U.S. astronomer Peter Thomas (Cornell University, Ithaca, New York, U.S.A.) and fellow colleagues used data from the NASA (U.S.)/ESA (Europe)/ASI (Italy) Cassini robotic spacecraft, taken during flybys in 2005 and 2006, especially the flyby on September 25, 2005, to make detailed models of Hyperion’s shape and surface.

Their results show that the body has a very low density with over 40% of the moon composed of empty space while the other 60% is mostly water ice and small amounts of rocks and dust. Astronomers often describe it as a loose pile of rubble.

When space rocks, such as meteors, collide with Hyperion they simply plummet through its surface and into its interior without sending a lot of material up into space. Consequently, deep craters are formed. These colliding rocks have peppered its surface for so long that it now looks like a piece of Swiss cheese or, as the researchers say, a “giant sea sponge”.

The Thomas team found that the sponge-like appearance of Hyperion is due to its porosity. They performed laboratory experiments here on the Earth in which projectiles were fired at porous objects. The results were similar to what has happened on Hyperion’s surface: many sharp-edged craters without too much material being ejected upwards.

Thomas reported to New Scientist magazine on July 4, 2007, “The craters stay a little fresher and deep as they accumulate, so the surface looks more like a sponge rather than smoothing everything out.” [New Scientist, subscription required]

Carbon dioxide was also found on Hyperion but attached to a molecule of water—unlike most pure forms of the compound. This arrangement allows it to remain on the moon for millions of years, rather than to evaporate off into space. The researchers think that carbon dioxide on Hyperion may be the key point in why the body is so porous.

Hyperion is about 360 kilometers (225 miles) in length and about 250 kilometers (160 miles) in width. Because of its irregular shape and the gravitational effects from Saturn and its other moon, Hyperion tumbles in a crazy and chaotic rotation while in its orbit around the planet.

It is one of the most irregularly shaped (non-spherical) bodies in the solar system. Its largest crater is 120 kilometers (75 miles) in diameter and about 10 kilometers (6 miles) in depth. The craters are deep, sharp-edged holes with dark material at the bottom of most of them. It is thought by astronomers to be the oldest body in the Saturnian system—at about 4.6 million years old. Fifty-nine moons are known to orbit Saturn.

The Cassini-Hyperion webpage by NASA’s Jet Propulsion Laboratory is: http://saturn.jpl.nasa.gov/science/moons/moonDetails.cfm?pageID=6.

More information on Hyperion appears on The Planetary Society’s webpage: http://www.planetary.org/explore/topics/our_solar_system/saturn/hyperion.html.

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