Iapetus
Cassini mosaic image of Iapetus[lower-alpha 1]
Discovery
Discovered byG. D. Cassini
Discovery dateOctober 25, 1671
Designations
Designation
Saturn VIII
Pronunciation/aɪˈæpətəs/[1]
Named after
Ἰαπετός Īapetus
AdjectivesIapetian /aɪəˈpiːʃən/[2]
Orbital characteristics
3560820 km
Eccentricity0.0276812 [3]
79.3215 d
3.26 km/s
Inclination
  • 17.28° (to the ecliptic)
  • 15.47° (to Saturn's equator)
  • 8.13° (to Laplace plane)[4]
Satellite ofSaturn
Physical characteristics
Dimensions1,492.0 × 1,492.0 × 1,424 km [5]
Mean radius
734.4±2.8 km [5][6]
6700000 km2
Mass(1.8056591±0.0000544)×1021 kg[6]
Mean density
1.0887±0.0127 g/cm3[6]
0.223 m/s2 (0.0228 g) (0.138 Moons)
0.573 km/s
79.3215 d
(synchronous)
zero
Albedo0.05–0.5[7]
Temperature90–130 K
10.2–11.9[8]

    Iapetus (/aɪˈæpətəs/) is a moon of Saturn. With an estimated diameter of 1,469 km, it is the third-largest moon of Saturn and the eleventh-largest in the Solar System.[lower-alpha 2] Named after the Titan Iapetus, the moon was discovered in 1671 by Giovanni Domenico Cassini.

    A relatively low-density body made up mostly of ice, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its leading hemisphere, which is dark, and its trailing hemisphere, which is bright, as well as a massive equatorial ridge running three-quarters of the way around the moon.

    History

    Discovery

    Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.[10][11]

    Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side.[12]

    Etymology

    Iapetus is named after the Titan Iapetus from Greek mythology. The name was suggested by John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[13] in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan Cronus (whom the Romans equated with their god Saturn).

    The name has a largely obsolete variant, Japetus[13][14] /ˈdʒæpɪtəs/,[15] with an adjectival form Japetian.[14] These occur because there was no distinction between the letters ⟨i⟩ and ⟨j⟩ in Latin, and authors rendered them differently.

    When first discovered, Iapetus was among four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV (the other three were Tethys, Dione and Rhea). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII. With the discovery of Hyperion in 1848, Iapetus became Saturn VIII, which is still its Roman numerical designation today.[16] Geological features on Iapetus are generally named after characters and places from the French epic poem The Song of Roland.[16]

    Orbit

    Side view of Iapetus's orbit (red) compared to the other large moons, showing its unusually high inclination

    The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see. The cause of this highly inclined orbit is unknown; however, the moon is not likely to have been captured. One suggestion for the cause of Iapetus' orbital inclination is an encounter between Saturn and another planet.[17]

    Formation

    The moons of Saturn are typically thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. However, a proposed model on the formation of Titan suggests that Titan was instead formed in a series of giant impacts between pre-existing moons. Iapetus and Rhea are thought to have formed from part of the debris of these collisions.[18] More-recent studies, however, suggest that all of Saturn's moons inward of Titan are no more than 100 million years old; thus, Iapetus is unlikely to have formed in the same series of collisions as Rhea and all the other moons inward of Titan, and—along with Titan—may be a primordial satellite.[19]

    Physical characteristics

    The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.[20]

    Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles.[21] Its unique equatorial ridge (see below) is so high that it visibly distorts Iapetus's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.

    Iapetus is heavily cratered, and Cassini images have revealed large impact basins, at least five of which are over 350 km (220 mi) wide. The largest, Turgis, has a diameter of 580 km (360 mi);[22] its rim is extremely steep and includes a scarp about 15 km (9.3 mi) high.[23] Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding.[24]

    Two-tone coloration

    Natural-color image of Iapetus

    The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5–0.6, almost as bright as Europa). Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9—beyond the capacity of the best telescopes in the 17th century. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it. The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag from the sublimation (evaporation) of ice from the warmer areas of the moon's surface, further darkened by exposure to sunlight.[25][26][27] It contains organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets; Earth-based observations have shown it to be carbonaceous, and it probably includes cyano-compounds such as frozen hydrogen cyanide polymers.

