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Named for a titan in Greek mythology, Iapetus is the outermost of Saturn's major moons (though the planet has many more minor moons extending far out into space beyond Iapetus). With a semi-major axis of more than three and a half million kilometres, the moon's orbit carries it around the ringed planet at a distance more than double that of Saturn's largest moon Titan. Unlike the other significant moons, Iapetus' orbital path is inclined far out the plane of Saturn's rings and other moons, implying that it had an encounter with another body in the far distant past that caused a major shift in its orbit.
Iapetus is a body composed largely of water ice, and for that reason much of its surface is light in colour. A very distinct exception, however, is the wide Cassini Regio, a dark region covering a significant portion of the moon's surface. Iapetus is tidally locked to Saturn, meaning that it always maintains the same orientation in its orbit. The dark region is on the leading hemisphere; that is, the part of the moon that always faces in the direction of its orbit. The dark area is thought to be the result of temperature differences on the moon's surface, with the leading hemisphere being slightly warmer than the trailing hemisphere. This slight temperature difference (the dark region can reach 129 K or -144°C), conditions are suitable for the sublimation of the ice to a gaseous form, leaving behind a 'lag' or dark sediment. At the poles, and on the trailing hemisphere of the moon, temperatures are too low for this sublimation process to have a significant effect, leaving the surface ice bright white in colour.
Iapetus is generally spheroidal in shape, but highly oblate (that is, it bulges significantly around its equator). A curious feature of the moon is its equatorial ridge, a line of mountains up to ten kilometres in height that run along the equator. This ridge is especially prominent in the dark Cassini Regio, and its origins remain mysterious. Several hypotheses have been put forward to explain the origins of the strange ridge, including the idea that the ridge was formed by extremely rapid rotation of the moon during its formation, or even that the moon at one time had its own miniature ring system that accreted onto its surface. For the present, the true origins of this unique feature remain unknown.
The light side of the moon is divided into two zones, Roncevaux Terra to the north and Saragossa Terra to the south. In common with almost all surface features on Iapetus, these take their names from the Song of Roland (the exception being the dark Cassini Regio, which is named for the moon's discoverer). Saragossa Terra contains one of Iapetus' most distinctive features, the huge crater Engelier, whose diameter of some 500 km dominates the southern parts of the light side. Nonetheless, Engelier is not Iapetus' largest crater: within the dark surface of Cassini Rego lies the even larger Turgis, which stretches 580 km from rim to rim.
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