Editing Iapetus (moon)

{{Short description|Moon of Saturn}}
{{Redirect|Saturn VIII|the rocket|Saturn C-8}}
{{Infobox planet
| name = Iapetus
| mpc_name = Saturn VIII
| 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>
| named_after = [[Iapetus|Ἰαπετός]] ''Īapetus''
| image = Iapetus as seen by the Cassini probe - 20071008.jpg
| 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]]
| discovered = October 25, 1671
| semimajor = {{val|3560820|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>
| 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 = {{plainlist |
* 17.28° (to the ecliptic)
* 15.47° (to Saturn's equator)
*  8.13° (to [[Laplace plane]])<ref name="jpl-ssd">{{cite web| date = 2009-12-17| title = Planetary Satellite Mean Orbital Parameters| publisher = JPL/NASA| author = Jacobson, R.A. (2009) SAT317| url = http://ssd.jpl.nasa.gov/?sat_elem#saturn| access-date = 2011-01-15}}
</ref>}}
| satellite_of = [[Saturn]]
| 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|6777600|u=km<sup>2</sup>}}
| 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)
| escape_velocity = {{V2|1.805|734.5|3}} km/s
| rotation = {{val|79.3215|u=d}} <br> ([[Synchronous rotation|synchronous]])
| axial_tilt = zero
| albedo = 0.05–0.5<ref name="fact">{{cite web | first = David R.| last = Williams| title = Saturnian Satellite Fact Sheet| work = NASA| url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html| access-date = 2007-11-04}}
</ref>
| single_temperature = 90–130 [[Kelvin|K]]
| magnitude = 10.2–11.9<ref name="Observatorio ARVAL" />
}}

'''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 natural satellites|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 [[Titans|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 on Iapetus|equatorial ridge]] running three-quarters of the way around the moon.

==History==
=== 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 [[Christiaan 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>

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is [[tidal locking|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.<ref>{{cite book|title= Moons: A Very Short Introduction|first1=David A.|last1=Rotherty|publisher=[[Oxford University Press]]|date=January 1, 2016|isbn=9780198735274|page=102}}</ref>

=== Name ===
[[File:John_F._Hershel.png|left|thumb|upright|[[John Herschel]], the astronomer who suggested that the moons of Saturn be named after the Titans and Giants]]Iapetus is named after the [[Titans|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'',<ref name="Lassell" /> in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan [[Cronus]] (whom the [[Roman mythology|Romans]] equated with their god [[Saturn (mythology)|Saturn]]); and [[Giants (Greek mythology)|Giants]], the massive but lesser relatives of the Titans who sided with the Titans against [[Zeus]] and the [[Twelve Olympians|Olympian Gods]].<ref>https://library.si.edu/digital-library/book/resultsastronom00hers - Page 415</ref>

The name has a largely obsolete variant, '''Japetus'''<ref name="Lassell">{{cite journal | last = Lassell| first = William| date = January 14, 1848| title = Satellites of Saturn| journal = [[Monthly Notices of the Royal Astronomical Society]]| volume = 8| issue = 3| pages = 42–43| url = http://adsabs.harvard.edu//full/seri/MNRAS0008//0000042.000.html| bibcode = 1848MNRAS...8...42L|doi=10.1093/mnras/8.3.42 | doi-access = free}}</ref><ref name=Hill1952/> {{IPAc-en|ˈ|dʒ|æ|p|ᵻ|t|ə|s}},<ref>{{cite book|first1=Noah|last1=Webster|author-link=Noah Webster|date=1884|url=https://books.google.com/books?id=5YVDAQAAMAAJ|publisher=[[Merriam-Webster]]|editor=Dorsey Gardner|title=A Practical Dictionary of the English Language}}{{page needed|date=October 2022}}</ref>
with an adjectival form ''Japetian''.<ref name=Hill1952>George William Hill (1952) ''The Radiant Universe'', p. 280</ref>
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 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/>

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the [[dwarf planet]] symbols, proposed a Greek [[iota]] (the initial of Iapetus) combined with the crook of the Saturn symbol as the symbol of Iapetus ([[File:Iapetus symbol (fixed width).svg|16px]]). This symbol is not widely used.<ref name=moons>{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}</ref>

