J. Geophys. Res. http://doi.org/fxqc8r (2012)

A striking mountainous ridge, 20 km high and 200 km wide, runs along the equator of Saturn's satellite Iapetus. The ridge may have formed from a rain of debris from a shattered secondary moon.

Andrew Dombard at the University of Illinois at Chicago, and colleagues used numerical modelling and calculations to investigate a range of circumstances that could have potentially led to the formation and preservation of Iapetus's unusual equatorial ridge. Their preferred scenario is one in which Iapetus once had a moon of its own, formed during an impact with a large passing object. Tidal forces imparted by Iapetus would have caused the orbit of its moon to decay, before finally ripping the moon apart to form a ring of debris above Iapetus's equator. Over time, the debris would have rained down onto Iapetus, creating the ridge.

This scenario can explain why the ridge is placed on the equator, and the equator alone, and why such a ridge has so far only been observed on Iapetus and no other object in the solar system.