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'''Iapetus''' is the third largest [[moon]] of [[Saturn]], and eleventh in the solar system, discovered by [[Giovanni Cassini]] in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the ''Cassini'' mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.
{{Planet
|image=Iapetus dark ridge.jpg
|caption=Iapetus, showing Cassini Regio and the equatorial ridge, taken by Cassini
|date=October 25, 1671<ref name=usgs>"[http://planetarynames.wr.usgs.gov/append7.html Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers]." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.</ref>
|discname=Giovanni Domenico Cassini<ref name=usgs/><ref name=Hamilton>Hamilton, Calvin J. "[http://www.solarviews.com/eng/iapetus.htm Entry for Iapetus]." ''The Views of the Solar System'', 2004. Accessed June 16, 2008.</ref>
|origname=A Titan, god of mortality and wounding
|primary=Saturn
|order=22
|periapsis=3,460,515 km<ref name=calc>Calculated</ref>
|apoapsis=3,662,085 km<ref name=calc/>
|semimajor=3,561,300 km<ref name=saturnsatfact>Williams, David R. "[http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturniansatfact.html Saturnian Satellite Fact Sheet]." National Space Science Data Center, [[National Aeronautics and Space Administration|NASA]], November 23, 2007. Accessed June 4, 2008.</ref>
|eccentricity=0.0283<ref name=saturnsatfact/>
|sidereal=79.330183 da<ref name=saturnsatfact/>
|orbitspeed=3.27 km/s<ref name=Hamilton/>
|inclination=14.72°<ref name=saturnsatfact/>
|siderealday=79.330183 da<ref name=saturnsatfact/>
|rotatespeed=0.000658 km/s<ref name=calc/>
|axialtilt=0°<ref name=saturnsatfact/>
|mass=1.81 * 10<sup>21</sup> kg<ref name=saturnsatfact/>
|density=1090 kg/m³<ref name=saturnsatfact/>
|meanradius=718 km<ref name=saturnsatfact/>
|equatorradius=748 km<ref name=Thomas>Thomas, P.C., Burns, J.A., Helfenstein, P., ''et al.'' "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4NDMNGC-4&_user=10&_coverDate=10%2F31%2F2007&_rdoc=21&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%236821%232007%23998099997%23668725%23FLA%23display%23Volume)&_cdi=6821&_sort=d&_docanchor=&_ct=29&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8101cb113c75c3b948580ff1b4750474 Shapes of the saturnian icy satellites and their significance]." ''Icarus'' 190(2):573-584, October 2, 2007. {{doi|10.1016/j.icarus.2007.03.012}} Accessed June 16, 2008.</ref>
|polarradius=712.6 km<ref name=Thomas/>
|surfacegrav=0.234 m/s²<ref name=calc/>
|escapespeed=0.579 km/s<ref name=calc/>
|surfacearea=6,478,266 km²<ref name=calc/>
|mintemp=70 K<ref name=tempimg>"[http://photojournal.jpl.nasa.gov/catalog/PIA07004 Catalog Page for PIA07004: Iapetus Thermal Radiation Image]." JPL Photojournal, [[Jet Propulsion Laboratory|JPL]], [[National Aeronautics and Space Administration|NASA]], January 10, 2005. Accessed June 16, 2008.</ref>
|meantemp=100 K<ref name=tempimg/>
|maxtemp=130 K<ref name=tempimg/>
|composition=Water ice and small amounts of rock
|compauth=<ref name=Hamilton/>
|color=Light and dark gray
|albedo=0.5<ref name=saturnsatfact/>
}}'''Iapetus''' or '''Japetus''' or '''Saturn VIII''' is the twenty-second [[moon]] of [[Saturn]], the second moon of Saturn to be discovered, and one of the most remarkable objects in all the [[Solar system]].
