Iapetus (moon): Difference between revisions

Iapetus
	Iapetus as imaged by the Cassini orbiter in false color, September 2007. Iapetus's unusual coloration can be seen, with the equatorial ridge on the right limb. The large craters Engelier and Gerin are near the bottom.
Discovery
Discovered by	G. D. Cassini
Discovery date	October 25, 1671
Designations
Designation	Saturn VIII
Pronunciation	/aɪˈæpətəs/
Named after	Ἰαπετός Īapetus
Adjectives	Iapetian /aɪəˈpiːʃən/
Orbital characteristics
Aphelion	3462250 km
Perihelion	3659390 km
Semi-major axis	3560820 km
Eccentricity	0.0276812
Orbital period (sidereal)	79.3215 d
Average orbital speed	3.26 km/s
Inclination	17.28° (to the ecliptic); 15.47° (to Saturn's equator); 8.13° (to Laplace plane);
Satellite of	Saturn
Physical characteristics
Dimensions	1,492.0 × 1,492.0 × 1,424 km
Mean radius	734.4±2.8 km
Surface area	6777600 km2
Mass	1.80565×1021 kg
Mean density	1.0887±0.0127 g/cm3
Surface gravity	0.223 m/s2 (0.0228 g) (0.138 Moons)
Escape velocity	0.573 km/s
Synodic rotation period	79.3215 d ; (synchronous)
Axial tilt	zero
Albedo	0.05–0.5
Temperature	90–130 K
Apparent magnitude	10.2–11.9

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Iapetus (/aɪˈæpətəs/) is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System.^[a] 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. This is the first moon that Cassini discovered; the second moon of Saturn to be discovered after Christaan Huygens spotted Titan 16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.

Cassini discovered Iapetus when the moon was on the western side of Saturn, but when he tried viewing it on the eastern side some months later, he 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]

Name

Iapetus is named after the Titan Iapetus from Greek mythology. The name was suggested by John Herschel (son of William Herschel) 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); and Giants, the massive but lesser relatives of the Titans who sided with the Titans against Zeus and the Olympian Gods.^[14]

The name has a largely obsolete variant, Japetus^[13]^[15] /ˈdʒæpɪtəs/,^[16] with an adjectival form Japetian.^[15] These occurred because there was no distinction between the letters ⟨i⟩ and ⟨j⟩ in Latin, and authors rendered them differently.

When first discovered, Iapetus was among the 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 because it was the fifth known Saturnian moon in order of distance from Saturn at that time. 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.^[17] Geological features on Iapetus are generally named after characters and places from the French epic poem The Song of Roland.^[17]

Orbit

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 also has 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's orbital inclination is an encounter between Saturn and another planet in the distant past.^[18]

Despite being, on average, 2.4 times further from Saturn than Hyperion, the next moon inward, Iapetus is tidally locked to Saturn while Hyperion is not.^[19]

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.^[20] 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.^[21]

Physical characteristics

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.^[22]

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles.^[23] 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);^[24] its rim is extremely steep and includes a scarp about 15 km (9.3 mi) high.^[25] Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding.^[26]

Two-tone coloration

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.^[27]^[28]^[29] 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.^[30] 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.^[31] The dark material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas,^[32] according to Cassini radar imaging and the fact that very small meteor impacts have punched through to the ice underneath.^[29]^[33]

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.^[29]^[34] 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.^[29] 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.^[34]^[35] 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.^[29]

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.^[27]^[28]

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.^[28]^[29] 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.^[29] 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.^[28] 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.^[28] 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,^[36] 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,^[37] supporting the model.^[38] 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.^[39] 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.^[23]

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, although its existence had been inferred from the moon's polar images by Voyager 2.^[40] 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.^[41] Within the bright regions there is no ridge, but there are a series of isolated 10 km (6.2 mi) peaks along the equator.^[42] 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's 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's interior, or that it is a result of convective overturn.^[43]

Exploration

The first spacecraft to visit Saturn, Pioneer 11, did not provide any images of Iapetus and it came no closer than 1,030,000 km (640,000 mi) from the moon.^[45] Nonetheless, Pioneer 11 was humanity's first attempt to obtain direct measurements from the objects within the Saturnian system.

Voyager 1 arrived at Saturn on November 12, 1980, and it became the first probe to return pictures of Iapetus that clearly show the moon's two-tone appearance from a distance of 2,480,000 km (1,540,000 mi)^[46] as it was exiting the Saturnian system.^[47]

Voyager 2 became the next probe to visit Saturn on August 22, 1981, and made its closest approach to Iapetus at a distance of 909,000 km (565,000 mi). It took photos of Iapetus's north pole as it entered the Saturnian system - opposite the approach direction of Voyager 1.

The latest probe to visit Iapetus was the Cassini orbiter which went into orbit around Saturn starting on July 1, 2004.^[49] Iapetus has been imaged many times from moderate distances by Cassini but its great distance from Saturn makes close observation difficult.

Cassini made its first targeted flyby of Iapetus on Dec. 31, 2004, at a distance of 123,400 km (76,700 mi) around the time when the spacecraft was settling in its orbit around Saturn.^[50] Cassini did not cross Iapetus's orbit when it flew by and remained inside the moon's orbit. Cassini's subsequent flybys of Titan would make the spacecraft's orbit smaller, preventing Cassini from flying close to Iapetus for months.

Cassini made a second flyby of Iapetus on November 12, 2005, at a distance of 415,000 km (258,000 mi),^[51] also without crossing the moon's orbit.

