Triton (moon) 44371 222862580 2008-07-01T13:57:48Z Kheider 2155897 list absmag (H) of -1.2 since this is the only visited (captured) [[plutoid]]. {{otheruses|Triton (disambiguation)}} <!-- Scroll down to edit the contents of this page. Additional parameters for this template are available at [[Template:Infobox Planet]]. --> {{Infobox Planet | name = Triton | image = [[Image:Triton moon mosaic Voyager 2 (large).jpg|300px]] | bgcolour = #a0ffa0 | discovery = yes | discoverer = [[William Lassell]] | discovered = [[October 10]], [[1846]] | semimajor = 354,800 km | eccentricity = 0.000016<ref name="fact">{{cite web | title = Neptunian Satellite Fact Sheet | publisher = NASA | author = David R. Williams | date = [[23 November]] [[2006]] | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html | accessdate = 2008-01-18 }}</ref> | period = −5.877 d<br/>([[Retrograde and direct motion|retrograde]]) | inclination = 130.267° (to the [[ecliptic]])<br/>157.340° (to Neptune's equator)<ref name=inclin>{{cite web | title = Constraints on the Orbital Evolution of Triton | author= Matija Ćuk, Brett J. Gladman | url = http://adsabs.harvard.edu/abs/2005ApJ...626L.113C | year = 2005 | date = June 2005 | journal = The Astrophysical Journal | volume = 626 | issue = 2 | pages = L113-L116 | accessdate = 2008-01-14 }}</ref><br/>130.063° (to Neptune's orbit) | satellite_of = [[Neptune]] | physical_characteristics = yes | mean_radius = 1353.4 ± 0.9 km<ref name="SSD">{{cite web | title = Planetary Satellite Physical Parameters | work = Solar System Dynamics | url = http://ssd.jpl.nasa.gov/?sat_phys_par | accessdate = 2006-05-10 }}</ref> (0.2122 Earths) | surface_area = 23,018,000 km²{{Ref_label|A|a|none}} | volume = 10,384,000,000 km³{{Ref_label|B|b|none}} | mass = 2.14{{e|22}} [[Kilogram|kg]] (0.00359 Earths){{Ref_label|C|c|none}} | density = 2.061 [[Gram|g]]/[[Cubic centimetre|cm³]]<ref name="SSD" /> | surface_grav = 0.779 [[Acceleration|m/s²]]{{Ref_label|D|d|none}} | escape_velocity = 1.455 km/s{{Ref_label|E|e|none}} | sidereal_day = 5 d, 21 h, 2 min, 53s<ref name=book /> | rotation = [[Synchronous rotation|synchronous]] | axial_tilt = 0 | albedo = 0.76<ref name="SSD" /> | magnitude = 13.47<ref name=magnitude>{{cite web | title = Classic Satellites of the Solar System | url = http://www.oarval.org/ClasSaten.htm | publisher = Observatorio ARVAL | accessdate = 2007-09-28 }}</ref> | abs_magnitude = −1.2<ref name=fischer>{{cite web | date = 12.2.2006 | title = Kuiperoids & Scattered Objects | publisher = Argelander-Institut für Astronomie | author = Daniel Fischer | url = http://www.astro.uni-bonn.de/~dfischer/planeten/jwd.html | accessdate = 2008-07-01}}</ref> | single_temperature = 38&nbsp;[[kelvin|K]]<ref name=book /><br/> | adjectives = Tritonian | atmosphere = yes | surface_pressure = 0.0014–9&nbsp;[[Pascal (unit)|kPa]]<ref name=book /><br />(1/70,000th the surface pressure on Earth)<ref name=solarsystemexploration/> | atmosphere_composition = [[nitrogen]]; [[methane]] traces.<ref name=nature2 /> }} '''Triton''' ({{pronEng|ˈtraɪtən}}, or as in Greek ''Τρίτων),''<ref>{{cite web|title=Theogonia Heseodou|work= Neoteron Enkuklopaidikon Lexikon|url=http://www.astro.gr/arxaia-sofia/Hesiod.htm|accessdate=2008-02-19}}</ref> is the largest [[natural satellite|moon]] of the [[planet]] [[Neptune]], discovered on [[October 10]], [[1846]] by [[William Lassell]]. It is the only large moon in the [[Solar System]] with a [[Retrograde and direct motion|retrograde orbit]], which is an orbit in the opposite direction to its planet's rotation. At 2,700&nbsp;km in diameter, it is the [[List of moons by diameter|seventh-largest moon]] in the Solar System. Triton comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and twelve other known moons. It is also more massive than all the Solar System's 159 known smaller moons combined.{{Ref_label|F|f|none}}{{Ref_label|G|g|none}} Because of its [[retrograde]] orbit (unique for an object of its size) and similar composition to [[Pluto]], Triton is thought to have been captured from the [[Kuiper belt]].<ref name="Agnor06"/> Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a substantial core of rock and metal.