==Discovery==
== Discovery and naming ==
[[Giovanni Domenico Cassini]] discovered Iapetus in 1671. However, he noted that he could reliably observe Iapetus only on the western side of [[Saturn]], and never on the eastern side. In 1705 he observed Iapetus on the eastern side, with an improved telescope, and noted that it was two visual magnitudes dimmer on that side.
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, so that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and the dark hemisphere on the other side. The dark hemisphere was later named Cassini Regio in his honour.
After his discovery of Iapetus, Cassini discovered three other moons ([[Tethys]], [[Dione]], and [[Rhea]]) in the ensuing years, during the reign of "Sun-King" [[Louis XIV]] of [[France]].<ref name=usgs/> Cassini named these moons the "Sidera Lodoicea" in honor of the king. Later generations (especially after the French Revolution) would not retain such regal flattery.<ref name=Boulay>Boulay, J. C. "[http://jcboulay.free.fr/astro/sommaire/astronomie/univers/galaxie/etoile/systeme_solaire/saturne/satellites/page_saturne_satellites1.htm Saturne: les satellites]." ''Astronomie-astronautique''. Accessed June 5, 2008.</ref>
Sir John Herschel, son of the astronomer [[William Herschel]], suggested the current names of the seven largest satellites of Saturn, including Iapetus. [[Titan]] received a generic name, and the other six received names of the Titans of mythology. Iapetus was one of four brothers who conspired with Kronos or Cronus to depose Uranus, father of all the gods.<ref name=Lasell>Lassell, William. "[http://articles.adsabs.harvard.edu/full/seri/MNRAS/0008//0000042.000.html Satellites of Saturn]." ''Monthly Notices of the Royal Astronomical Society'', 8(3):42-43, January 14, 1848. Accessed June 4, 2008.</ref>
Iapetus was among four moons of Saturn labelled the ''Sidera Lodoicea'' by their discoverer [[Giovanni Domenico Cassini|Giovanni Cassini]] after 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 by numbers, with Iapetus as '''Saturn V'''. Once [[Mimas (moon)|Mimas]] and [[Enceladus (moon)|Enceladus]] were discovered in 1789, the numbering scheme was extended and Iapetus became '''Saturn VII''', and subsequently ''Saturn VIII'' after the discovery of [[Hyperion (moon)|Hyperion]] in 1848. It is still known by that number today.
== Orbital characteristics ==
Iapetus is in an eccentric orbit around Saturn, at an average distance of 3,561,300 km. This distance is nearly three times the average distance of [[Titan]] and 2.5 times the average distance of [[Hyperion]]. Its sidereal month is about 79.33 days.
The orbit of Iapetus is inclined to Saturn's equator by more than 14 degrees, more than any of the other large moons of Saturn. Only the much smaller irregular moons of Saturn have orbits more inclined than that of Iapetus.
The low [[density]] of Iapetus indicates that it is mostly composed of [[ice]], with only a small (~20%) amount of rocky materials.<ref>Castillo-Rogez, J. C., with coauthors Matson, D. L.; Sotin, C.; Johnson, T. V.; Lunine, J. I.; Thomas, P. C.: <u>Iapetus’ geophysics: Rotation rate, shape, and equatorial ridge</u>. Icarus, 2007; volume 190, 179–202 <[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4NC4MCH-3&_user=10&_coverDate=09%2F30%2F2007&_rdoc=15&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%236821%232007%23998099998%23665908%23FLA%23display%23Volume)&_cdi=6821&_sort=d&_docanchor=&view=c&_ct=22&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=bd916410e8f013e963db0882ca52c451]></ref>
== Rotational characteristics ==
Iapetus is in [[tidal lock]] with Saturn.
Iapetus is heavily cratered, and ''Cassini'' images have revealed large impact basins in the dark region, at least five of which are over 350 km wide. The largest has a [[diameter]] over 500 km; its rim is extremely steep and includes a [[escarpment|scarp]] over 15 km high.