Cassini then made a third and more distant flyby of Iapetus on January 22, 2006, at a distance of 1,300,000 km (810,000 mi).^[52]^[53]

The fourth flyby happened on April 8, 2006, at a distance of approximately 866,000 km (538,000 mi), and this time, Cassini crossed Iapetus' orbit.^[54] After this, Cassini's orbit was made smaller once again, preventing the probe from approaching Iapetus for more than a year this time.

Cassini's closest flyby of Iapetus happened on September 10, 2007, at a minimum range of 1,227 km (762 mi).^[30] It approached Iapetus from its night side.^[55]

After this encounter, Cassini made no further targeted flybys of Iapetus.

Notes

^ 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

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External links

Listen to this article (19 minutes)

This audio file was created from a revision of this article dated 22 August 2013, and does not reflect subsequent edits.

Cassini mission page – Iapetus
Discussion of Iapetus dated October 2007
Iapetus Profile at NASA's Solar System Exploration site
The Planetary Society: Iapetus
Google Iapetus 3D, interactive map of the moon
NASA Astronomy Picture of the Day: Saturn's Iapetus: Moon with a Strange Surface (1 February 2005)
Mirror Objects in the Solar System?—refereed article discussing the speculative mirror matter, and Iapetus in this context
A Moon with a View—Richard C. Hoagland's imaginative discussion of Iapetus's oddities
New attempts to crack Saturn's 'walnut' moon—equatorial ridge formation theories
Cassini images of Iapetus Archived 2011-07-25 at the Wayback Machine
Images of Iapetus at JPL's Planetary Photojournal
Movie of Iapetus's rotation at the National Oceanic and Atmospheric Administration site
Iapetus basemap (May 2008) from Cassini images Archived 2010-02-17 at the Wayback Machine
Iapetus atlas (October 2008) from Cassini images
Iapetus nomenclature and Iapetus map with feature names from the USGS planetary nomenclature page
Flight over Iapetus (video)

[10] 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]

[1] "Iapetus". Merriam-Webster.com Dictionary. Merriam-Webster.

[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

[jpl-ssd-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)

[Roatsch_et_al._2009-5] 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.

