The mysterious equatorial ridge on Saturn's moon Iapetus is either a fossil ring system that fell to the surface, or a pile up of crustal rocks formed as the satellite changed its shape. These are the latest theories from planetary scientists.
The ridge, revealed by the Cassini probe, is unlike anything else in the solar system. It is up to 20 kilometres high and stretches 1300 km along the moon's equator, resembling the ridge on a walnut.
Counts of impact craters show the ridge must be nearly as old as the crust on the adjacent plains, which are thought to have solidified about 4.5 billion years ago.
The first possible explanation for the ridge's formation is the slowing of Iapetus's spin from less than 10 hours per rotation when it formed to the present speed of 80 days.
The initial rapid spin would have produced strong centrifugal force that pushed the equator of the largely fluid young satellite outwards. It was about 1.5 times wider across the equator than from pole to pole, says Julie Castillo at the Jet Propulsion Laboratory in California, US.
Then tidal (gravitational) friction with Saturn slowed the spin, so Iapetus became more spherical. This caused the surface area to shrink, leaving the moon with an excess of solid crust, which Castillo says piled up along the equator.
Falling down
Alternatively, the ridge might have formed from debris that fell from rings that once orbited the equator of Iapetus, says Wing-Huen Ip of the National Central University in Chung Li, Taiwan, (Geophysical Research Letters, DOI: 10.1029/2005GL025386).
The rings may have been remnants of the disc of dust and rock from which Iapetus originally condensed. Or the rings may have been formed after a large body hit Iapetus during the final stages of its formation, splashing debris into orbit which formed rings and a satellite which later escaped or broke up.
In either case, Ip says, the ring material would have fallen back to the surface in a narrow strip along the equator. He told New Scientist that he has yet to work out how the ring material could have hit slowly enough to build up a ridge rather than blast craters along the equator.
In a spin
Ip argues his scenario is more plausible because a hot, fast-spinning young Iapetus could not have dissipated rotational energy fast enough for Castillo's scenario to work.
Castillo, however, says that Ip uses too simple a model to calculate cooling, and that Iapetus needed only to slow its spin down to once every 20 hours in order to become spherical enough to form an equatorial ridge. Her group has submitted a journal paper describing a complex numerical model of the thermal and dynamic effects that they believe produced the equatorial ridge.
So the jury remains out. "The question for both models is: How do you get so much material in such a very small area?" says Paul Schenk of the Lunar and Planetary Institute in Houston, Texas, US.
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The Cassini space probe will make a close flyby today of Saturn's third largest moon Iapetus. For reasons that are not well-understood, one side of Iapetus is much darker than the other side. Explanations for the dark side include meteorite deposition as well as possible debris from other moons of Saturn. 

Also, the Cassini probe had previously discovered an equatorial ridge on Iapetus during a flyby in 2004 which compounds Iapetus' already non-spherical shape. The origin of the ridge is also not well understood and scientists have proposed a variety of explanations for its presence. This second flyby will be approximately  1,600 km from the surface and will be about 100 times closer than the previous flyby.  Scientists hope that the detailed observation using radar and photography will provide insight into Iapetus's odd shape and coloring.

Iapetus was first discovered by Giovanni Cassini, for whom the probe is named, in 1671. He was also responsible for discovering that one side of Iapetus was substantially darker than the other.  

This flyby will likely be the last close flyby of one of Saturn's moons by the Cassini probe.