== 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'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, 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==
The moons of Saturn are typically thought to have formed through [[Accretion disc|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 (moon)|Titan]] suggests that Titan was instead formed in a series of [[giant impact]]s between pre-existing moons. Iapetus and Rhea are thought to have formed from part of the debris of these collisions.<ref>{{cite web |title=Giant impact scenario may explain the unusual moons of Saturn |work=Space Daily |date=2012 |url=http://www.spacedaily.com/reports/Giant_impact_scenario_may_explain_the_unusual_moons_of_Saturn_999.html |access-date=2012-10-19 }}</ref> 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.<ref>{{Cite web | url=http://www.space.com/32378-saturn-rings-and-moons-younger-than-dinosaurs.html | title=Saturn's Moons and Rings May be Younger Than the Dinosaurs| website=[[Space.com]]| date=25 March 2016}}</ref>

== Physical characteristics ==
{{main|List of geological features on Iapetus}}

[[File:Iapetus,_Earth,_Moon_size_comparison.jpg|left|thumb|Size comparison between Iapetus (lower left), the [[Moon]] (upper left) and Earth]][[File:Iapetus_map_labelled.png|thumb|right|250px|A map of the surface of Iapetus with its surface features labeled with their official names. 0° longitude marks the side of the moon that always faces Saturn.]]
The low [[density]] of Iapetus indicates that it is mostly composed of [[ice]], with only a small (~20%) amount of rocky materials.<ref name="Castillo2007">{{cite journal| last=Castillo-Rogez| first=J. C.|author2=Matson, D. L. |author3=Sotin, C. |author4=Johnson, T. V. |author5=Lunine, Jonathan I. |author6= Thomas, P. C. | title=Iapetus' geophysics: Rotation rate, shape, and equatorial ridge| journal=Icarus| date=2007| volume=190| issue=1| pages=179–202| doi=10.1016/j.icarus.2007.02.018| bibcode=2007Icar..190..179C}}</ref>

Unlike most of the large moons, its overall shape is neither [[sphere|spherical]] nor [[ellipsoid]], but has a bulging waistline and squashed poles.<ref name="Science News">Cowen, R. (2007). Idiosyncratic Iapetus, ''Science News'' vol. 172, pp. 104–106. [http://www.sciencenews.org/articles/20070818/bob8ref.asp references] {{Webarchive|url=https://web.archive.org/web/20071013165655/http://www.sciencenews.org/articles/20070818/bob8ref.asp |date=2007-10-13 }}</ref> 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 [[impact crater|cratered]], and ''Cassini'' images have revealed large impact basins, at least five of which are over {{cvt|350|km||}} wide. The largest, [[Turgis (crater)|Turgis]], has a [[diameter]] of {{cvt|580|km||}};<ref name="USGS">{{cite web | title = Iapetus: Turgis | publisher = USGS Astrogeology | work = Gazetteer of Planetary Nomenclature | url = http://planetarynames.wr.usgs.gov/Feature/14488 | access-date = 2009-01-10 }}</ref> its rim is extremely steep and includes a [[escarpment|scarp]] about {{cvt|15|km||}} high.<ref name="scarp">{{cite web| date = 2004-12-31| title = PIA06171: Giant Landslide on Iapetus| publisher = NASA/JPL/Space Science Institute (photojournal)| url = http://photojournal.jpl.nasa.gov/catalog/PIA06171| access-date = 2009-01-10}}</ref>
Iapetus is known to support long-runout landslides or [[sturzstrom]]s, possibly supported by ice sliding.<ref>{{cite news | url=https://www.bbc.co.uk/news/science-environment-19011011| title=BBC News - Saturn moon Iapetus' huge landslides stir intrigue| work=[[BBC Online]]| access-date=30 July 2012}}</ref>

