Could Iapetus have been formed from the "gentle" collision of two asteroids, one bright and one dark? Its inclined orbit suggests that it's not one of Saturn's original satellites but was only recently captured - too recent for its orbit to be pulled into the equatorial plane by Saturn's bulge. And the equatorial ridge might have been puished up by the collision.... the overall shape of Iapetus is neither spherical nor ellipsoid - unusual for a large moon; parts of its globe appear to be squashed flat, and its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance ... In the seventeenth century, Giovanni Cassini observed that he could see Iapetus only on one side of Saturn and not on the other. He drew the conclusion that one side of Iapetus was darker than the other, a conclusion confirmed by images from the ''Voyager'' and ''Cassini'' spacecraft.
The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere is dark ... with a slight reddish-brown coloring, while most of the trailing hemisphere and poles is bright ... The pattern of coloration is analogous to a spherical yin-yang symbol...
The orbit of Iapetus is somewhat unusual. Although it is the Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.