Hyperion: Return of the Sponge Moon

A false-color view of Hyperion reveals crisp details across the tumbling moon's surface. Color differences could represent varying surface materials. The view was obtained during Cassini's close flyby on Sept. 26, 2005.

Hyperion is one of the many odd moons of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. Since its discovery, it had been distinguished only by its irregular shape and chaotic rotation. And then in 2005, NASA's Cassini spacecraft discovered that this strange moon has a sponge-like appearance, due to its extreme porosity.

The amazing discoveries continue. In July, 2007, NASA's Cassini spacecraft has revealed for the first time surface details of Hyperion, including cup-like craters filled with hydrocarbons that may indicate more widespread presence in our solar system of basic chemicals necessary for life.

What had previously been theorized about Hyperion is that its pitted appearance was due to "Sun cups". This effect is a common phenomenon at the Earth's poles. Material accumulates, heats up in sunlight, melts, creates a pit and then more material accumulates. The process continues until a deep crater-like spot occurs. But now, scientists speculate that it seems that the craters on Hyperion really are craters--- the deep pits are caused by the extremely low density of this intriguing moon. Hyperion is nearly 40% porous--so porous in fact, that if a rock hits the surface, the ejecta from the impact falls into the original hole, causing deep big craters.

This color map from Cassini???s flyby in September, 2005 shows the composition of a portion of Hyperion's surface determined with the Visual and Infrared Mapping Spectrometer aboard the Cassini spacecraft. Blue shows the maximum exposure of frozen water, red denotes carbon dioxide ice ("dry ice"), magenta indicates regions of water plus carbon dioxide, yellow is a mix of carbon dioxide and an unidentified material.

Hyperion yielded some of its secrets to the battery of instruments aboard Cassini as the spacecraft flew close by in September 2005. Water and carbon dioxide ices were found, as well as dark material that fits the spectral profile of hydrocarbons.

A paper appearing in the July 5 issue of Nature reports details of Hyperion's surface craters and composition observed during this flyby, including keys to understanding the moon's origin and evolution over 4.5 billion years. This is the first time scientists were able to map the surface material on Hyperion.

According to Dale Cruikshank, a planetary scientist at NASA???s Ames Research Center, ??????the molecules when embedded in ice and exposed to ultraviolet light, form new molecules of biological significance. This doesn't mean that we have found life, but it is a further indication that the basic chemistry needed for life is widespread in the universe."

"Most of Hyperion's surface ice is a mix of frozen water and organic dust, but carbon dioxide ice is also prominent. The carbon dioxide is not pure, but is somehow chemically attached to other molecules," explained Cruikshank.

What does this mean about habitability?

This is a color composition map of a portion of Hyperion's surface about 75 kilometers (45 miles) on a side--a closeup of the map data from the image shown above. This map used data from Cassini's Visual and Infrared Mapping Spectrometer to depict regions of frozen water, carbon dioxide ice, unidentified material, and various mixtures.

"The Hyperion flyby was a fine example of Cassini's multi-wavelength capabilities. In this first-ever ultraviolet observation of Hyperion, the detection of water ice tells us about compositional differences of this bizarre body," said Amanda Hendrix, Cassini scientist on the ultraviolet imaging spectrograph at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Because Cassini???s ultraviolet imaging spectrograph and visual and infrared mapping spectrometer captured compositional variations in Hyperion's surface, they confirmed presence of frozen water found by earlier ground-based observations, and also discovered solid carbon dioxide (dry ice) mixed in unexpected ways with the ordinary ice. The craters form and are allowed to remain nearly as pristine as the day they were created. Scientists determined that Hyperion is composed mostly of water ice and that the bottoms of its craters are covered in a dark red gunk that could be the key to resolving some of the moon's other strange properties. Prior spacecraft data from other moons of Saturn, as well as Jupiter's moons Ganymede and Callisto, suggest that the carbon dioxide molecule is "complexed," or attached with other surface material in multiple ways. "We think that ordinary carbon dioxide will evaporate from Saturn's moons over long periods of time," said Cruikshank, "but it appears to be much more stable when it is attached to other molecules."

Hyperion, Saturn's eighth largest moon, has a chaotic spin and orbits Saturn every 21 days. The July 5 issue of Nature also includes new findings from the imaging team about Hyperion's strange, spongy-looking appearance.

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