Hyperion (moon)

Hyperion is the eighth-largest moon of Saturn. It is distinguished by its highly irregular shape, chaotic rotation, low density, and unusual sponge-like appearance. It was the first non-rounded moon to be discovered.

Discovery and naming Hyperion was independently discovered by William Cranch Bond and his son George Phillips Bond in the United States, and William Lassell in the United Kingdom in September 1848. The moon is named after the Titan Hyperion, the god of watchfulness and observation, and the elder brother of Cronus (the Greek equivalent of the Roman god Saturn). 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. William Lassell, who saw Hyperion two days after William Bond, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it. He also beat Bond to publication.

Orbit and rotation

Hyperion orbits Saturn at a distance of 1.48 million kilometers, between Titan and Iapetus. Unlike other regular moons, its orbit is moderately eccentric. It participates in a 3:4 orbital resonance with Titan; for every four orbits Titan makes, Hyperion makes 3. Conjunctions with Titan happen at the apocenter of Hyperion's orbit. The influence of Titan keeps Hyperion's orbit eccentric, preventing it from becoming more circular over time. The Voyager 2 images and subsequent ground-based photometry indicated that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable. Its Lyapunov time is around 30 days. Though its rotation is predictable over short timescales, over longer timeframes it becomes impossible to predict. Hyperion, together with Pluto's moons Nix and Hydra, is among only a few moons in the Solar System known to rotate chaotically, although it is expected to be common in binary asteroids. It is also the only regular planetary natural satellite in the Solar System known to not be tidally locked. Hyperion is unique among the large moons because of its highly irregular shape, fairly eccentric orbit, and proximity to the much larger 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 fact that its rotation is not locked probably accounts for the relative uniformity of Hyperion's surface, in contrast to many of Saturn's other moons, which have contrasting trailing and leading hemispheres.

Physical characteristics

Shape and size

Hyperion is one of the largest bodies known to be highly irregularly shaped (non-ellipsoidal, and especially not in hydrostatic equilibrium) in the Solar System. The only larger planetary moons known to be irregular in shape are Neptune's moons Proteus and Nereid. Hyperion has about 15% of the mass of Mimas, the least massive known ellipsoidal body. Although Hyperion is the eighth-largest moon of Saturn, it is only the ninth-most massive. Phoebe has a smaller radius, but it is more massive than Hyperion and thus denser. A possible explanation for the irregular shape is that Hyperion is a fragment of a larger body that was broken up by a large impact in the distant past. A proto-Hyperion could have been 350–1,000 km (220–620 mi) in diameter (which ranges from a little below the size of Mimas to a little below the size of Tethys). Over about 1,000 years, ejecta from a presumed Hyperion breakup would have impacted Titan at low speeds, building up volatiles in the atmosphere of Titan.

Composition and structure

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 (0.33), 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. The latest analyses of data obtained by Cassini during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of it is empty space. The new analyses also confirmed that Hyperion is composed mostly of water ice with very little rock.

Surface features

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. Scientists attribute Hyperion's unusual, sponge-like appearance to the fact that it has an unusually low density for such a large object. Its low density makes Hyperion quite porous, with a weak surface gravity. These characteristics mean impactors tend to compress the surface, rather than excavating it, and most material that is blown off the surface never returns. The steep slopes of the craters cause mass wasting on Hyperion despite its low gravity. This can be caused by the temperature difference of the surface between day and night, as well as nearby impacts. Hyperion has a porosity of about 0.42 ± 0.06. It was suggested in July 2007 that Hyperion's large porosity allows craters to remain nearly unchanged over the eons. Many of its smaller craters appear to have polygon-shaped edges, rather than circular ones, which also contributes to its sponge-like appearance. This may provide information about hidden subsurface faults, as impacts may preferentially blow away material along existing fracture lines in the surface. A famous example of this happening on Earth is Meteor Crater. Shortly after the Voyager 2 flyby, the International Astronomical Union (IAU) assigned official names to four craters and one dorsum (ridge) on Hyperion in 1982. These five remain the only named features on Hyperion. Most features since are unnamed, due to the lack of a approved coordinate system which causes difficulty in describing the absolute location of features (see below).

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