other major moon, three times farther away than Titan and about ten times farther than all the rest of the medium-sized icy moons. It is also the only one of Saturn's major moons with an inclined orbit, 15 degrees away from the ring plane.
Iapetus is the highest-contrast body in the solar system. Its leading hemisphere is very dark, as dark as asphalt, while its trailing hemisphere is bright, similar to the solar system's other icy satellites. The cause of the black smudge on one face of Iapetus is unknown. The common explanation from before the Cassini mission was that dust from Phoebe -- the darkest object in the Saturnian system except for Iapetus's smudge -- spirals in to Iapetus and is deposited on the surface. However, Cassini's spectrometetric measurements indicate that the dark stuff on Phoebe is a better match to the composition of Iapetus' bright side.
The dust-from-outside hypothesis has a second problem: if true, then one entire hemisphere of Iapetus should be dark and the other bright. Closer examination reveals that the dark stuff wraps around to Iapetus' trailing side at the equator, while the poles are bright. The data suggest that the dark and bright coloration may be as much an endogenous process (caused within Iapetus) rather than an exogenous one (having an outside origin). One possibility is that some initial amount of dark material deposited on the leading hemisphere Iapetus' equator could have absorbed solar energy and accelerated evaporation of ice there. The evaporated water would preferentially condense on Iapetus' poles.
Another oddity about Iapetus is its topography. It is the only body in the solar system with a self-defined equator; a startlingly linear ridge of tall mountains (a feature that is casually known as Iapetus' "belly band" exactly girdles its equator). In addition, it has a flattened shape that indicates that it once spun much faster than it does now. But judging from the number of huge impact basins and smaller craters covering Iapetus' surface, there has been little in the way of geologic activity on Iapetus recently.
Features on Iapetus are named for people and places from Dorothy Sayers' translation of Chanson de Roland.
Flybys of Iapetus
Click to enlarge > NASA / JPL / SSI
Cassini
December 31, 2005 at 18:49 UTC
“0CIA” nontargeted flyby
Closest approach altitude 123,402 kilometers (76,670 miles)
Click to enlarge > NASA / JPL / SSI
Cassini
November 12, 2005 at 14:36 UTC
"17IA" and "18IA" nontargeted flyby
Closest approach altitude about 416,000 kilometers (258,000 miles)
Future Flybys
Cassini
September 10, 2007 at 12:34 UTC
“49IA” targeted flyby [I1]
Closest approach altitude 1,229 kilometers (764 miles)
Map of Iapetus
The Cassini space probe will make a close flyby today of Saturn's third largest moon Iapetus. For reasons that are not well-understood, one side of Iapetus is much darker than the other side. Explanations for the dark side include meteorite deposition as well as possible debris from other moons of Saturn. 

Also, the Cassini probe had previously discovered an equatorial ridge on Iapetus during a flyby in 2004 which compounds Iapetus' already non-spherical shape. The origin of the ridge is also not well understood and scientists have proposed a variety of explanations for its presence. This second flyby will be approximately  1,600 km from the surface and will be about 100 times closer than the previous flyby.  Scientists hope that the detailed observation using radar and photography will provide insight into Iapetus's odd shape and coloring.

Iapetus was first discovered by Giovanni Cassini, for whom the probe is named, in 1671. He was also responsible for discovering that one side of Iapetus was substantially darker than the other.  

This flyby will likely be the last close flyby of one of Saturn's moons by the Cassini probe.