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Last Updated: Monday, 10 January 2005, 16:50 GMT
Iapetus moon bulges at equator
A prominent topographic ridge circles the equator of Iapteus

The Cassini spacecraft's flyby of Saturn's moon Iapetus has revealed a bizarre geological feature in its images: a bulging ridge at its equator.

Mission scientists have started to release detailed images of the moon's surface, which is sharply divided into a bright half and a dark half.

The ridge is around 13km (8 miles) high in some places - taller than Mount Everest, the tallest peak on Earth.

Data from the pass may help solve how the moon looks the way it does.

Dr Carolyn Porco, leader of Cassini's imaging team, says Iapetus is one of the most interesting targets Cassini will visit during its four-year tour of Saturn.

Landslide victory

The ridge is unique in the Solar System and its formation could be related to whatever process "painted" the moon's surface with dark material.

It is not yet clear whether the ridge around Iapetus arose through compression, like a mountain belt that has folded upward, or through material from inside Iapetus erupting on to the surface and locally accumulating, forming the ridge.

The images also reveal a spectacular landslide from a 15km- (9 mile-) high scarp on the edge of an ancient impact basin located on Iapetus' dark side.

Iapetus, Nasa/JPL/Space Science Institute
Iapetus is still very much a mystery to scientists

Debris from the slip extends for roughly 60km along a flat-floored impact crater. This could suggest the material is fine-grained and that some unknown mechanical force may have helped spread it over this large distance.

The images also reveal in close-up detail the sharp transition between the dark and light sides of Iapetus, which Dr Porco describes as being like "a stitched baseball".

It is clear that this bizarre appearance is caused by dark material coating a light surface, not the other way round. But how this situation came about is a mystery.

The dark substance could have started off as a cloud of material ejected by an impact on an outer moon of Saturn such as Phoebe. Over time, the orbiting dust could have moved inward towards the ringed planet.

Eventually, Iapetus flew through the cloud, which "painted" it on one side with the material.

The images show a giant landslide from a 15km-high scarp

In another scenario, the dark material could have originated within the moon itself and was extruded on to the surface - possibly through a form of icy volcanism. If so, it could be related to whatever geological process formed the ridge.

At 0130GMT on 1 January, Cassini flew by the frigid moon at a distance of 123,400km on its closest approach.

Cassini got roughly 10 times closer to the moon than the Voyager 2 probe did in 1981.

The best images taken of Iapetus by Voyager had a resolution of about 8km per pixel, while on this pass Cassini got down to a resolution of about 1km per pixel.

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