    Images from the Cassini orbiter, which passed within 1,227 km (762 miles), show that both Cassini Regio and the Terra's are heavily cratered.[28] The color dichotomy of scattered patches of light and dark material in the transition zone between Cassini Regio and the bright areas exists at very small scales, down to the imaging resolution of 30 metres (98 ft). There is dark material filling in low-lying regions, and light material on the weakly illuminated pole-facing slopes of craters, but no shades of grey.[29] The dark material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas,[30] according to Cassini radar imaging and the fact that very small meteor impacts have punched through to the ice underneath.[27][31]

    View of Cassini Regio. Large craters visible include Falsaron (upper left), Turgis (above and right of center) and Ganelon (lower right)

    Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus would have had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 129 K (−144 °C) in the dark Cassini Regio compared to 113 K (−160 °C) in the bright regions.[27][32] The difference in temperature means that ice preferentially sublimates from Cassini Regio, and deposits in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio.[27] It is estimated that over a period of one billion years at current temperatures, dark areas of Iapetus would lose about 20 meters (70 ft) of ice to sublimation, while the bright regions would lose only 10 cm (4 in), not considering the ice transferred from the dark regions.[32][33] This model explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini Regio. The redistribution of ice is facilitated by Iapetus's weak gravity, which means that at ambient temperatures a water molecule can migrate from one hemisphere to the other in just a few hops.[27]

    However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus. The core of this model is some 30 years old, and was revived by the September 2007 flyby.[25][26]

    The bright regions of Iapetus. Roncevaux Terra is at the top (north); Saragossa Terra with its prominent basin Engelier, Iapetus's second largest, is at the bottom.

    Light debris outside of Iapetus's orbit, either knocked free from the surface of a moon by micrometeoroid impacts or created in a collision, would spiral in as its orbit decays. It would have been darkened by exposure to sunlight. A portion of any such material that crossed Iapetus's orbit would have been swept up by its leading hemisphere, coating it; once this process created a modest contrast in albedo, and so a contrast in temperature, the thermal feedback described above would have come into play and exaggerated the contrast.[26][27] In support of the hypothesis, simple numerical models of the exogenic deposition and thermal water redistribution processes can closely predict the two-toned appearance of Iapetus.[27] A subtle color dichotomy between Iapetus's leading and trailing hemispheres, with the former being more reddish, can in fact be observed in comparisons between both bright and dark areas of the two hemispheres.[26] In contrast to the elliptical shape of Cassini Regio, the color contrast closely follows the hemisphere boundaries; the gradation between the differently colored regions is gradual, on a scale of hundreds of kilometers.[26] The next moon inward from Iapetus, chaotically rotating Hyperion, also has an unusual reddish color.

    The largest reservoir of such infalling material is Phoebe, the largest of the outer moons. Although Phoebe's composition is closer to that of the bright hemisphere of Iapetus than the dark one,[34] dust from Phoebe would only be needed to establish a contrast in albedo, and presumably would have been largely obscured by later sublimation. The discovery of a tenuous disk of material in the plane of and just inside Phoebe's orbit was announced on 6 October 2009,[35] supporting the model.[36] The disk extends from 128 to 207 times the radius of Saturn, while Phoebe orbits at an average distance of 215 Saturn radii. It was detected with the Spitzer Space Telescope.

    Overall shape

    Current triaxial measurements of Iapetus give it radial dimensions of 746 km × 746 km × 712 km (464 mi × 464 mi × 442 mi), with a mean radius of 734.5 ± 2.8 km (456.4 ± 1.7 mi).[5] However, these measurements may be inaccurate on the kilometer scale as Iapetus's entire surface has not yet been imaged in high enough resolution. The observed oblateness would be consistent with hydrostatic equilibrium if Iapetus had a rotational period of approximately 16 hours, but it does not; its current rotation period is 79 days.[37] A possible explanation for this is that the shape of Iapetus was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation, until it became tidally locked.[21]

    Equatorial ridge

    Closeup of the equatorial ridge

    A further mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km (810 mi) long, 20 km (12 mi) wide, and 13 km (8.1 mi) high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004. Peaks in the ridge rise more than 20 km (12 mi) above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km (120 mi) and sections with three near parallel ridges.[38] Within the bright regions there is no ridge, but there are a series of isolated 10 km (6.2 mi) peaks along the equator.[39] The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives Iapetus a walnut-like appearance.