=== Two-tone coloration ===
{{multiple image
 | align = right
 | total_width = 300
 | image1 = Iapetus 1.png
 | image2 = Iapetus 2.png
}}
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 [[Shades of brown|reddish-brown]] coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5–0.6, almost as bright as [[Europa (moon)|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 [[telescope]]s 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 deposit|lag]] from the [[Sublimation (chemistry)|sublimation]] (evaporation) of ice from the warmer areas of the moon's surface, further darkened by exposure to sunlight.<ref name="Mason">{{cite web |last= Mason |first= J. |author2= Martinez, M. |author3= Balthasar, H. |title= Cassini Closes in on the Centuries-old Mystery Of Saturn's Moon Iapetus |work= CICLOPS website newsroom |publisher= [[Space Science Institute]] |date= 2009-12-10 |url= http://ciclops.org/view.php?id=6033 |access-date= 2009-12-22 |archive-date= 2012-02-03 |archive-url= https://web.archive.org/web/20120203132024/http://ciclops.org/view.php?id=6033 |url-status= dead }}</ref><ref name="Denk">{{cite journal |last1= Denk |first1= T. |title= Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging| journal = [[Science (journal)|Science]] |volume= 327 |issue= 5964 | pages = 435–439| date = 2010-01-22
| doi = 10.1126/science.1177088| pmid = 20007863 | bibcode = 2010Sci...327..435D| display-authors = 1| last2 = Neukum| first2 = G.| last3 = Roatsch| first3 = T.| last4 = Porco| first4 = C. C.| last5 = Burns| first5 = J. A.| last6 = Galuba| first6 = G. G.| last7 = Schmedemann| first7 = N.| last8 = Helfenstein| first8 = P.| last9 = Thomas| first9 = P. C. |s2cid= 165865 }}</ref><ref name="Spencer">{{cite journal| last = Spencer | first = J. R. |author2=Denk, T.| title = Formation of Iapetus' Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration| journal = [[Science (journal)|Science]] | volume = 327 | issue = 5964 | pages = 432–435| date = 2010-01-22 | doi = 10.1126/science.1177132 | pmid = 20007862|bibcode = 2010Sci...327..432S | citeseerx = 10.1.1.651.4218 | s2cid = 20663944 }}</ref> It contains [[organic matter|organic compounds]] similar to the substances found in primitive [[meteorite]]s or on the surfaces of [[comet]]s; Earth-based observations have shown it to be [[carbon]]aceous, and it probably includes cyano-compounds such as frozen [[hydrogen cyanide]] [[polymer]]s.

[[File:Iapetus_Colored_Map.jpg|thumb|upright=4|center|A colored map of the surface of Iapetus by the Lunar and Planetary Institute clearly showing the dichotomy between its light and dark hemisphere.]]

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)]]

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" />

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 motion|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.<ref name="Mason" /><ref name="Denk" />

[[File:Color hemispheres map of Iapetus PIA18436 Nov. 2014 (Cropped).png|thumb|200px|The bright regions of Iapetus. [[Roncevaux Terra]] is at the top (north); while [[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 [[Poynting–Robertson effect|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.<ref name="Denk" /><ref name="Spencer" /> 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.<ref name="Spencer" /> 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.<ref name="Denk" /> 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.<ref name="Denk" /> The next moon inward from Iapetus, chaotically rotating [[Hyperion (moon)|Hyperion]], also has an unusual reddish color.