<ref name=book /> The core makes up two-thirds of its total mass. Triton has a mean density of 2.061&nbsp;g/cm<sup>3</sup><ref name="SSD" /> and is composed of approximately 15–35% water ice.<ref name= book /> Triton is one of the few moons in the Solar System known to be geologically active. Its crust is dotted with geysers believed to erupt [[nitrogen]].<ref name=solarsystemexploration>{{cite web | url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Triton | publisher = [[NASA]] | accessdate = 2007-09-21 | title = Neptune: Moons: Triton }}</ref> As a consequence, its surface is relatively young, with a complex geological history revealed in intricate and mysterious tectonic terrains.<ref name= book /> Triton has a tenuous nitrogen atmosphere less than 1/70,000<sup>th</sup> the pressure of Earth's atmosphere at [[sea level]].<ref name=solarsystemexploration/> ==Discovery and naming== [[Image:William Lassell.jpg|thumb|left|150 px|William Lassell, the discoverer of Triton]] The moon was discovered by [[United Kingdom|British]] [[astronomer]] [[William Lassell]] on [[October 10]] [[1846]],<ref name="LassellDiscovery">{{cite journal | title = Lassell's Satellite of Neptune | author= William Lassell | year = 1847 | date = November 12 1847 | journal = Monthly Notices of the Royal Astronomical Society | volume = 8 | issue = 1 | url = http://adsabs.harvard.edu/abs/1847MNRAS...8....9B <!--- Mistake in Harvard abstracts shows Bond as author and next page, page is actually 8 not 9 and Lassell is the author ---> | pages = 8 }}</ref> just 17&nbsp;days after Neptune itself was discovered by [[Germany|German]] astronomers [[Johann Gottfried Galle]] and [[Heinrich Louis d'Arrest]]. A brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When [[John Herschel]] received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell did so and discovered Triton just eight days later.<ref>{{cite journal | title = Discovery of Supposed Ring and Satellite of Neptune | author = William Lassell | year = 1846 | date = [[November 13]] [[1846]] | journal = Monthly Notices of the Royal Astronomical Society | volume = 7 | issue = 9 | url = http://adsabs.harvard.edu/abs/1846MNRAS...7..157L | pages = 157 }} *{{cite journal | title = Physical observations on Neptune | author = William Lassell | year = 1846 | date = [[December 11]] [[1846]] | journal = Monthly Notices of the Royal Astronomical Society | volume = 7 | issue = 10 | pages = 167&ndash;168 | url = http://adsabs.harvard.edu/abs/1847MNRAS...7..297L }} *{{cite journal | title = Observations of Neptune and his satellite | authorlink1 = William Lassell | year = 1847 | date = 1847 | journal = Monthly Notices of the Royal Astronomical Society | volume = 7 | issue = 17 | pages = 307&ndash;308 | url = http://adsabs.harvard.edu/abs/1847MNRAS...7..307L }}</ref><!--- Vol. 7, No. 16 is June 11, Vol. 8, No. 1 is November 12; Vol. 7, No. 17 was a special issue published "during the vacation" ---><ref name="LassellDiscovery" /> Lassell also claimed to have discovered rings. However, although Neptune does have rings, they are so faint and dark that it is unlikely he actually observed them.<ref name="Smithetal1984">{{cite journal | author= Robert W. Smith, Richard Baum | title = William Lassell and the Ring of Neptune: A Case Study in Instrumental Failure | journal = Journal of History of Astronomy | volume = 15 | issue = 42 | pages = 1–17 | year = 1984 | url = http://adsabs.harvard.edu/abs/1984JHA....15....1S }}</ref> Triton is named after the Greek sea god ''[[Triton (mythology)|Triton]]'' (Τρίτων), the son of [[Poseidon]] (the Greek god comparable to the Roman [[Neptune (mythology)|Neptune]]). The name was first proposed by [[Camille Flammarion]] in his 1880 book ''Astronomie Populaire'',<ref>{{cite web | author = Flammarion, Camille | title = ''Astronomie populaire'', p. 591 | date = 1880 | url = http://gallica.bnf.fr/ark:/12148/bpt6k94887w/f610.table | accessdate = 2007-04-10 }}</ref> although it was not officially adopted until many decades later.