    It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are many hypotheses, but none explain why the ridge is confined to Cassini Regio. Theories include that the ridge is a remnant of Iapetus' oblate shape during its early life, that it was created by the collapse of a ring system, that it was formed by icy material welling from Iapetus' interior, or that it is a result of convective overturn.[40]

    Exploration

    Iapetus has been imaged many times from moderate distances by the Cassini orbiter. Its great distance from Saturn makes close observation difficult. Cassini made one close flyby, at a minimum range of 1,227 km (762 mi), on September 10, 2007.[28]

    The monolith featured during the climax of Arthur C. Clarke's 1968 novel 2001: A Space Odyssey is located on Iapetus.

    A team of scientists explore Iapetus in The Saturn Game, a science-fiction novella by Poul Anderson (1981).

    Iapetus is also the setting for Czech writer Julie Nováková's story "The Long Iapetan Night", published in the November 2020 edition of Asimov's Science Fiction Magazine.[41]

    See also

    Notes

    1. ↑ The bright trailing hemisphere, with part of the dark area appearing on the right (the equatorial ridge is in profile on the right limb). The large crater Engelier is near the bottom; to its lower right can be seen the rim of a partly obliterated, slightly smaller older crater, Gerin.
    2. ↑ The moons more massive than Iapetus are: the Moon, the 4 Galilean moons (Ganymede, Callisto, Io, and Europa), Titan, Rhea, Titania, Oberon, and Triton.[9]