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@@ Line 1: / Line 1: @@
 {{Short description|Moon of Saturn}}
-{{Redirect|Japetus|the mythological figure|Iapetus|the insect genus|Japetus (planthopper)}}
 {{Redirect|Saturn VIII|the rocket|Saturn C-8}}
 {{Infobox planet
-| name                     = Iapetus
+| name = Iapetus
-| mpc_name                 = Saturn VIII
+| mpc_name = Saturn VIII
-| pronounced               = {{IPAc-en|aɪ|ˈ|æ|p|ə|t|ə|s}}<ref>{{MW|Iapetus}}</ref>
+| pronounced = {{IPAc-en|aɪ|ˈ|æ|p|ə|t|ə|s}}<ref>{{MW|Iapetus}}</ref>
-| adjectives               = Iapetian {{IPAc-en|aɪ|ə|ˈ|p|iː|ʃ|ə|n}}<ref>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</ref>
+| adjectives = Iapetian {{IPAc-en|aɪ|ə|ˈ|p|iː|ʃ|ə|n}}<ref>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</ref>
-| named_after              = [[Iapetus|Ἰαπετός]] ''Īapetus''
+| named_after = [[Iapetus|Ἰαπετός]] ''Īapetus''
-| image                    = Iapetus as seen by the Cassini probe - 20071008.jpg
+| image = Iapetus as seen by the Cassini probe - 20071008.jpg
-| caption                  = A ''[[Cassini–Huygens|Cassini]]'' mosaic image of Iapetus{{efn|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 (crater)|Gerin]].}}
+| caption = Iapetus as imaged by the ''[[Cassini–Huygens|Cassini]]'' orbiter in false color, September 2007. Iapetus's unusual coloration can be seen, with the equatorial ridge on the right limb. The large craters [[Engelier]] and [[Gerin (crater)|Gerin]] are near the bottom.
-| discoverer               = [[Giovanni Domenico Cassini|G. D. Cassini]]
+| discoverer = [[Giovanni Domenico Cassini|G. D. Cassini]]
-| discovered               = October 25, 1671
+| discovered = October 25, 1671
-| semimajor                = {{val|3560820|u=km}}
+| semimajor = {{val|3560820|u=km}}
+| perihelion = {{val|3659390|u=km}}
-| eccentricity             = {{val|0.0276812}} <ref>[http://home.gwi.net/~pluto/mpecs/ss08.htm#elements Pseudo-MPEC for Saturn VIII] {{webarchive |url=https://web.archive.org/web/20120222025902/http://home.gwi.net/~pluto/mpecs/ss08.htm#elements |date=February 22, 2012 }}</ref>
+| aphelion = {{val|3462250|u=km}}
-| period                   = {{val|79.3215|u=[[Day|d]]}} <!--Computed using the http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html µ value-->
+| eccentricity = {{val|0.0276812}} <ref>[http://home.gwi.net/~pluto/mpecs/ss08.htm#elements Pseudo-MPEC for Saturn VIII] {{webarchive |url=https://web.archive.org/web/20120222025902/http://home.gwi.net/~pluto/mpecs/ss08.htm#elements |date=February 22, 2012 }}</ref>
+| period = {{val|79.3215|u=[[Day|d]]}} <!--Computed using the http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html μ value-->
 | avg_speed = 3.26 km/s<!---{{efn|name=calculated}}--->
-| inclination              =
+| inclination = {{plainlist |
-  {{plainlist |
 * 17.28° (to the ecliptic)
 * 15.47° (to Saturn's equator)
@@ Line 25: / Line 25: @@
 | dimensions = 1,492.0&thinsp;×&thinsp;1,492.0&thinsp;×&thinsp;1,424 km <ref name="Roatsch et al. 2009" />
 | mean_radius = {{val|734.4|2.8|u=km}} <ref name="Roatsch et al. 2009" /><ref name="Jacobson2022"/>
-| surface_area = {{val|6700000|u=km<sup>2</sup>}}
+| surface_area = {{val|6777600|u=km<sup>2</sup>}}
-| mass = {{val|1.8056591|0.0000544|e=21|u=kg}}<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |pages=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref>
+| mass = {{val|1.80565|e=21|u=kg}}<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |pages=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref>
 | density = {{val|1.0887|0.0127|u=g/cm<sup>3</sup>}}<ref name="Jacobson2022"/>
 | surface_grav = {{Gr|1.805|734.5|3}} [[Acceleration|m/s<sup>2</sup>]] ({{val|0.0228|u=[[G-force|''g'']]}}) (0.138 [[Moon]]s)
@@ Line 38: / Line 38: @@
 }}
-'''Iapetus''' ({{IPAc-en|aɪ|ˈ|æ|p|ə|t|ə|s}}) is a [[natural satellite|moon]] of [[Saturn]]. With an estimated diameter of 1,469 km, it is the third-largest moon of Saturn and the [[List of moons|eleventh-largest in the Solar System]].{{efn|The moons more massive than Iapetus are: the Moon, the 4 [[Galilean moons]] (Ganymede, Callisto, Io, and Europa), Titan, Rhea, Titania, Oberon, and Triton.<ref>{{cite web|last1=Park|first1=Ryan|last2=Chamberlin|first2=Alan B.|year=2021|url=http://ssd.jpl.nasa.gov/?sat_phys_par|website=[[Jet Propulsion Laboratory]]|title=Planetary Satellite Physical Parameters|publisher=[[NASA]]}}</ref>}} Named after the [[Titan (mythology)|Titan]] [[Iapetus (mythology)|Iapetus]], the moon was discovered in 1671 by [[Giovanni Domenico Cassini]].
+'''Iapetus''' ({{IPAc-en|aɪ|ˈ|æ|p|ə|t|ə|s}}) is the outermost of [[Moons of Saturn|Saturn's large moons]]. With an estimated diameter of {{convert|1,469|km|mi|abbr=on}}, it is the third-largest moon of Saturn and the [[List of moons|eleventh-largest in the Solar System]].{{efn|The moons more massive than Iapetus are: the Moon, the 4 [[Galilean moons]] (Ganymede, Callisto, Io, and Europa), Titan, Rhea, Titania, Oberon, and Triton.<ref>{{cite web|last1=Park|first1=Ryan|last2=Chamberlin|first2=Alan B.|year=2021|url=http://ssd.jpl.nasa.gov/?sat_phys_par|website=[[Jet Propulsion Laboratory]]|title=Planetary Satellite Physical Parameters|publisher=[[NASA]]}}</ref>}} Named after the [[Titan (mythology)|Titan]] [[Iapetus (mythology)|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 on Iapetus|equatorial ridge]] running three-quarters of the way around the moon.
@@ Line 45: / Line 45: @@
 === Discovery ===
 [[File:Giovanni Cassini.jpg|left|thumb|upright|Giovanni Domenico Cassini, discoverer of Iapetus]]
-Iapetus was discovered by [[Giovanni Domenico Cassini]], an Italian-born French astronomer, in October 1671. This is the first moon that Cassini discovered; the second moon of Saturn to be discovered after [[Christaan Huygens]] spotted [[Titan_(moon)|Titan]] 16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.
+Iapetus was discovered by [[Giovanni Domenico Cassini]], an Italian-born French astronomer, in October 1671. This is the first moon that Cassini discovered; the second moon of Saturn to be discovered after [[Christaan Huygens]] spotted [[Titan (moon)|Titan]] 16 years prior in 1655; and the sixth extraterrestrial moon to be discovered in human history.
