Published online 4 July 2007 | Nature | doi:10.1038/news070702-11

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Snapshots of Hyperion show its holey nature.

Hyperion up close. NASA/JPL/Space Science

Saturn's moon Hyperion looks like a sponge — it is full of holes.

This image of Saturn's largest irregularly-shaped moon (see picture) was snapped by the Cassini spacecraft on a close flyby in September 2005, and shows Hyperion in all its porous glory.

The structure, revealed in Nature today1, comes as something of a surprise: "We certainly hadn't predicted it," says Peter Thomas, at Cornell University in Ithaca, New York, who led the research team.

The holey look indicates that the moon was probably formed by the gradual accumulation of small particles. It has probably stayed in its irregular, lumpy form because it is too small to be squashed by its own gravity, says Thomas. The moon measures just 360 by 280 by 225 kilometres. Large objects generally get packed into more compact spheres by gravity pulling the material together.

The porous nature of the object means that Hyperion's craters are slightly deeper than would be expected for a solid object, since incoming objects met less resistance and penetrated more deeply.

The pictures made it easy to come up with a reasonable explanation for Hyperion's sponginess, Thomas says, adding that it is something that could be predicted only with the benefit of hindsight.

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Hyperion's density — worked out by Thomas to be 544 kilograms per cubic metre, which is low for a moon — indicates that the object is spongy all the way through. "The low density of Hyperion ensures that much of its interior is porous," says Dale Cruikshank, from the NASA Ames Research Center in Moffett Field, California.

Cruikshank's own work on Hyperion, also published in Nature today2, shows that the moon is made of ice contaminated with hydrocarbon-containing complex organic materials. "The ice is 'dirty'," he says.

Cruikshank adds that other low-density objects might also turn out to be porous. "There are some other low-density bodies in the outer Solar System, including some of Saturn's small, close-in satellites," he says. But these haven't yet been snapped in high enough resolution to know for sure whether they are spongy like Hyperion.

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