Hyperion /haɪˈpɪəriən/ 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.[12] 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.[13] He also beat Bond to publication.[14]

Physical characteristics

Hyperion compared to Ceres and the Moon[6]

Shape

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.[b] 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. The largest crater 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.[15] 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).[16] 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.[16]

Composition

True-color image of Hyperion, taken by the Cassini spacecraft

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.[7] 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.[6]

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 Hyperion's surface. Early images from the Cassini orbiter suggested an unusual appearance, but it was not until Cassini'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-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.[17]

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. 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.[18]

The IAU assigned official names to four craters and one dorsum (ridge) on Hyperion in 1982.

Coordinate system

Due to its chaotic and random rotation, as well as its non-spherical irregular shape, scientists have found it difficult to assign a coordinate system to Hyperion. At the time when Voyager 2 was passing through the Saturnian system, the chaotic nature of its rotation had not yet been reliably determined.[19] Regardless, the first coordinate system was established without taking the chaotic rotation into account;[20] indeed, it was made when information on the shape and orientation of Hyperion was not yet even available, and is entirely arbitrary.[21] This coordinate system was used by the IAU when it assigned names and approved official coordinates for the four craters and one dorsum, and still holds official status today.[20][c] Information about this coordinate system remains ambiguous; in fact, the origin point and the shape model used by the system are actually not known, which has prompted many authors to use their own coordinate systems.[20] Some of these systems use an instantaneous rotation axis of Hyperion to define the coordinate system, while others use its shape as the basis.

Static charge

Hyperion's surface is electrically charged and was the first discovered to be so other than the Moon's surface.[22]

Orbit and rotation

Animation of Hyperion's orbit.
   Saturn ·    Hyperion ·   Titan

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.[23][24][25] Hyperion, together with Pluto's moons Nix and Hydra,[26][27] is among only a few moons in the Solar System known to rotate chaotically, although it is expected to be common in binary asteroids.[28] 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.[29]

Exploration

Image of Hyperion processed to bring out details. It was taken by the Cassini space probe.

Hyperion has been imaged several times from moderate distances by the Cassini orbiter. The first close targeted flyby occurred at a distance of 500 km (310 mi) on 26 September 2005.[17] 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).[30] Cassini's last flyby was on 31 May 2015 at a distance of about 34,000 km (21,000 mi).[17]

See also

Notes

  1. 1 2 From the long 'i' in Latin Hyperīon, 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 as Keats' poem.
  2. ↑ There are about ten asteroids and an unknown number of irregular Trans-Neptunian objects larger than Hyperion.
  3. ↑ Hyperion has not been mentioned in an IAU report since 1990.