<ref>{{cite web | title = Camile Flammarion | work = Hellenica | url = http://www.mlahanas.de/Physics/Bios/CamilleFlammarion.html | accessdate = 2008-01-18 }}</ref> Until the discovery of the second moon [[Nereid (moon)|Nereid]] in 1949, Triton was commonly known as simply "the satellite of Neptune". Lassell did not name his own discovery, although he suggested names a few years later to his subsequent discovery of an eighth moon of [[Saturn]] ([[Hyperion (moon)|Hyperion]]). The third and fourth moons of [[Uranus]] ([[Ariel (moon)|Ariel]] and [[Umbriel (moon)|Umbriel]]), which Lassell discovered in 1851, were named by [[John Herschel]].<ref>{{cite web | title = Planet and Satellite Names and their Discoverers | work = International Astronomical Union | url = http://www.indwes.edu/Faculty/bcupp/solarsys/Names.htm | accessdate = 2008-01-13 }}</ref> ==Orbit and rotation== Triton is unique among all large moons in the Solar System for its [[Retrograde and direct motion|retrograde orbit]] around its planet (''i.e.'', it orbits in a direction opposite to the planet's rotation). Most of the outer [[irregular moon]]s of [[Jupiter]] and [[Saturn]] also have retrograde orbits, as do some of [[Uranus]]' outer moons. However, these moons are all quite small in comparison; the largest of them ([[Phoebe (moon)|Phoebe]]){{Ref_label|H|h|none}} has only 8% of the diameter (and 0.03% of the mass) of Triton. Triton's axis of rotation is also unusual, tilted 157 degrees with respect to Neptune's axis, which is in turn inclined 30 degrees from the plane of Neptune's orbit.<ref name=inclin /> The net result of these two axial tilts is that Triton's rotational axis lies in the plane of Neptune's orbit, and hence during Neptune's year each pole points almost directly toward the Sun, much like Uranus'. As Neptune orbits the Sun, Triton's polar regions take turns facing the sun, probably resulting in radical seasonal changes as one pole then the other moves into the sunlight. The proximity of such a large moon to Neptune has resulted in [[Tidal acceleration#Tidal deceleration|tidal deceleration]], which has altered the moon's orbit and rotation through gravitational interaction. Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. However, [[viscoelasticity|viscoelastic]] damping from tides alone are not believed to be capable of circularizing Triton's orbit in the time since the origin of the system, and [[Drag (physics)|gas drag]] from a prograde debris disc is likely to have played a substantial role.<ref name=inclin /> Tidal interactions have also meant that Triton's already close orbit to Neptune is slowly decaying further, and predictions are that, some 3.6&nbsp;billion years from now, Triton will pass within Neptune's [[Roche limit]].<ref name="Chyba">{{cite journal | author = Christopher F. Chyba, D G Jankowski, P D Nicholson | title = Tidal evolution in the Neptune-Triton system | journal = Astronomy and Astrophysics | year = 1989 | date = [[July]] [[1989]] | volume = 219 | issue = 1–2 | pages = L23–L26 | url = http://adsabs.harvard.edu/abs/1989A&A...219L..23C | accessdate = 2006-05-10 }}</ref> This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a [[planetary ring|ring]] system similar to that found around [[Saturn]].<ref name="Chyba"/> ==Capture== [[Image:Outersolarsystem objectpositions labels comp.png|thumb|250 px|The [[Kuiper belt]], from which Triton is believed to have originated]] Moons in retrograde orbits cannot have formed out of the same region of the [[solar nebula]] as the planets they orbit, but must have been captured from elsewhere. Triton is therefore suspected of being captured from the [[Kuiper belt]].<ref name="Agnor06"/> The Kuiper belt is a ring of small icy objects extending outward from just inside the orbit of Neptune to about 55&nbsp;[[astronomical unit|AU]] from the Sun. Believed to be the point of origin for the majority of short-period [[comet]]s observed from Earth, it is also home to several large, planet-like bodies including [[Pluto]], which is now recognized as the largest in a population of Kuiper belt objects (the [[Plutino]]s) locked in orbital step with Neptune. Triton is only slightly larger than Pluto and nearly identical in composition, which has led to the hypothesis that the two share a common origin.