Hyperion (, or as in Greek Ὑπερίων) is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance.

Name

The moon is named after Hyperion, a Titan in Greek mythology. It is also designated Saturn VII. The adjectival form of the name is Hyperionian.

Hyperion's discovery came shortly after John Herschel had suggested names for the seven previously-known satellites of Saturn in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. Lassell, who saw Hyperion two days after Bond, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it. He also beat Bond to publication.

Physical characteristics

Shape

Hyperion is one of the largest highly irregular (non-spherical) bodies in the solar system (second to Proteus). The largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for the irregular morphology is that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past, an event which has been linked to the enigmatic darkening of Iapetus.

Composition

Like most of Saturn's moons, Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition. However, unlike most of Saturn's moons, Hyperion has a low albedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.

Hyperion has a porosity of about 0.46 .

Surface features

Voyager 2 passed through the Saturn system but photographed Hyperion only from a distance. It discerned individual craters and an enormous ridge but was not able to make out the texture of the moon's surface. Early images from the Cassini orbiter suggested an unusual appearance, but it was not until Cassini's sole targeted flyby of Hyperion on September 25, 2005 that the moon's oddness was revealed in full.

Hyperion's surface is covered with deep, sharp-edged craters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater. The reddish substance contains long chains of carbon and hydrogen and appears very similar to material found on other Saturnian satellites, most notably Iapetus.

The latest analyses of data obtained by NASA's Cassini spacecraft during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of the moon is empty space. It was suggested in July 2007 that this porosity allows craters to remain nearly unchanged over the eons. The new analyses also confirmed that Hyperion is composed mostly of water ice with very little rock. "We find that water ice is the main constituent of the surface, but it's dirty water ice," said Dale Cruikshank, a researcher at NASA Ames Research Center.

Rotation

The Voyager 2 images and subsequent ground based photometry indicate that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable. Hyperion is the only known moon in the solar system that rotates chaotically, but simulations suggest that other irregular satellites may have done so in the past. It is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near another large moon, Titan. These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely. The odd rotation probably accounts for the relative uniformity of Hyperion's surface, in contrast to many of Saturn's other moons which have contrastive trailing and leading hemispheres.

Exploration

Hyperion has been imaged several times from moderate distances by the Cassini orbiter. There was one close targeted fly-by, at a distance of 500 km on 2005 September 26; there are no plans for any others.

References

External links

• Cassini mission Hyperion page
• Hyperion Profile by NASA's Solar System Exploration
• The Planetary Society: Hyperion
• NASA: Saturn's Hyperion, A Moon With Odd Craters