 Cassini discovered Iapetus when the moon was on the western side of Saturn, but when he tried viewing it on the eastern side some months later, he 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 [[apparent magnitude|magnitudes]] dimmer on that side.<ref>{{cite book|last1=Van Helden|first1=Albert|title=Saturn through the telescope: A brief historical survey|location=[[Tucson, Arizona]]|bibcode=1984satn.book...23V|publisher=[[University of Arizona Press]]|pages=23–43|year=1984}}</ref><ref>{{cite book|last1=Harland|first1=David M.|author-link1=David M. Harland|title=Mission to Saturn: Cassini and the Huygens Probe|location=Chichester|publisher=[[Springer Publishing]]|isbn=978-1852336561|year=2002}}{{page needed|date=October 2022}}</ref>
@@ Line 58: / Line 58: @@
 These occurred because there was no distinction between the letters {{angbr|i}} and {{angbr|j}} in Latin, and authors rendered them differently.
-When first discovered, Iapetus was among four the [[Moons of Saturn|Saturnian moons]] labelled the ''[[Sidera Lodoicea]]'' by their discoverer Giovanni Cassini after [[Louis XIV of France|King Louis XIV]] (the other three were [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]] and [[Rhea (moon)|Rhea]]). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being '''Saturn V''' because it was the fifth known Saturnian moon in terms of distance from Saturn at that time. Once [[Mimas (moon)|Mimas]] and [[Enceladus (moon)|Enceladus]] were discovered in 1789, the numbering scheme was extended and Iapetus became '''Saturn&nbsp;VII'''. With the discovery of [[Hyperion (moon)|Hyperion]] in 1848, Iapetus became ''Saturn VIII'', which is still its Roman numerical designation today.<ref name=NASADepth>{{cite web|url=https://solarsystem.nasa.gov/moons/saturn-moons/iapetus/in-depth/|title=Iapetus: In Depth|last1=Davis|first1=Phil|last2=Dunford|first2=Bill|last3=Boeck|first3=Moore|website=Solar System Exploration: Our Galactic Neighbourhood|publisher=[[NASA]]|date=December 19, 2019|accessdate=March 1, 2023}}</ref> Geological features on Iapetus are generally named after characters and places from the [[Chanson de geste|French epic poem]] ''[[The Song of Roland]]''.<ref name=NASADepth/>
+When first discovered, Iapetus was among the four [[Moons of Saturn|Saturnian moons]] labelled the ''[[Sidera Lodoicea]]'' by their discoverer Giovanni Cassini after [[Louis XIV of France|King Louis XIV]] (the other three were [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]] and [[Rhea (moon)|Rhea]]). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being '''Saturn V''' because it was the fifth known Saturnian moon in order of distance from Saturn at that time. Once [[Mimas (moon)|Mimas]] and [[Enceladus (moon)|Enceladus]] were discovered in 1789, the numbering scheme was extended and Iapetus became '''Saturn&nbsp;VII'''. With the discovery of [[Hyperion (moon)|Hyperion]] in 1848, Iapetus became '''Saturn VIII''', which is still its Roman numerical designation today.<ref name=NASADepth>{{cite web|url=https://solarsystem.nasa.gov/moons/saturn-moons/iapetus/in-depth/|title=Iapetus: In Depth|last1=Davis|first1=Phil|last2=Dunford|first2=Bill|last3=Boeck|first3=Moore|website=Solar System Exploration: Our Galactic Neighbourhood|publisher=[[NASA]]|date=December 19, 2019|accessdate=March 1, 2023}}</ref> Geological features on Iapetus are generally named after characters and places from the [[Chanson de geste|French epic poem]] ''[[The Song of Roland]]''.<ref name=NASADepth/>
 == Orbit ==
 [[File:Iapetus orbit (side).jpg|thumb|right|250px|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 (moon)|Titan]]. It also has the most inclined orbital plane of the regular satellites; only the irregular outer satellites like [[Phoebe (moon)|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 in the distant past.<ref name=Nesvorny_2014>{{cite journal|last1=Nesvorný|first1=David|last2=Vokrouhlický|first2=David|last3=Deienno|first3=Rogerio|last4=Walsh|first4=Kevin J.|title=Excitation of the Orbital Inclination of Iapetus during Planetary Encounters|journal=The Astronomical Journal|date=2014|volume=148|issue=3|page=52|doi=10.1088/0004-6256/148/3/52|bibcode=2014AJ....148...52N|arxiv = 1406.3600 |s2cid=54081553}}</ref>
+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 (moon)|Titan]]. It also has the most inclined orbital plane of the regular satellites; only the irregular outer satellites like [[Phoebe (moon)|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's orbital inclination is an encounter between Saturn and another planet in the distant past.<ref name=Nesvorny_2014>{{cite journal|last1=Nesvorný|first1=David|last2=Vokrouhlický|first2=David|last3=Deienno|first3=Rogerio|last4=Walsh|first4=Kevin J.|title=Excitation of the Orbital Inclination of Iapetus during Planetary Encounters|journal=The Astronomical Journal|date=2014|volume=148|issue=3|page=52|doi=10.1088/0004-6256/148/3/52|bibcode=2014AJ....148...52N|arxiv = 1406.3600 |s2cid=54081553}}</ref>
-Despite being, on average, 2.4 times further from Saturn than Hyperion, the next moon inward that is closer to its parent planet, Iapetus is tidally locked to Saturn while Hyperion is not.<ref>{{cite journal |last1=Buratti |first1=Bonnie J. |title=Hyperion |journal=Nature Astronomy |date=September 2017 |volume=1 |issue=9 |pages=574 |doi=10.1038/s41550-017-0243-9 |bibcode=2017NatAs...1..574B |s2cid=256706837 }}</ref>
+Despite being, on average, 2.4 times further from Saturn than Hyperion, the next moon inward, Iapetus is tidally locked to Saturn while Hyperion is not.<ref>{{cite journal |last1=Buratti |first1=Bonnie J. |title=Hyperion |journal=Nature Astronomy |date=September 2017 |volume=1 |issue=9 |pages=574 |doi=10.1038/s41550-017-0243-9 |bibcode=2017NatAs...1..574B |s2cid=256706837 }}</ref>
 ==Formation==
@@ Line 89: / Line 89: @@
 Images from the ''Cassini'' orbiter, which passed within {{convert|1,227|km|mi|abbr=in}}, show that both Cassini Regio and the Terra's are heavily cratered.<ref name="Cassini Solstice Mission"/> 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 {{convert|30|m||}}. 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.<ref>{{cite web|url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2761 |archive-url=https://web.archive.org/web/20091231061042/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2761 |url-status=dead |archive-date=2009-12-31 |title=Cassini–Huygens: Multimedia-Images |publisher=Saturn.jpl.nasa.gov |access-date=2012-07-30}}</ref> The dark material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas,<ref>{{cite web|url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2762 |archive-url=https://web.archive.org/web/20100622191340/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2762 |url-status=dead |archive-date=2010-06-22 |title=Cassini–Huygens: Multimedia-Images |publisher=Saturn.jpl.nasa.gov |access-date=2012-07-30}}</ref> according to Cassini radar imaging and the fact that very small [[meteor]] impacts have punched through to the ice underneath.<ref name="Spencer" /><ref name="jpl779">{{cite web| title = Cassini Is on the Trail of a Runaway Mystery| work = Mission News| publisher = [[NASA]]| date = 2007-10-08| url = http://www.nasa.gov/mission_pages/cassini/media/cassini20071008.html| access-date = 2009-10-08| archive-date = 2022-05-01| archive-url = https://web.archive.org/web/20220501081912/https://www.nasa.gov/mission_pages/cassini/media/cassini20071008.html| url-status = dead}}</ref>