<ref name="Cruikshank2004">{{cite journal | author = Dale P. Cruikshank | title = Triton, Pluto, Centaurs, and Trans-Neptunian Bodies | work = NASA Ames Research Center | url = http://books.google.co.uk/books?id=MbmiTd3x1UcC&pg=PA421&dq=Triton,+Pluto,+Centaurs,+and+Trans-Neptunian+Bodies&lr=&sig=PjnT8J95_lO9r3_3ivBDXm4JK4o | year = 2004 | publication-date = [[January]] [[2005]] | publisher = Springer | isbn = 1402033621 | accessdate = 2008-01-13 }}</ref> The proposed capture of Triton may explain several features of the Neptunian system including the extremely eccentric orbit of Neptune's moon [[Nereid (moon)|Nereid]] and the scarcity of moons as compared to the other gas giants. Triton's initially [[Orbital eccentricity|eccentric]] orbit would have intersected irregular moons and [[Perturbation (astronomy)|disrupted]] those of smaller natural moons, dispersing them through [[gravitation]]al interactions.<ref name=inclin /> The circularization of Triton's eccentric post-capture orbit would have resulted in heating of the moon by [[tidal force]]s. These would have kept Triton liquid for a billion years, which is supported by evidence of differentiation in the moon's interior.<ref name=solarsystemexploration/> There are two ways which Triton's capture may have occurred. In order to be gravitationally captured by a planet, a moon must be slowed down by losing energy. An early theory of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely).<ref name= book /> Another hypothesis suggests that, before its capture, Triton may have had a massive companion similar to Pluto's moon [[Charon (moon)|Charon]] with which it formed a binary. When the binary encountered Neptune, Triton's companion was expelled, providing the required mechanism to capture Triton in an orbit around the planet. This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects.<ref name=binary>{{cite web | title = Binary Kuiper Belt Objects | work=University of Hawaii |author = Dave Jewitt | url = http://www.ifa.hawaii.edu/~jewitt/kb/binaries.html | year = 2005 | accessdate = 2007-06-24 }}</ref> The event was brief but gentle, saving Triton from collisional disruption; and events like this may have been common during the formation of Neptune, or later when it [[Planetary migration|migrated outward]].<ref name="Agnor06">{{cite journal | author = Craig B Agnor, Douglas P Hamilton | title = Neptune's capture of its moon Triton in a binary–planet gravitational encounter | journal = Nature | year = 2006 | date = May 2006 | volume = 441 | issue = 7090 | pages = 192–194 | doi = 10.1038/nature04792 | url = http://www.nature.com/nature/journal/v441/n7090/abs/nature04792.html | accessdate = 2006-05-10 }}</ref> ==Physical characteristics== Triton has a similar size, density {{nowrap|(2.061&nbsp;g/cm³)}}, temperature and chemical composition to that of [[Pluto]].<ref name=voyager>{{cite web | date = [[June 1]], [[2005]] | title = Triton (Voyager) | publisher = NASA (Voyager The Interstellar Mission) | url = http://voyager.jpl.nasa.gov/science/neptune_triton.html | accessdate = 2007-12-09 }}</ref> As with Pluto, 55% of Triton's surface is covered with frozen nitrogen, with [[water]] ice comprising 15&ndash;35% and [[dry ice]] (frozen [[carbon dioxide]]) forming the remaining 10–20%. Traces include 0.1% [[methane]] and 0.05% [[carbon monoxide]] ice.<ref name=book /> There could be [[ammonia]] on the surface that resulted from possible ammonia [[hydrate|dihydrate]] in the [[lithosphere]].<ref name=ammonia>{{cite journal | author= Javier Ruiz | title = Heat flow and depth to a possible internal ocean on Triton | journal = Icarus | volume = 166 | issue = 2 | pages = 436–439 | year = 2003 | date = December 2003 | doi = 10.1016/j.icarus.2003.09.009 | url = http://adsabs.harvard.edu/abs/2003Icar..166..436R | accessdate=2008-01-16 }}</ref> This density means Triton is probably about 30–45% [[ice|water ice]], with the remainder being rocky material.<ref name= book /> Triton's surface area is 23&nbsp;million&nbsp;km², which is 4.5% of [[Earth]], or 15.5% of Earth's land area. Triton is very bright, reflecting 60–95% of the sunlight that reaches it. By comparison, Earth's moon reflects only 11%.