Iapetus, the outermost of Saturn's major regular moons, is extraordinary because of its great contrast in surface brightness.

It was discovered by the Italian-born French astronomer Gian Domenico Cassini in 1671 and named for one of the Titans of Greek mythology.

Iapetus has a radius of 718 km (446 miles) and orbits Saturn once every 79.3 Earth days at a distance of 3,561,300 km (2,212,900 miles). Its bulk density of 1.0 grams per cubic cm implies that it must be made mostly of ices. The closer moons of Saturn orbit within roughly one degree of Saturn's equatorial plane, but, at Iapetus's orbit and beyond, the gravitational influence of Saturn's equatorial bulge becomes less important, permitting larger orbital inclinations. It has

been suggested that Iapetus's 15° average inclination is a relic of the tilt of the long-vanished gaseous disk from which Saturn's major regular moons formed.

Tidal interactions with Saturn have synchronized the rotation of Iapetus with its orbital period. As a result, the moon always keeps the same face to Saturn and always leads with the same face in its orbital motion. Remarkably, the leading hemisphere is extremely dark, reflecting only a few percent of the sunlight falling on it, whereas the trailing hemisphere is as much as 10 times a better reflector. The reflectance at the poles is higher still. Iapetus displays the greatest variation in brightness of any object known in the solar system. Cassini himself wrote that, as Iapetus traveled in its orbit, he could observe it on one side of Saturn but not on the other, and he speculated correctly about the reason for this discrepancy.

Although the U.S. Voyager spacecraft flybys revealed impact craters only on Iapetus's bright trailing side, subsequent higherresolution Cassini spacecraft images show craters on the leading side as well. The surface material on the bright side is very nearly pure water ice, possibly mixed with other ices. The material coating the surface of the dark side, which has a reddish hue, appears to be an opaque layer of complex organic molecules mixed with iron-bearing minerals that have been altered by water. The reflectivity difference is caused by dark material composed of particles that originated elsewhere in the Saturnian system collecting on the moon's leading hemisphere and absorbing more sunlight, which heats up this region enough to cause significant sublimation of water ice over geologic time. The water vapour then condenses onto the trailing hemisphere and freezes.

The Cassini spacecraft imaged a remarkable narrow ridge encircling much of Iapetus's equator. Models suggest that it was formed by motions of a thin, active ice lithosphere when deeper layers of the moon were warm. On the other hand, the moon's observed impact basins and other topography generally require a thicker lithosphere. Possibly most of the features were formed when temperatures within the moon were changing rapidly during its first few million years of existence.

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