-[[File:Iapetus Spins and Tilts.jpg|thumb|right|200px|View of Cassini Regio. The large craters that are visible include [[List_of_geological_features_on_Iapetus#Craters|Falsaron]] (upper left), [[Turgis (crater)|Turgis]] (above and right of center) and Ganelon (lower right)]]
+[[File:Iapetus Spins and Tilts.jpg|thumb|right|200px|View of Cassini Regio. The large craters that are visible include [[List of geological features on Iapetus#Craters|Falsaron]] (upper left), [[Turgis (crater)|Turgis]] (above and right of center) and Ganelon (lower right)]]
 Because of its slow rotation of 79&nbsp;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 {{cvt|129|K|C|0|lk=in}} in the dark Cassini Regio compared to {{cvt|113|K|C|0}} in the bright regions.<ref name="Spencer" /><ref name="jpl2776">{{cite web |url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2776 |title=Cassini–Huygens: Multimedia-Images |publisher=Saturn.jpl.nasa.gov |access-date=2012-07-30 |archive-url=https://web.archive.org/web/20150107231932/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2776 |archive-date=2015-01-07 |url-status=dead }}</ref> The difference in temperature means that ice preferentially sublimates from Cassini Regio, and [[Deposition (phase transition)|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.<ref name="Spencer" /> It is estimated that over a period of one billion years at current temperatures, dark areas of Iapetus would lose about {{convert|20|m|-1|sp=us}} of ice to sublimation, while the bright regions would lose only {{convert|10|cm|0|abbr=on}}, not considering the ice transferred from the dark regions.<ref name="jpl2776" /><ref>{{cite web|url=https://www.sciencedaily.com/releases/2009/12/091210173611.htm |title=Dark Side of a Saturnian Moon: Iapetus Is Coated With Foreign Dust |publisher=Sciencedaily.com |date=2009-12-11 |access-date=2012-07-30}}</ref> 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.<ref name="Spencer" />
@@ Line 108: / Line 108: @@
 [[File:Iapetus equatorial ridge.jpg|right|thumb|200px|Closeup of the equatorial ridge]]
 A further mystery of Iapetus is the [[equatorial ridge]] that runs along the center of Cassini Regio, about {{cvt|1300|km||}} long, {{cvt|20|km||}} wide, and {{cvt|13|km||}} high. It was discovered when the ''Cassini'' spacecraft imaged Iapetus on December 31, 2004, although its existence had been inferred from the moon's polar images by Voyager 2.<ref>{{cite web | url=https://www.planetary.org/articles/1151 | title=Iapetus' "Voyager Mountains" }}</ref> Peaks in the ridge rise more than {{cvt|20|km||}} 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 {{cvt|200|km||}} and sections with three near parallel ridges.<ref name="porco2005">{{cite journal| last=Porco | first=C. C. | author-link = Carolyn Porco | author2=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, [[Carl D. Murray|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| date=2005-02-25
-| title=Cassini imaging science: Initial results on Phoebe and Iapetus| journal=Science| volume=307| issue=5713| pages=1237–1242| pmid=15731440| doi=10.1126/science.1107981| id=2005Sci...307.1237P|bibcode = 2005Sci...307.1237P | s2cid=20749556 | url=https://authors.library.caltech.edu/36635/7/Porco_Iapetus_SOM.pdf}}</ref>  Within the bright regions there is no ridge, but there are a series of isolated {{cvt|10|km||}} peaks along the equator.<ref>{{cite web |url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2760 |title=Cassini–Huygens: Multimedia-Images |publisher=Saturn.jpl.nasa.gov |access-date=2012-07-30 |archive-url=https://web.archive.org/web/20110610025306/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2760 |archive-date=2011-06-10 |url-status=dead }}</ref> The ridge system is heavily cratered, indicating that it is ancient. The prominent [[equatorial bulge]] gives Iapetus a [[Walnut#Nuts and kernels|walnut]]-like appearance.
+| title=Cassini imaging science: Initial results on Phoebe and Iapetus| journal=Science| volume=307| issue=5713| pages=1237–1242| pmid=15731440| doi=10.1126/science.1107981| id=2005Sci...307.1237P|bibcode = 2005Sci...307.1237P | s2cid=20749556 | url=https://authors.library.caltech.edu/36635/7/Porco_Iapetus_SOM.pdf}}</ref> Within the bright regions there is no ridge, but there are a series of isolated {{cvt|10|km||}} peaks along the equator.<ref>{{cite web |url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2760 |title=Cassini–Huygens: Multimedia-Images |publisher=Saturn.jpl.nasa.gov |access-date=2012-07-30 |archive-url=https://web.archive.org/web/20110610025306/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2760 |archive-date=2011-06-10 |url-status=dead }}</ref> The ridge system is heavily cratered, indicating that it is ancient. The prominent [[equatorial bulge]] gives Iapetus a [[Walnut#Nuts and kernels|walnut]]-like appearance.
-[[File:Iapetus_Voyager2_flyby_sequence.jpg|thumb|upright=4|center|A series of images of Iapetus' north pole taken by Voyager 2 as it flew by. The white dots at the lower part of the images (Iapetus' equator) hinted at the presence of high mountains there which will later turn out to be the equatorial bulge and duly named Voyager Mountains.]]
+[[File:Iapetus_Voyager2_flyby_sequence.jpg|thumb|upright=4|center|A series of images of Iapetus's north pole taken by Voyager 2 as it flew by. The white dots at the lower part of the images (Iapetus's equator) hinted at the presence of high mountains there which will later turn out to be the equatorial bulge and duly named Voyager Mountains.]]
-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.<ref>