<ref>{{cite web | title = Lunar Albedo | author = Jeff Medkeff | work = Sky and Telescope Magazine | year = 2002 | url = http://jeff.medkeff.com/astro/lunar/obs_tech/albedo.htm | accessdate = 2008-02-04 }} </ref> Triton's reddish colour is believed to be the result of methane ice which reduces to carbon under bombardment from [[ultraviolet]] radiation.<ref name= book /> Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like [[Earth]], into a solid [[core (geology)|core]], a [[mantle (geology)|mantle]] and a [[crust (geology)|crust]]. [[Water]], the most abundant [[Volatiles|volatile]] in the Solar System, comprises the moon's mantle, which lies over a core of rock and metal. There is enough rock in Triton's interior for solid-state [[convection]] to be occurring within its mantle, powered by [[radioactive decay]]. The heat may even be sufficient to maintain a "subterranean ocean" similar to that which is hypothesized to exist underneath the surface of [[Europa (moon)|Europa]].<ref name= book/> The possible presence of a layer of liquid water suggests the possibility, if unlikely, of life.<ref>{{cite journal | title = Assessing the Plausibility of Life on Other Worlds | author= Louis Neal Irwin, Dirk Schulze-Makuch | journal = Astrobiology | volume = 1 | issue = 2 | pages = 143&ndash;60 | year = 2001 | date = June 2001 | doi = 10.1089/153110701753198918 | url = http://adsabs.harvard.edu/abs/2001AsBio...1..143I | accessdate = 2008-01-29 }}</ref> ==Atmosphere== {{main|Atmosphere of Triton}} [[Image:Tritoncloud.jpg|thumb|300px|right|A cloud over the limb of Triton]] Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface.<ref name=nature2>{{cite journal | title = Ultraviolet Spectrometer Observations of Neptune and Triton | author = A L Broadfoot, S K Bertaux, J E Dessler et al. | url = http://adsabs.harvard.edu/abs/1989Sci...246.1459B | date = [[December 15]] [[1989]] | journal = Science | issn = 0036-8075 | volume = 246 | pages = 1459–1466 | accessdate = 2008-01-15 | doi = 10.1126/science.246.4936.1459 | pmid = 17756000 }}</ref><ref name=grand>{{cite book | title = The Grand Tour: A Traveler's Guide to the Solar System | author = Ron Miller | authorlink = Ron Miller (artist and author) | coauthors = William K. Hartmann | year = 2005 | month = May | pages = 172&ndash;73 | publisher = Workman Publishing | location = Thailand | edition = 3rd | isbn = 0-7611-3547-2 }}</ref> Like Pluto's atmosphere, the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface.<ref name="Cruikshank2004" /> The surface temperature is at least {{nowrap|35.6 [[kelvin|K]]}} {{nowrap|(−237.6 °C)}} because Triton's [[Nitrogen#Notable characteristics of elemental nitrogen|nitrogen ice]] is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature.<ref name="Duxburyetal1993">{{cite journal | title = The Phase Composition of Triton's Polar Caps | url = http://adsabs.harvard.edu/abs/1993Sci...261..748D | author = N S Duxbury, R H Brown | journal = Science | issn = 0036-8075 | volume = 261 | issue = 5122 | pages = 748–751 | date = August 1993 | accessdate = 2008-01-24 | doi = 10.1126/science.261.5122.748 | pmid = 17757213}}</ref> An upper limit with kelvins in the low 40s can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere.<ref>{{cite journal | title = Spectroscopic Determination of the Phase Composition and Temperature of Nitrogen Ice on Triton | author= Kimberly Tryka, Robert Brown, V. Anicich et al. | journal = Science | issn = 0036-8075 | volume = 261 | issue = 5122 | pages = 751–754 | url = http://adsabs.harvard.edu/abs/1993Sci...261..751T | date = August 1993 | accessdate = 2008-01-24 | doi = 10.1126/science.261.5122.751 | pmid = 17757214 }}</ref> This temperature range is colder than [[Pluto]]'s average equilibrium temperature of {{nowrap|44 K}} {{nowrap|(−229 °C)}}. Triton's surface atmospheric pressure is only about {{nowrap|1.4–1.9}} [[pascal (unit)|pascal]] {{nowrap|(0.014–0.019}} [[Bar (unit)|millibar]]).<ref name=book/> Turbulence at Triton's surface creates a [[troposphere]] (a "weather region") rising to an altitude of 8&nbsp;km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometre in size.