The largest reservoir of such infalling material is [[Phoebe (moon)|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,<ref name="Hendrix 2005">{{cite journal| last=Hendrix| first=A. R.|author2=Hansen, C. J.| title=Iapetus and Phoebe as Measured by the Cassini UVIS| journal=36th Annual Lunar and Planetary Science Conference| pages=2272|date=March 14–18, 2005| url=http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2272.pdf| bibcode=2005LPI....36.2272H}}</ref> 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 [[Phoebe ring|tenuous disk of material]] in the plane of and just inside Phoebe's orbit was announced on 6 October 2009,<ref>[http://www.sciencenews.org/view/generic/id/48097/title/Largest_known_planetary_ring_discovered Largest known planetary ring discovered] {{Webarchive|url=https://web.archive.org/web/20110822023022/http://www.sciencenews.org/view/generic/id/48097/title/Largest_known_planetary_ring_discovered |date=2011-08-22 }}, Science News</ref> supporting the model.<ref>[https://www.newscientist.com/article/dn17928-largest-ring-in-solar-system-found-around-saturn.html Largest ring in solar system found around Saturn], New Scientist</ref> 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 {{cvt|746|x|746|x|712|km||}}, with a mean radius of {{cvt|734.5|±|2.8|km}}.<ref name="Roatsch et al. 2009" /> 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.<ref name="Thomas2010"/> A possible explanation for this is that the shape of Iapetus was frozen by formation of a thick [[crust (geology)|crust]] shortly after its formation, while its rotation continued to slow afterwards due to [[Tidal acceleration|tidal dissipation]], until it became [[tidal locking|tidally locked]].<ref name="Science News" />

=== Equatorial ridge ===
{{Main|Equatorial ridge on Iapetus}}
[[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.

[[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'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}}
*{{Cite journal|last=|first=|date=January 21, 2005|title=Ring around a moon|url=https://www.science.org/doi/10.1126/science.307.5708.349c|journal=Science|volume=307|issue=5708|pages=349|doi=10.1126/science.307.5708.349c|s2cid=210274907|via=}}
*{{cite journal | last1 = Ip | first1 = W.-H | year = 2006 | title = On a ring origin of the equatorial ridge of Iapetus | journal = Geophysical Research Letters | volume = 33 | issue = 16| page = L16203 | doi = 10.1029/2005GL025386 | bibcode=2006GeoRL..3316203I| doi-access = free }}
*{{cite journal|url=https://www.sciencedirect.com/science/article/abs/pii/S0019103511002041|title=Ridge formation and de-spinning of Iapetus via an impact-generated satellite|journal=[[Icarus (journal)|Icarus]]|last1=Levison|first1=Harold F.|author-link=Harold F. Levison|last2=Walsh|first2=Kevin J.|last3=Barr|first3=Amy C.|last4=Dones|first4=Luke|date=August 2011|volume=214|issue=2|pages=773–778|doi=10.1016/j.icarus.2011.05.031|publisher=[[Elsevier]]|arxiv=1105.1685 |bibcode=2011Icar..214..773L |s2cid=5849043 }}
*{{cite journal| last=Czechowski | first=L. |author2=J.Leliwa-Kopystynski| date=2013-09-25| title= Remarks on the Iapetus' bulge and ridge | journal=Earth, Planets and Space| volume=65| issue=8 | pages=929–934 | doi=10.5047/eps.2012.12.008|bibcode = 2013EP&S...65..929C | doi-access=free| url=http://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-834.pdf |archive-url=https://web.archive.org/web/20150109095658/http://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-834.pdf |archive-date=2015-01-09 |url-status=live}}</ref>

== Exploration ==
[[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 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>

[[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}}.<ref>https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_2/distance?time=1981-08-23T01:17:50.573+00:00&rate=0&to=iapetus</ref> It took photos of Iapetus's north pole as it entered the Saturnian system <ref>https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_2?time=1981-08-22T23:02:30.949+00:00&rate=0</ref>- 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 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'' 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's 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 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'' made no further targeted flybys of Iapetus.

== See also ==
* [[Planet#Objects formerly considered planets|Former classification of planets]]
* [[Saturn in fiction#Other_moons|Iapetus in fiction]]
* [[List of tallest mountains in the Solar System]]
* [[Lists of astronomical objects]]
* [[Subsatellite]]

==Notes==
{{Notelist}}

== References ==
{{Reflist|30em|
refs =
<ref name="Roatsch et al. 2009">
{{cite book| doi = 10.1007/978-1-4020-9217-6_24| last1 = Roatsch| first1 = T.| last2 = Jaumann| first2 = R.| last3 = Stephan| first3 = K.| last4 = Thomas| first4 = P. C.| year = 2009| chapter = Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data| title = Saturn from Cassini-Huygens| pages = 763–781| isbn = 978-1-4020-9216-9| ref = {{sfnRef|Roatsch Jaumann et al.|2009}}}}
</ref>