+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's 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's interior, or that it is a result of convective overturn.<ref>
 *{{cite journal| last=Kerr| first=Richard A.
 | date=2006-01-06| title=How Saturn's Icy Moons Get a (Geologic) Life| journal=Science| volume=311| issue=5757| page=29| doi=10.1126/science.311.5757.29| pmid=16400121| s2cid=28074320| doi-access=free}}
@@ Line 121: / Line 121: @@
 == Exploration ==
-[[File:Voyager_1_Iapetus.png|right|thumb|200px|One of the first-ever images that clearly shows Iapetus' light and dark areas, taken by Voyager 1, 1980 on Nov. 12, 1980 from a distance of 3.2 million kilometers (1.9 million miles).<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia02268-iapetus | title=Iapetus | website=[[Jet Propulsion Laboratory]] }}</ref>]]
+[[File:Voyager_1_Iapetus.png|right|thumb|200px|One of the first-ever images that clearly shows Iapetus's light and dark areas, taken by Voyager 1 on Nov. 12, 1980, from a distance of {{convert|3.2|e6km|e6mi|abbr=off}}.<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia02268-iapetus | title=Iapetus | website=[[Jet Propulsion Laboratory]] }}</ref>]]
-The first-ever spacecraft to visit Saturn, [[Pioneer 11]], did not provide any images of Iapetus and it came no closer than 1,030,000 km (640,000 miles) from the moon.<ref>{{cite web | url=https://eyes.nasa.gov/apps/solar-system/#/sc_pioneer_11/distance?to=iapetus&rate=0&time=1979-08-29T05:04:53.987+00:00 | title=Eyes on the Solar System - NASA/JPL }}</ref> Nonetheless, Pioneer 11 was humanity's first attempt to obtain direct measurements from the objects within the Saturnian system.
+The first spacecraft to visit Saturn, [[Pioneer 11]], did not provide any images of Iapetus and it came no closer than {{convert|1,030,000|km|mi|abbr=on}} from the moon.<ref>{{cite web | url=https://eyes.nasa.gov/apps/solar-system/#/sc_pioneer_11/distance?to=iapetus&rate=0&time=1979-08-29T05:04:53.987+00:00 | title=Eyes on the Solar System - NASA/JPL }}</ref> Nonetheless, Pioneer 11 was humanity's first attempt to obtain direct measurements from the objects within the Saturnian system.
-[[Voyager 1]] arrived at Saturn on November 12, 1980 and it became the first probe to return pictures of Iapetus that clearly show the moon's two-tone appearance from a distance of {{convert|2,480,000|km|mi|abbr=on}}. <ref>{{cite web | url=https://voyager.jpl.nasa.gov/mission/science/saturn/ | title=Voyager - Saturn Approach }}</ref> as it was exiting the Saturnian system.<ref>{{cite web | url=https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_1/distance?time=1980-11-14T18:36:21.176+00:00&rate=0&to=iapetus | title=Eyes on the Solar System - NASA/JPL }}</ref>
+[[Voyager 1]] arrived at Saturn on November 12, 1980, and it became the first probe to return pictures of Iapetus that clearly show the moon's two-tone appearance from a distance of {{convert|2,480,000|km|mi|abbr=on}}<ref>{{cite web | url=https://voyager.jpl.nasa.gov/mission/science/saturn/ | title=Voyager - Saturn Approach }}</ref> as it was exiting the Saturnian system.<ref>{{cite web | url=https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_1/distance?time=1980-11-14T18:36:21.176+00:00&rate=0&to=iapetus | title=Eyes on the Solar System - NASA/JPL }}</ref>
-[[File:Voyager_2_Iapetus.jpg|right|thumb|200px|Voyager 2's image of the north pole of Iapetus taken on August 22, 1981 from a distance of {{convert|910,000|km|mi|abbr=on}}. The crater on the lower part along the [[terminator (solar)|terminator]] is [[Roland_(crater)|Roland]]<ref>{{cite web | url=https://nssdc.gsfc.nasa.gov/imgcat/html/object_page/vg2_4391335.html | title=Iapetus - Voyager 2 }}</ref>]]
+[[File:Voyager_2_Iapetus.jpg|right|thumb|200px|Voyager 2's image of the north pole of Iapetus taken on August 22, 1981, from a distance of {{convert|910,000|km|mi|abbr=on}}. The crater on the lower part along the [[terminator (solar)|terminator]] is [[Roland (crater)|Roland]]<ref>{{cite web | url=https://nssdc.gsfc.nasa.gov/imgcat/html/object_page/vg2_4391335.html | title=Iapetus - Voyager 2 }}</ref>]]
-[[Voyager 2]] became the next probe to visit Saturn on August 22, 1981 and made its closest approach to Iapetus at a distance of {{convert|909,000|km|mi|abbr=on}}. It took photos of Iapetus' north pole as it entered the Saturnian system - opposite the approach direction of Voyager 1.
+[[Voyager 2]] became the next probe to visit Saturn on August 22, 1981, and made its closest approach to Iapetus at a distance of {{convert|909,000|km|mi|abbr=on}}. It took photos of Iapetus's north pole as it entered the Saturnian system - opposite the approach direction of Voyager 1.
 The latest probe to visit Iapetus was the [[Cassini–Huygens|''Cassini'' orbiter]] which went into orbit around Saturn starting on July 1, 2004.<ref>{{cite web | url=https://science.nasa.gov/mission/cassini/the-journey/timeline/ | title=Timeline - NASA Science }}</ref> Iapetus has been imaged many times from moderate distances by ''Cassini'' but its great distance from Saturn makes close observation difficult.
-''Cassini'' made its first targeted flyby of Iapetus on  Dec. 31, 2004 at a distance of {{convert|123,400|km|mi|abbr=on}} around the time when the spacecraft was settling in its orbit around Saturn.<ref>{{cite web | url=https://solarsystem.nasa.gov/news/12305/cassini-caps-off-2004-with-flyby-of-icy-moon-iapetus/ | title=Cassini Caps off 2004 with Flyby of Icy Moon Iapetus }}</ref> ''Cassini'' did not crossed Iapetus' orbit when it flew by and remained inside the moon's orbit. ''Cassini's'' subsequent flybys  of Titan would make the spacecraft's orbit smaller, preventing Cassini from flying close to Iapetus for months.
+''Cassini'' made its first targeted flyby of Iapetus on Dec. 31, 2004, at a distance of {{convert|123,400|km|mi|abbr=on}} around the time when the spacecraft was settling in its orbit around Saturn.<ref>{{cite web | url=https://solarsystem.nasa.gov/news/12305/cassini-caps-off-2004-with-flyby-of-icy-moon-iapetus/ | title=Cassini Caps off 2004 with Flyby of Icy Moon Iapetus }}</ref> ''Cassini'' did not cross Iapetus's orbit when it flew by and remained inside the moon's orbit. ''Cassini's'' subsequent flybys of Titan would make the spacecraft's orbit smaller, preventing Cassini from flying close to Iapetus for months.
-''Cassini'' made a second flyby of Iapetus on November 12, 2005 at a distance of {{convert|415,000|km|mi|abbr=on}},<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia07766-iapetus-spins-and-tilts | title=Iapetus Spins and Tilts | website=[[Jet Propulsion Laboratory]] }}</ref> also without crossing the moon's orbit.