<ref name=nature>{{cite journal | title = Voyager 2 at Neptune: Imaging Science Results | author = B A Smith, L A Soderblom et al. | journal = Science | issn = 0036-8075 | volume = 246 | pages = 1422–1449 | date = [[December 15]] [[1989]] | url = http://adsabs.harvard.edu/abs/1989Sci...246.1422S | accessdate = 2008-01-15 | doi = 10.1126/science.246.4936.1422 | pmid = 17755997 }}</ref> Unlike other atmospheres, Triton's has no [[stratosphere]], and instead consists of a [[thermosphere]] from 8 to 950&nbsp;km above the surface, and an exosphere above that.<ref name= book /> The temperature of Triton's upper atmosphere, at 95&nbsp;±&nbsp;5&nbsp;kelvins, is higher than the temperature at the surface due to heat deposited from space.<ref name=nature2 /> A haze permeates most of Triton's troposphere, believed to be composed largely of [[hydrocarbon]]s and [[nitrile]]s created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface.<ref name=book /> In the 1990s, observations from [[Earth]] were made of Triton's [[Limb darkening|limb]] as the moon [[occultation|passed in front of stars]]. These observations indicated the presence of a denser atmosphere than was thought from ''[[Voyager 2]]'' data.<ref name="Hubblesite">{{cite journal | url = http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/23/text/ | title = Hubble Space Telescope Helps Find Evidence that Neptune's Largest Moon Is Warming Up | work = Hubblesite | author = D Savage, D Weaver, D Halber | accessdate = 2007-12-31 | date = [[June 24]] [[1998]] | id = STScI-1998-23 }}</ref> Other observations have shown an increase in temperature by 5% from 1989 to 1998.<ref name="MIT Triton">{{cite web | title = MIT researcher finds evidence of global warming on Neptune's largest moon | url = http://web.mit.edu/newsoffice/1998/triton.html | date = [[1998-06-24]] | publisher = [[Massachusetts Institute of Technology]] | accessdate = 2007-12-31 }}</ref> These observations indicate Triton is approaching an unusually warm summer season that only happens once every few hundred years. Theories for this warming include a change of frost patterns on Triton's surface and a change in ice albedo, which would allow more heat to be absorbed.<ref name="Scienceagogo.com">{{cite journal | title = Solar System Satellites and Summary | url = http://adsabs.harvard.edu/full/2003ASPC..291...93M | date = [[1998-06-28]] | publisher = Space Telescope Science Institute | accessdate = 2008-02-24 | author = Melissa MacGrath }}</ref> Another theory argues the changes in temperature are a result of deposition of dark, red material from geological processes on the moon. Because Triton's [[Bond albedo]] is among the highest within the [[Solar System]], it is sensitive to small variations in spectral [[albedo]].<ref name="Nature">{{cite journal | title = Does global warming make Triton blush? | url = http://www.bio.indiana.edu/~palmerlab/Journals/170.pdf | author = Bonnie J. Buratti, Michael D Hicks, Ray L Newburn Jr. | journal = [[Nature (journal)|Nature]] | volume = 397 | issue = 6716 | year = 1999 | date = [[1999-01-21]] | doi = 10.1038/16615 | format = [[Portable Document Format|PDF]] | accessdate=2007-12-31 | pages = 219 }}</ref> ==Surface features== All detailed knowledge of the surface of Triton was acquired in a single encounter by the ''Voyager 2'' spacecraft in 1989. The 40% of Triton's surface imaged by ''Voyager'' revealed rocky outcrops, canyons and icy melt, mainly frozen [[methane]]. Triton is relatively flat; its observed topography never varies beyond a kilometer.<ref name=book>{{cite book | title = Encyclopedia of the Solar System | chapter = Triton | author = William B. McKinnon, Randolph L Kirk | publisher = Academic Press | year = 2007 | editor = Lucy Ann Adams McFadden, Lucy-Ann Adams, Paul Robert Weissman, Torrence V. Johnson | edition = 2<sup>nd</sup> | publication-place = Amsterdam; Boston | isbn = 0120885891 | pages = 483&ndash;502 }}</ref> There are relatively few [[impact crater]]s on Triton. Recent analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from 50&nbsp;million years old to just 6&nbsp;million years old.