== Physical characteristics ==
Iapetus is slightly more dense than [[water]]. It therefore is likely composed of water ice with a small mineral component.
===Two-tone coloration===
=== Two colors ===
Iapetus is best known for the surprisingly different colors of its leading and trailing hemispheres. Its leading hemisphere has an albedo of about 0.05, while the trailing hemisphere has an albedo as high as 0.6. The dark region is named Regio Cassini, in honor of the discoverer. The bright region is named Roncevaux Terra. The boundary between these two regions is sharp, but not perfectly so In the border between the two regions, the dark material often covers only one side of a crater or fills the floor of another.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/iapetus.html Entry for Iapetus]." ''The <s>Nine</s> 8 Planets'', May 13, 2005. Accessed June 16, 2008.</ref>
The difference in coloring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright. 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 pattern of coloration is analogous to a spherical [[yin-yang]] symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region Roncevaux Terra.
The origin of the dark material remains unsettled. Some astronomers speculate that the material came from [[Phoebe]], a member of the [[Norse group]] of moons. Yet the colors of Phoebe and the dark region of Iapetus are not alike.<ref name=Arnett/><ref name=Hamilton/>
==References==
The dark material is either several meters thick or else is constantly renewed. Cassini Regio has no bright-rimmed craters consistent with erosion of the dark material.<ref name=Hamilton/>
[[Category:Astronomy]]
=== Shape and surface features ===
Iapetus is an oblate spheroid.<ref name=Thomas/> The observed oblateness is far more pronounced than astronomers would expect of a body having a sidereal day of more than 79 Earth days. Moreover, Iapetus has a ridge of mountains, 20 km high, along its equator. To assume such a shape, Iapetus ought to have had a sidereal day of 10 hours, slowing later to 16 hours. Its crust would then have had to freeze and thicken over a period of 100 million to 900 million years, according to [[uniformitarianism|uniformitarian]] theories. But during this time, Iapetus would have required a source of internal heat sufficient to melt a vast quantity of water ice. Astronomers who have considered the problem believe that this internal heat source was radioactive decay, specifically of <sup>26</sup>[[Aluminum|Al]] and <sup>60</sup>[[iron|Fe]].<ref name=Cowen>Cowen, Ron. "[http://www.sciencenews.org/view/feature/id/8823/title/Idiosyncratic_Iapetus Idiosyncratic Iapetus]." ''Science News'' 172:104-106, August 14, 2007. Accessed June 16, 2008.</ref>
The ridge system itself is an enigma. Astronomers have suggested at least two models for its formation:
# It formed during the process described above, as the crust buckled from the rapid rotation as the body cooled.<ref name=Kerr>Kerr, Richard A. "[http://www.sciencemag.org/cgi/content/summary/311/5757/29 How Saturn's Icy Moons Get a (Geologic) Life]." ''Science'' 311(5757):29, January 6, 2006. {{doi|10.1126/science.311.5757.29}} Accessed June 16, 2008.</ref>
# Iapetus initially had rings, which then collapsed onto the equator and persist today as the ridge.<ref name=Ip>Ip, Wing-Huen. "[http://www.agu.org/pubs/crossref/2006/2005GL025386.shtml On a ring origin of the equatorial ridge of Iapetus]." ''Geophys. Res. Lett.'' 33(L16203), 2006. {{doi|doi:10.1029/2005GL025386}} Accessed June 16, 2008.</ref>
The ridge and all the rest of the surface is covered with multiple impact craters that show little or no erosion. These include a 400-kilometer-wide basin within Regio Cassini at about 30 degrees north latitude,<ref name=basinimg>"[http://photojournal.jpl.nasa.gov/catalog/PIA06166 PIA06166: Encountering Iapetus]." JPL Photojournal, [[Jet Propulsion Laboratory|JPL]], [[National Aeronautics and Space Administration|NASA]], 2005. Accessed June 16, 2008.</ref> and a 450-kilometer-wide basin in Roncevaux Terra at about 50 degrees south latitude.<ref name=basinimg2>"[http://photojournal.jpl.nasa.gov/catalog/PIA08384 PIA08384: The Other Side of Iapetus]." JPL Photojournal, [[Jet Propulsion Laboratory|JPL]], [[National Aeronautics and Space Administration|NASA]], September 10, 2007. Accessed June 16, 2008.</ref> Iapetus has nine such very large impact basins in all.