+''Cassini'' made a second flyby of Iapetus on November 12, 2005, at a distance of {{convert|415,000|km|mi|abbr=on}},<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia07766-iapetus-spins-and-tilts | title=Iapetus Spins and Tilts | website=[[Jet Propulsion Laboratory]] }}</ref> also without crossing the moon's orbit.
-''Cassini'' then made a third and more distant flyby of Iapetus on January 22, 2006 at a distance of {{convert|1,300,000|km|mi|abbr=on}}. <ref>{{cite web | url=https://science.nasa.gov/resource/to-the-relief-of-iapetus/ | title=To the Relief of Iapetus - NASA Science }}</ref><ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia08125-to-the-relief-of-iapetus | title=To the Relief of Iapetus | website=[[Jet Propulsion Laboratory]] }}</ref>
+''Cassini'' then made a third and more distant flyby of Iapetus on January 22, 2006, at a distance of {{convert|1,300,000|km|mi|abbr=on}}.<ref>{{cite web | url=https://science.nasa.gov/resource/to-the-relief-of-iapetus/ | title=To the Relief of Iapetus - NASA Science }}</ref><ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia08125-to-the-relief-of-iapetus | title=To the Relief of Iapetus | website=[[Jet Propulsion Laboratory]] }}</ref>
 The fourth flyby happened on April 8, 2006, at a distance of approximately {{convert|866,000|km|mi|abbr=on}}, and this time, Cassini crossed Iapetus' orbit.<ref>{{cite web | url=https://www.jpl.nasa.gov/images/pia08164-from-dark-to-bright-and-red-to-white | title=From Dark to Bright and Red to White | website=[[Jet Propulsion Laboratory]] }}</ref> After this, ''Cassini's'' orbit was made smaller once again, preventing the probe from approaching Iapetus for more than a year this time.
-''Cassini's'' closest flyby of Iapetus happened on September 10, 2007, at a minimum range of {{convert|1,227|km|mi|abbr=on}}.<ref name="Cassini Solstice Mission">{{cite web|title=Iapetus|url=http://saturn.jpl.nasa.gov/science/moons/iapetus/|website=Cassini Solstice Mission|publisher=NASA|access-date=6 July 2015|url-status=dead|archive-url=https://web.archive.org/web/20150326045917/http://saturn.jpl.nasa.gov/science/moons/iapetus/|archive-date=2015-03-26}}</ref> It approached Iapetus from its night side.<ref>{{cite web | url=https://science.nasa.gov/resource/approaching-iapetus/ | title=Approaching Iapetus - NASA Science }}</ref>
+''Cassini's'' closest flyby of Iapetus happened on September 10, 2007, at a minimum range of {{convert|1,227|km|mi|abbr=on}}.<ref name="Cassini Solstice Mission">{{cite web|title=Iapetus|url=http://saturn.jpl.nasa.gov/science/moons/iapetus/|website=Cassini Solstice Mission|publisher=NASA|access-date=6 July 2015|url-status=dead|archive-url=https://web.archive.org/web/20150326045917/http://saturn.jpl.nasa.gov/science/moons/iapetus/|archive-date=2015-03-26}}</ref> It approached Iapetus from its night side.<ref name="Approaching Iapetus - NASA Science">{{cite web | url=https://science.nasa.gov/resource/approaching-iapetus/ | title=Approaching Iapetus - NASA Science }}</ref>
-After this encounter, ''Cassini'' did not make any targeted flyby of the moon any further.
-== In popular culture ==
-The monolith featured during the climax of Arthur C. Clarke's 1968 novel ''[[2001: A Space Odyssey (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|Asimov's Science Fiction Magazine]].<ref>{{cite web |title=Publications - Julie Novakova |url=https://www.julienovakova.com/publications/}}</ref>
-== Gallery ==
-<gallery mode="packed" heights="200">
-File:Iapetus_Taijitu_1.jpg|Iapetus seemingly in the shape of the famous [[Taijitu]] (also known as the Yin-Yang symbol). This was imaged by Voyager 1 on November 12, 1980.
-File:Iapetus by Voyager 2 - enhanced.jpg|Enhanced version of an image of Iapetus by Voyager 2. The left side of Iapetus in this image is its side that is always facing away from Saturn. <ref>{{cite web | url=https://nssdc.gsfc.nasa.gov/imgcat/html/object_page/vg2_4391335.html | title=Iapetus - Voyager 2 }}</ref>
-File:Iapetus_Roland_and_Turgis.png|An image of a half-illuminated Iapetus, taken by Cassini  on November 12, 2005 from a distance of  417,000 kilometers (259,000 miles) during its flyby of Saturn. The big crater at the center is [[Turgis_(crater)|Turgis]], while the partly-shadowed crater with a central peak to the north is [[List_of_geological_features_on_Iapetus#Craters|Roland]].<ref>{{cite web | url=https://science.nasa.gov/resource/a-moon-with-two-dark-sides/ | title=A Moon with Two Dark Sides - NASA Science }}</ref>
-File:Iapetus_crescent_true_color.png|An image of Iapetus from its nightside that was photographed by an approaching Cassini, showing it as a slender crescent with its massive equatorial ridge clearly visible at the middle. This image was taken on September 10, 2007 at a distance of 83,000 kilometers (51,600 miles). <ref>{{cite web | url=https://science.nasa.gov/resource/approaching-iapetus/ | title=Approaching Iapetus - NASA Science }}</ref>
-File:Iapetus_splatter.png|A super-close image of Iapetus's transition area between its light and dark side, showing  white materials sharply spattering into the dark materials. Imaged by Cassini on September 10, 2007 at a distance of 9,240 kilometers (5740 miles) somewhere in the area of the [[Carassone Montes]].<ref>{{cite web | url=https://www.esa.int/ESA_Multimedia/Images/2007/09/Bright_and_Dark_mountains_on_Iapetus | title=Bright and Dark mountains on Iapetus }}</ref>
-File:Saturn-Iapetus-yin-yang.jpg|A much clearer image of Iapetus's Taijitu (Yin-Yang) appearance, taken by Cassini on August 30, 2013 from a distance of 2.5 million kilometers (1.5 million miles). The crater near the terminator at the lower center is [[Ganelon_(crater)| Ganelon]].
+After this encounter, ''Cassini'' made no further targeted flybys of Iapetus.
-File:Final_Cassini_Iapetus.jpg|One of Cassini's final images of Iapetus taken on May 30, 2017, a few months before Cassini plunge into Saturn, at a distance of 2.5 million kilometers (1.5 million miles). The clearly-visible  craters right of center on the white side are [[Naimon_(crater)|Naimon]], [[Astor_(crater)|Astor]] and [[Ivon_(crater)|Ivon]] (left to right).
-</gallery>
 == See also ==
 * [[Planet#Objects formerly considered planets|Former classification of planets]]
@@ Line 176: / Line 151: @@
 ==Notes==
 {{Notelist}}
 == References ==