<ref name="Observatorio ARVAL">{{cite web| author = Observatorio ARVAL| title = Classic Satellites of the Solar System| publisher = Observatorio ARVAL| date = April 15, 2007| url = http://www.oarval.org/ClasSaten.htm| access-date = 2011-12-17| ref = {{sfnRef|Observatorio ARVAL}}| archive-url = https://web.archive.org/web/20110920010842/http://www.oarval.org/ClasSaten.htm| archive-date = September 20, 2011| url-status = dead}}</ref>

<ref name="Thomas2010">{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 2015-09-25| archive-date = 2018-12-23| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref>}}

== External links ==
{{Spoken Wikipedia|Iapetus (moon).ogg|date=2013-08-22}}
{{Commons|Iapetus (moon)|Iapetus}}
* [https://web.archive.org/web/20090123111031/http://saturn.jpl.nasa.gov/science/moons/iapetus/ ''Cassini'' mission page&nbsp;– Iapetus]
* [https://web.archive.org/web/20080228114042/http://saturn.jpl.nasa.gov/multimedia/products/pdfs/CHARM_20071030_Iapetus.pdf Discussion of Iapetus dated October 2007]
* [https://web.archive.org/web/20070801204721/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Iapetus Iapetus Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* [https://web.archive.org/web/20060808065916/http://planetary.org/explore/topics/saturn/iapetus.html The Planetary Society: Iapetus]
* [https://www.google.com/maps/space/iapetus/@-1.3366732,138.0552194,11488532m/ Google Iapetus 3D], interactive map of the moon
* {{APOD |date=1 February 2005 |title=Saturn's Iapetus: Moon with a Strange Surface}}
* [http://th-www.if.uj.edu.pl/acta/vol33/abs/v33p1325.htm Mirror Objects in the Solar System?]—refereed article discussing the speculative [[mirror matter]], and Iapetus in this context
* {{usurped|1=[https://web.archive.org/web/20090529013037/http://www.enterprisemission.com/moon1.htm A Moon with a View]}}—[[Richard C. Hoagland]]'s imaginative discussion of Iapetus's oddities
* [http://www.newscientistspace.com/article.ns?id=dn9884 New attempts to crack Saturn's 'walnut' moon]—equatorial ridge formation theories
* [http://ciclops.org/search.php?x=19&y=3&search=Iapetus ''Cassini'' images of Iapetus] {{Webarchive|url=https://web.archive.org/web/20110725173137/http://ciclops.org/search.php?x=19&y=3&search=Iapetus |date=2011-07-25 }}
* [http://photojournal.jpl.nasa.gov/target/Iapetus Images of Iapetus at JPL's Planetary Photojournal]
* Movie of [https://web.archive.org/web/20100601171842/http://sos.noaa.gov/videos/Iapetus.mov Iapetus's rotation] at the National Oceanic and Atmospheric Administration site
* [http://ciclops.org/view.php?id=5221 Iapetus basemap (May 2008) from ''Cassini'' images] {{Webarchive|url=https://web.archive.org/web/20100217194917/http://ciclops.org/view.php?id=5221 |date=2010-02-17 }}
* [http://ciclops.org/view.php?id=5219 Iapetus atlas (October 2008) from ''Cassini'' images]
* [http://planetarynames.wr.usgs.gov/Page/IAPETUS/target Iapetus nomenclature] and [http://planetarynames.wr.usgs.gov/images/iapetus_comp.pdf Iapetus map with feature names] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]
* [https://web.archive.org/web/20111203154619/http://www.nasaimages.org/luna/servlet/detail/NVA2~1~1~994~101040:Flight-over-Iapetus Flight over Iapetus] (video)
{{Clear}}

{{Iapetus|state=uncollapsed}}
{{Moons of Saturn}}
{{Solar System moons (compact)}}
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[[Category:Iapetus (moon)| ]]
[[Category:Astronomical objects discovered in 1671|16711025]]
[[Category:Discoveries by Giovanni Domenico Cassini]]
[[Category:Moons with a prograde orbit]]