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Space Topics: SaturnIapetusTwo-Faced Moon
Size: 1,460 km - 3rd largest moon of Saturn 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
Cassini
Cassini Future FlybysCassini Map of Iapetus
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