Editing Iapetus (moon) (section)

=== Two-tone coloration ===
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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]].