The Walnut-Shaped Moon

Iapetus and its giant ridge
This Cassini image shows Iapetus's leading hemisphere, which is as dark as an asphalt parking lot. Scientists were amazed to find a giant ridge girdling at least one-third of the moon's circumference. The highest peaks along the ridge are about 20 kilometers (12 miles) high, which rivals Mars's giant volcanoes in height. The impact basin in the center is about 400 km (250 miles) across.
Researchers working with the Cassini spacecraft have recently published a paper in the journal Icarus that proposes an explanation for why Saturn's moon Iapetus has that distinctive mountain ridge along its equator.

Julie Castillo and Cassini project scientist Dennis Matson (NASA/Jet Propulsion Laboratory) say that the moon rotated very fast in its youth and it bulged out accordingly. Its shape was "frozen in time" as the body cooled. They go on to say that radioactive isotopes imply that Iapetus's age is 4.564 billion years old.

No doubt this paper will keep the Iapetus community thinking until September 10th. That's when Cassini will make its closest flyby of the moon yet — just 1,000 kilometers (620 miles) above the surface.

Learn more from the press release or the paper itself.