    References

    1. ↑ "Iapetus". Merriam-Webster.com Dictionary.
    2. ↑ Schenk et al. "Saturn's Other Icy Moons: Geologically Complex Worlds." In Schek et al. (2018) Enceladus and the Icy Moons of Saturn, p. 248
    3. ↑ Pseudo-MPEC for Saturn VIII Archived February 22, 2012, at the Wayback Machine
    4. ↑ Jacobson, R.A. (2009) SAT317 (2009-12-17). "Planetary Satellite Mean Orbital Parameters". JPL/NASA. Retrieved 2011-01-15.{{cite web}}: CS1 maint: numeric names: authors list (link)
    5. 1 2 3 Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.
    6. 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.
    7. ↑ Williams, David R. "Saturnian Satellite Fact Sheet". NASA. Retrieved 2007-11-04.
    8. ↑ 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.
    9. ↑ Park, Ryan; Chamberlin, Alan B. (2021). "Planetary Satellite Physical Parameters". Jet Propulsion Laboratory. NASA.
    10. ↑ Van Helden, Albert (1984). Saturn through the telescope: A brief historical survey. Tucson, Arizona: University of Arizona Press. pp. 23–43. Bibcode:1984satn.book...23V.
    11. ↑ Harland, David M. (2002). Mission to Saturn: Cassini and the Huygens Probe. Chichester: Springer Publishing. ISBN 978-1852336561.
    12. ↑ Rotherty, David A. (January 1, 2016). Moons: A Very Short Introduction. Oxford University Press. p. 102. ISBN 9780198735274.
    13. 1 2 Lassell, William (January 14, 1848). "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.
    14. 1 2 George William Hill (1952) The Radiant Universe, p. 280
    15. ↑ Webster, Noah (1884). Dorsey Gardner (ed.). A Practical Dictionary of the English Language. Merriam-Webster.
    16. 1 2 Davis, Phil; Dunford, Bill; Boeck, Moore (December 19, 2019). "Iapetus: In Depth". Solar System Exploration: Our Galactic Neighbourhood. NASA. Retrieved March 1, 2023.
    17. ↑ Nesvorný, David; Vokrouhlický, David; Deienno, Rogerio; Walsh, Kevin J. (2014). "Excitation of the Orbital Inclination of Iapetus during Planetary Encounters". The Astronomical Journal. 148 (3): 52. arXiv:1406.3600. Bibcode:2014AJ....148...52N. doi:10.1088/0004-6256/148/3/52. S2CID 54081553.
    18. ↑ "Giant impact scenario may explain the unusual moons of Saturn". Space Daily. 2012. Retrieved 2012-10-19.
    19. ↑ "Saturn's Moons and Rings May be Younger Than the Dinosaurs". Space.com. 25 March 2016.
    20. ↑ Castillo-Rogez, J. C.; Matson, D. L.; Sotin, C.; Johnson, T. V.; Lunine, Jonathan I.; Thomas, P. C. (2007). "Iapetus' geophysics: Rotation rate, shape, and equatorial ridge". Icarus. 190 (1): 179–202. Bibcode:2007Icar..190..179C. doi:10.1016/j.icarus.2007.02.018.
    21. 1 2 Cowen, R. (2007). Idiosyncratic Iapetus, Science News vol. 172, pp. 104–106. references Archived 2007-10-13 at the Wayback Machine
    22. ↑ "Iapetus: Turgis". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 2009-01-10.
    23. ↑ "PIA06171: Giant Landslide on Iapetus". NASA/JPL/Space Science Institute (photojournal). 2004-12-31. Retrieved 2009-01-10.
    24. ↑ "BBC News - Saturn moon Iapetus' huge landslides stir intrigue". BBC Online. Retrieved 30 July 2012.
    25. 1 2 Mason, J.; Martinez, M.; Balthasar, H. (2009-12-10). "Cassini Closes in on the Centuries-old Mystery Of Saturn's Moon Iapetus". CICLOPS website newsroom. Space Science Institute. Archived from the original on 2012-02-03. Retrieved 2009-12-22.
    26. 1 2 3 4 5 Denk, T.; et al. (2010-01-22). "Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging". Science. 327 (5964): 435–439. Bibcode:2010Sci...327..435D. doi:10.1126/science.1177088. PMID 20007863. S2CID 165865.
    27. 1 2 3 4 5 6 7 Spencer, J. R.; Denk, T. (2010-01-22). "Formation of Iapetus' Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration". Science. 327 (5964): 432–435. Bibcode:2010Sci...327..432S. CiteSeerX 10.1.1.651.4218. doi:10.1126/science.1177132. PMID 20007862. S2CID 20663944.
    28. 1 2 "Iapetus". Cassini Solstice Mission. NASA. Archived from the original on 2015-03-26. Retrieved 6 July 2015.
    29. ↑ "Cassini–Huygens: Multimedia-Images". Saturn.jpl.nasa.gov. Archived from the original on 2009-12-31. Retrieved 2012-07-30.
    30. ↑ "Cassini–Huygens: Multimedia-Images". Saturn.jpl.nasa.gov. Archived from the original on 2010-06-22. Retrieved 2012-07-30.
    31. ↑ "Cassini Is on the Trail of a Runaway Mystery". Mission News. NASA. 2007-10-08. Archived from the original on 2022-05-01. Retrieved 2009-10-08.
    32. 1 2 "Cassini–Huygens: Multimedia-Images". Saturn.jpl.nasa.gov. Archived from the original on 2015-01-07. Retrieved 2012-07-30.
    33. ↑ "Dark Side of a Saturnian Moon: Iapetus Is Coated With Foreign Dust". Sciencedaily.com. 2009-12-11. Retrieved 2012-07-30.
    34. ↑ Hendrix, A. R.; Hansen, C. J. (March 14–18, 2005). "Iapetus and Phoebe as Measured by the Cassini UVIS" (PDF). 36th Annual Lunar and Planetary Science Conference: 2272. Bibcode:2005LPI....36.2272H.
    35. ↑ Largest known planetary ring discovered Archived 2011-08-22 at the Wayback Machine, Science News
    36. ↑ Largest ring in solar system found around Saturn, New Scientist
    37. ↑ 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 2018-12-23. Retrieved 2015-09-25.
    38. ↑ Porco, C. C.; E. Baker, J. Barbara, K. Beurle, A. Brahic, J. A. Burns, S. Charnoz, N. Cooper, D. D. Dawson, A. D. Del Genio, T. Denk, L. Dones, U. Dyudina, M. W. Evans, B. Giese, K. Grazier, P. Helfenstein, A. P. Ingersoll, R. A. Jacobson, T. V. Johnson, A. McEwen, C. D. Murray, G. Neukum, W. M. Owen, J. Perry, T. Roatsch, J. Spitale, S. Squyres, P. C. Thomas, M. Tiscareno, E. Turtle, A. R. Vasavada, J. Veverka, R. Wagner, R. West (2005-02-25). "Cassini imaging science: Initial results on Phoebe and Iapetus" (PDF). Science. 307 (5713): 1237–1242. Bibcode:2005Sci...307.1237P. doi:10.1126/science.1107981. PMID 15731440. S2CID 20749556. 2005Sci...307.1237P.{{cite journal}}: CS1 maint: multiple names: authors list (link)
    39. ↑ "Cassini–Huygens: Multimedia-Images". Saturn.jpl.nasa.gov. Archived from the original on 2011-06-10. Retrieved 2012-07-30.
    40. ↑
    41. ↑ "Publications - Julie Novakova".
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