=== Temperature range ===
[[Image:Iapetus temps.jpg|thumb|150px|left|Iapetus temperature map]]Temperatures on the leading (dark) hemisphere reach 130 K at local noon, because the dark material in the Regio Cassini can absorb more heat. Temperatures do not reach more than 100 K on the trailing hemisphere.<ref name=tempimg/>
== Problems for uniformitarian theories posed by Iapetus ==
The marked inclination of the orbit of Iapetus from the equatorial plane of Saturn defies explanation. According to the [[nebula hypothesis]], all the regular satellites of Saturn ought to remain in or near the same plane. But Iapetus is not likely to be a captured object. Its orbit, though eccentric, is still not consistent with such a model.
The markedly oblate shape of Iapetus, and its equatorial ridge, are inconsistent with its long sidereal day. Astronomers have modeled the possible evolutionary arrest of Iapetus shortly after its formation. But those same astronomers admit that the required sequence of events would have had to be exact, with virtually no tolerance for error.<ref name=Cowen/>
== Observation and Exploration ==
[[Voyager 1]] and [[Voyager 2]] each made rendezvous with Iapetus and took photographs. Those photographs revealed the first details of the contrasting colors of Iapetus's leading and trailing hemispheres.
The [[Cassini Huygens Mission]] Orbiter has made the closest rendezvous with Iapetus. Its camera first revealed the equatorial ridge, and its infrared sensors provided the first insights into the temperature on Iapetus.
No further rendezvous are currently planned.
== In fiction ==
One of the most remarkable appearances of Iapetus (or ''Japetus'' as the author spelled it) was in [[Arthur C. Clarke]]'s [[novel]] ''[[2001: A Space Odyssey]]'', based on the [[motion picture]] of the same name. This novel had as its theme the origins of [[human]] intelligence and a supposed history of contact with [[extraterrestrial life|extraterrestrial scientists]] in [[deep time]].<ref name=2001SO>Clarke, Arthur C. ''2001: A Space Odyssey'', reprint. Penguin Group USA, 2005. ISBN 9780451452733. Paperback.</ref> The author took especial note of Cassini's original astounding finding of radically different albedos on its different hemispheres. In this novel, either Iapetus was an artificial construct, or else extraterrestrial expeditionaries had seared the leading hemisphere and polished the trailing hemisphere to make it brilliant white. In the center of the white hemisphere the expeditionaries built a hyperspatial connecting "gateway" to their own dimension. They completed this project two million years ago and then waited with unfathomable patience for the first human [[astronaut]] to explore Iapetus and pilot a small vessel through their "gate."
The first edition of this novel appeared in the 1970s. The direct observation of Iapetus by Voyagers 1 and 2 rendered this speculation moot.
== Gallery ==
<gallery>
Image:Iapetus Mercator.jpg|Iapetus in Mercator projection
Image:Iapetus ridge overflight.jpg|Iapetus' equatorial ridge at close approach
Image:Iapetus spotted.jpg|Iapetus with impact craters filled with dark material at the edge of Regio Cassini
Image:Iapetus bright.jpg|Roncevaux Terra region of Iapetus, by Cassini
Image:Iapetus dark.jpg|Cassini regio region of Iapetus, by Cassini
</gallery>
== Video gallery ==
* [http://www.godtube.com/view_video.php?viewkey=db6e6784c6f1868e8d60 Iapetus ridge overflight] (<http://godtube.com/>)
== References ==
{{reflist|2}}
{{Solarsystem}}