v t e Natural satellites of the Solar System
Planetary satellites of	Earth Mars Jupiter Saturn Uranus Neptune
Dwarf planet satellites of	Orcus Pluto Haumea Quaoar Makemake Gonggong Eris
Minor-planet moons	Near-Earth Florence Didymos Dimorphos Moshup Squannit 1994 CC 2001 SN₂₆₃ Main belt Kalliope Linus Euphrosyne Daphne Peneius Eugenia Petit-Prince Sylvia Romulus Remus Minerva Aegis Gorgoneion Camilla Elektra Kleopatra Alexhelios Cleoselene Ida Dactyl Roxane Olympias Pulcova Balam Dinkinesh (Selam) Jupiter trojans Patroclus Menoetius Hektor Skamandrios Eurybates Queta TNOs Lempo Hiisi Paha 2002 UX₂₅ Sila–Nunam Salacia Actaea Varda Ilmarë Gǃkúnǁʼhòmdímà Gǃòʼé ǃHú 2013 FY₂₇
Ranked by size	Ganymede largest: 5268 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys Dysnomia Enceladus Miranda Vanth Proteus Mimas Ilmarë Nereid Hiʻiaka Actaea Hyperion Phoebe ...
Discovery timeline Inner moons Irregular moons List Planetary-mass moons Naming Subsatellite Regular moons Trojan moons

v t e Saturn
Outline
Geography	Dragon Storm Great White Spot Hexagon Magnetosphere Rings
Moons	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus Pandora Epimetheus Janus Aegaeon Mimas Methone Anthe Pallene Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Inuit group Kiviuq Paaliaq Siarnaq Gallic group Albiorix Norse group Phoebe
Astronomy	Delta Octantis Saturn-crossing minor planets
Exploration	Cassini–Huygens (Huygens) timeline retirement Pioneer 11 Voyager program Voyager 1 Voyager 2
Related	Fiction The Day the Earth Smiled In Saturn's Rings (2018 documentary)
Category