<ref name="surface age"/> ===Cryovolcanism=== {{main|Cryovolcano}} [[Image:Tritonfrost.gif|thumb|200 px|Bluish streaks across Triton's surface believed to have been left by eruptions from [[nitrogen]] geysers]] Triton is geologically active; its surface is young and has relatively few impact craters. When the ''Voyager 2'' probe studied Triton, it observed numerous [[cryovolcano|icy volcanoes]] or [[geyser]]s erupting [[liquid nitrogen]], dust, or methane compounds from beneath the surface in plumes up to 8&nbsp;km high.<ref name=voyager/> Although Triton is made of various ices, its subsurface processes are similar to those that produce [[volcano]]es and [[rift valley]]s on Earth, but with water and [[ammonia]] lavas as opposed to liquid rock.<ref name= book /> The volcanic activity is thought to be driven by seasonal heating from the Sun, unlike the [[tidal force|tidal]] heating responsible for the volcanoes of [[Io (moon)|Io]].<ref name=nature /> Solar heat is trapped under surface nitrogen ice, which creates a form of "solid [[greenhouse effect]]", slowly heating the subsurface until nitrogen beneath evaporates and erupts through the crust.<ref name=book /> Between 1977 and the ''Voyager'' flyby in 1989, Triton shifted from a reddish colour, similar to Pluto, to a far paler hue, suggesting that [[cryovolcanism]] in the intervening decade had covered older reddish material with lighter nitrogen frosts.<ref name= book /> Triton's entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy [[volcanism]]. The vast majority of surface features on Triton are [[endogenic]]—the result of internal geological processes rather than external processes such as impacts. Most are volcanic and extrusive in nature, rather than tectonic.<ref name=book /> [[Hili (plume)|Hili]] and [[Mahilani (plume)|Mahilani]] are two candidate [[cryovolcano]]es that have been observed on Triton. (They are named after a [[Zulu mythology|Zulu]] [[Tikoloshe|water sprite]] and a [[Tonga]]n sea spirit, respectively.)<ref>[http://planetarynames.wr.usgs.gov/index.html ''USGS Astrogeology Research Program: Gazetteer of Planetary Nomenclature''], search for "Hili" and "Mahilani"</ref> Triton thus joins the Earth, [[Io (moon)|Io]], and [[Enceladus (moon)|Enceladus]] as one of the few worlds of the Solar System with current known volcanic activity.<ref> {{cite journal | author=JS Kargel | title = Cryovolcanism on the icy satellites | journal = Earth, Moon, and Planet | volume = 67 | year = 1994 | publication-date = 1995 | doi = 10.1007/BF00613296 | pages = 101–113 | url = http://adsabs.harvard.edu/abs/1995EM%26P...67..101K }}</ref> ([[Venus]], [[Mars]], [[Titan (moon)|Titan]], and [[Dione (moon)|Dione]] may also be volcanically active.) The eruption of volatiles from Triton's equator and its deposition at the poles may redistribute enough mass over the course of 10,000&nbsp;years to cause [[polar wander]].<ref>{{cite journal | title = Polar wander on Triton and Pluto due to volatile migration | last = David Parry Rubincam | url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-493GSSP-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=079bb7c13784e9c8df7fe1c4c2452fcc | journal = Icarus | volume = 163 | issue = 2 | pages = 63&ndash;71 | doi = 10.1016/S0019-1035(03)00080-0 | year = 2002 | accessdate = 2008-02-16 }}</ref> ===Polar cap, plains and ridges=== [[Image:Triton (moon).jpg|thumb|200px|Triton's bright red south polar cap]] The southern polar region of Triton is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Little is known about the north pole because it was on the night side during the ''Voyager 2'' encounter. However, it is thought that Triton must also have a north polar cap.<ref name="Duxburyetal1993" /> The high plains found on Triton's eastern hemisphere cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape. The plains are dotted with pits, such as [[Leviathan Patera]], which are probably the vents from which this lava emerged. The composition of the lava is unknown, although a mixture of ammonia and water is suspected.<ref name= book /> Four roughly circular "walled plains" have been identified on Triton. They are the flattest regions so far discovered, with a variance in altitude of less than 200&nbsp;[[Metre|m]]. They are believed to have formed from eruption of icy lava.