References

  1. ↑ "Hyperion". Lexico UK English Dictionary. Oxford University Press. Archived from the original on March 22, 2020.
    "Hyperion". Merriam-Webster.com Dictionary. Merriam-Webster. OCLC 1032680871.
  2. ↑ "JPL (ca. 2008) Cassini Equinox Mission: Hyperion". Archived from the original on 2016-04-10. Retrieved 2011-01-30.
  3. ↑ Bill Yenne (1987) The Atlas of the Solar System, p. 144
  4. 1 2 3 Jacobson, Robert. A. (1 November 2022). "The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole*". The Astronomical Journal. 164 (5): 199. Bibcode:2022AJ....164..199J. doi:10.3847/1538-3881/ac90c9. S2CID 252992162.
  5. 1 2 Slodarzh, Nikolai A.; Zubarev, A. E.; Nadezhdina, Irina E.; Kozlova, N. A. (October 10–14, 2022). Hyperion (C7): control point network and shape model. Difficulties and solutions. The Thirteenth Moscow Solar System Symposium (PDF). pp. 201–202 – via Sapienza University of Rome.
  6. 1 2 3 Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025. Archived from the original (PDF) on 2011-09-27. Retrieved 2015-09-04.
  7. 1 2 Thomas, P.C.; Armstrong, J. W.; Asmar, S. W.; Burns, J. A.; Denk, T.; Giese, B.; Helfenstein, P.; Iess, L.; et al. (2007). "Hyperion's Sponge-like Appearance". Nature. 448 (7149): 50–56. Bibcode:2007Natur.448...50T. doi:10.1038/nature05779. PMID 17611535. S2CID 4415537.
  8. ↑ "Hyperion In Depth". NASA. December 2017. Retrieved 2019-02-05.
  9. ↑ D.R. Williams (18 September 2006). "Saturnian Satellite Fact Sheet". NASA. Retrieved 2007-11-04.
  10. ↑ "About Saturn & Its Moons: Moons – Hyperion". Cassini @ JPL/NASA. Archived from the original on 2012-06-04. Retrieved 2011-01-30.
  11. ↑ Observatorio ARVAL (April 15, 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Archived from the original on September 20, 2011. Retrieved 2011-12-17.
  12. ↑ Lassell, W. (14 January 1848). "Observations of satellites of Saturn". Monthly Notices of the Royal Astronomical Society. 8 (3): 42–43. Bibcode:1848MNRAS...8...42L. doi:10.1093/mnras/8.3.42. Retrieved 2011-12-18.
  13. ↑ W. Lassell (1848). "Discovery of a New Satellite of Saturn". Monthly Notices of the Royal Astronomical Society. 8 (9): 195–197. Bibcode:1848MNRAS...8..195L. doi:10.1093/mnras/8.9.195a.
  14. ↑ Bond, W.C. (1848). "Discovery of a new satellite of Saturn". Monthly Notices of the Royal Astronomical Society. 9 (1): 1–2. Bibcode:1848MNRAS...9....1B. doi:10.1093/mnras/9.1.1.
  15. ↑ R.A.J. Matthews (1992). "The Darkening of Iapetus and the Origin of Hyperion". Quarterly Journal of the Royal Astronomical Society. 33: 253–258. Bibcode:1992QJRAS..33..253M.
  16. 1 2 Farinella, P.; Marzari, F.; Matteoli, S. (1997). "The Disruption of Hyperion and the Origin of Titan's Atmosphere". Astronomical Journal. 113 (2): 2312–2316. Bibcode:1997AJ....113.2312F. doi:10.1086/118441.
  17. 1 2 3 "Cassini Prepares for Last Up-close Look at Hyperion". Jet Propulsion Laboratory. 28 May 2015. Retrieved 2015-05-29.
  18. ↑ "Key to Giant Space Sponge Revealed". Space.com. 4 July 2007. Retrieved October 26, 2007.
  19. ↑ Sokolov, A. I.; Nadezhdina, I. E.; Nyrtsov, M. V.; Zubarev, A.E.; Fleis, M. E.; Kozlova, N. A. (2024-02-01). "Mapping Hyperion in Projections of the Triaxial Ellipsoid Based on a New Reference Network and a Digital Terrain Model". Solar System Research. 58 (1): 112–121. doi:10.1134/S0038094624010106. ISSN 1608-3423.
  20. 1 2 3 Sokolov, A.I.; Nyrtsov, M.V.; Fleis, M.E.; Nadezhdina, I.E. (2024-09-15). "Investigation and cartographic representation of Hyperion space images photogrammetric processing results". Planetary and Space Science. 249 105945. Bibcode:2024P&SS..24905945S. doi:10.1016/j.pss.2024.105945.
  21. ↑ Stooke, Philip J. (1996-07-01). "Topography and geology of Hyperion". Earth, Moon, and Planets. 74 (1): 61–83. Bibcode:1996EM&P...74...61S. doi:10.1007/BF00118722. ISSN 1573-0794.
  22. ↑ "Cassini Caught in Hyperion's Particle Beam". www.jpl.nasa.gov. NASA Jet Propulsion Laboratory (JPL). Retrieved 2024-02-19.
  23. ↑ M. Tarnopolski (May 2015). "Nonlinear time-series analysis of Hyperion's lightcurves". Astrophysics and Space Science. 357 (2): 160. arXiv:1412.2423. Bibcode:2015Ap&SS.357..160T. doi:10.1007/s10509-015-2379-3. S2CID 56311141.
  24. ↑ M. Tarnopolski (Feb 2017). "Influence of a second satellite on the rotational dynamics of an oblate moon". Celestial Mechanics and Dynamical Astronomy. 127 (2): 121–138. arXiv:1607.07333. Bibcode:2017CeMDA.127..121T. doi:10.1007/s10569-016-9719-7. S2CID 118512400.
  25. ↑ M. Tarnopolski (Oct 2017). "Rotation of an oblate satellite: Chaos control". Astronomy & Astrophysics. 606: A43. arXiv:1704.02015. Bibcode:2017A&A...606A..43T. doi:10.1051/0004-6361/201731167. S2CID 119360690.
  26. ↑ M. R. Showalter, D. P. Hamilton (Jun 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889. S2CID 205243819.
  27. ↑ Kenneth Chang (2015-06-03). "Astronomers Describe Chaotic Dance of Pluto's Moons". New York Times.
  28. ↑ Nadoushan, M. J.; Assadian, N. (2015). "Widespread chaos in rotation of the secondary asteroid in a binary system". Nonlinear Dynamics. 81 (4): 2031. Bibcode:2015NonLD..81.2031J. doi:10.1007/s11071-015-2123-0. S2CID 124747289.
  29. ↑ Wisdom, J.; Peale, S.J.; Mignard, F. (1984). "The chaotic rotation of Hyperion". Icarus. 58 (2): 137–152. Bibcode:1984Icar...58..137W. CiteSeerX 10.1.1.394.2728. doi:10.1016/0019-1035(84)90032-0.
  30. ↑ "Saturn's Odd Pockmarked Moon Revealed in New Photos". Space.com. 30 August 2011. Retrieved 31 August 2011.
  31. Nadezhdina, Irina E.; Zubarev, A. E.; Slodarzh, Nikolai A.; Kozlova, N. A.; Chuikin, M. A. (October 10–14, 2022). Hyperion (C7) cartography: challenges and the first surface map. The Thirteenth Moscow Solar System Symposium (PDF). pp. 262–263 – via Sapienza University of Rome.
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