Hyperion/haɪˈpɪəriən/ is the eighth-largestmoon of Saturn. It is distinguished by its highly irregular shape, chaotic rotation, lowdensity, and its unusual sponge-like appearance. It was the first non-rounded moon to be discovered.
The moon is named after theTitanHyperion, the god of watchfulness and observation, and the elder brother ofCronus (the Greek equivalent of theRoman godSaturn). It is also designatedSaturn VII. The adjectival form of the name isHyperionian.
Hyperion's discovery came shortly afterJohn Herschel had suggested names for the seven previously known satellites of Saturn in his 1847 publicationResults of Astronomical Observations made at the Cape of Good Hope.[14]William Lassell, who saw Hyperion two days afterWilliam Bond, had already endorsed Herschel's naming scheme and suggested the name Hyperion in accordance with it.[15] He also beat Bond to publication.[16]
Hyperion is one of the largest bodies known to be highly irregularly shaped (non-ellipsoidal, and especially not inhydrostatic equilibrium) in theSolar System.[c] The only larger moon known to be irregular in shape is Neptune's moonProteus. Hyperion has about 15% of the mass ofMimas, the least massive known ellipsoidal body. The largestcrater on Hyperion is approximately 121.57 km (75.54 mi) in diameter and 10.2 km (6.3 mi) deep. 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.[17] 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 ofTethys).[18] Over about 1,000 years, ejecta from a presumed Hyperion breakup would have impactedTitan at low speeds, building upvolatiles in theatmosphere of Titan.[18]
Like most ofSaturn's moons, Hyperion's lowdensity 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 accretedpile of rubble in its physical composition. However, unlike most of Saturn's moons, Hyperion has a lowalbedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material fromPhoebe (which is much darker) that got pastIapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.
Hyperion has aporosity of about 0.46.[9] 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.[7]
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 Hyperion's surface. Early images from theCassini orbiter suggested an unusual appearance, but it was not untilCassini's first targeted flyby of Hyperion on 25 September 2005 that Hyperion's oddness was revealed in full.
Hyperion's surface is covered with deep, sharp-edgedcraters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater. The reddish substance contains long chains ofcarbon andhydrogen and appears very similar to material found on other Saturnian satellites, most notablyIapetus. 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.[19]
The latest analyses of data obtained byCassini during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of it is empty space. It was suggested in July 2007 that thisporosity 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.[20]
Hyperion is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near a much larger moon,Titan. These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4orbital 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 ofSaturn's other moons, which have contrasting trailing and leading hemispheres.[28]
Hyperion has been imaged several times from moderate distances by theCassini orbiter. The first close targeted flyby occurred at a distance of 500 km (310 mi) on 26 September 2005.[19]Cassini made another close approach to Hyperion on 25 August 2011 when it passed 25,000 km (16,000 mi) from Hyperion, and third close approach was on 16 September 2011, with closest approach of 58,000 km (36,000 mi).[29]Cassini's last flyby was on 31 May 2015 at a distance of about 34,000 km (21,000 mi).[19]
^abFrom the long 'i' in LatinHyperīon, one might expect the name to be stressed on the 'i' in English,/ˌhɪpəˈraɪən/, and indeed that pronunciation is recommended in Noah Webster (1884)A Practical Dictionary of the English Language, but recent sources universally stress the name of the 'e', as indeed do many older sources, such asKeats' poem.
^Computed from period, using theIAU-MPC NSES μ value.