<ref name=book /> The plains near Triton's eastern limb are dotted with black points, the ''[[Macula (planetary geology)|maculae]]''. Each of the maculae comprises a dark central patch surrounded by a white halo of material. They all have similar diameters of between 20 and 30&nbsp;km. Some speculate the maculae are outliers of the southern polar cap, which is in retreat in summer.<ref name=book /> There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles.<ref>{{cite journal | journal = Nature | volume = 393 | pages = 765–67 | year = 1998 | doi = 10.1038/31651 | title = Global warming on Triton | author = JL Elliot, HB Hammel, LH Wasserman, et al. }}</ref> Many also appear to be tectonic in nature and may result from extension or [[strike-slip fault]]ing.<ref name=linea>{{cite journal | title = Triton's Lineaments: Complex Morphology and Stress Patterns | author = Geoffrey Collins, Paul Schenk | location = Houston, TX | work = Abstracts of the 25th Lunar and Planetary Science Conference | date = [[March 14]] – [[March 18|18]], [[1994]] | url = http://adsabs.harvard.edu/full/1994LPI....25..277C | accessdate = 2008-01-25 | pages = 277 }}</ref> Some bear a strong resemblance to ridges on Europa, and may have a similar origin.<ref name=book /> In the equatorial region, long faults with parallel mountain ranges of ice expelled from the interior cross complex terrain with valleys. These ridges, or ''sulci,'' such as [[Yasu Sulci]], [[Ho Sulci]], and [[Lo Sulci]],<ref>{{cite journal | title = Working Group for Planetary System Nomenclature | author = K Aksnes, A Brahic, M Fulchignoni, M Ya Marov | location = Stony Brook, NY | journal = Reports on Astronomy | volume = 21A | pages = 613–19 | place = State University of New York | id = 1991IAUTA..21..613A | url = http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940014368_1994014368.pdf | format = PDF | date = 1990 | publication-date = 1991 | accessdate = 2008-01-25 }}</ref> are believed to be of intermediate age in Triton's geological history, and in many cases to have formed concurrently. They tend to be clustered in groups or "packets".<ref name=linea /> ===Cantaloupe terrain=== [[Image:PIA01537 modest.jpg|thumb|200px|The cantaloupe-skin terrain as seen from 130,000&nbsp;km by ''[[Voyager 2]]'']] Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because of its resemblance to the skin of a [[cantaloupe]] melon. Although it has few craters, it is believed that this is the oldest terrain on Triton.<ref name=cantaloupe/> It probably covers much of the western half of the moon.<ref name= book /> The cantaloupe terrain, which is mostly dirty frozen water, is known to exist only on Triton. It contains depressions {{nowrap|30–40 km}} in diameter.<ref name=cantaloupe/> The depressions (''cavi'') are probably not impact craters by meteorites because they are all of similar size and have smooth curves. The leading hypothesis as to their formation is [[diapir]]ism, the rising of "lumps" of less dense material through a stratum of denser material.<ref name= book /> Other hypotheses of the region's formation include that it formed by collapses, or by flooding caused by [[cryovolcanism]].<ref name=cantaloupe>{{cite journal | author = Joseph M. Boyce | title = A structural origin for the cantaloupe terrain of Triton | journal = In Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F (SEE N94-12015 01-91) | volume = 24 | pages = 165–66 | date = March 1993 | url = http://adsabs.harvard.edu/abs/1993LPI....24..165B | accessdate=2008-01-15 }}</ref> ===Impact craters=== [[Image:PIA01538.jpg|thumb|200px|The few craters that exist on Triton reveal intense geologic activity]] Due to constant erasure and modification by ongoing geological activity, [[impact crater]]s on Triton's surface are relatively rare. A census of Triton's craters imaged by ''Voyager 2'' found only 179 that were incontestably of impact origin, compared with 835 observed for [[Uranus|Uranus']] moon [[Miranda (moon)|Miranda]], which has only three percent of Triton's [[surface area]].<ref name=impact>{{cite journal | title=The Impact Cratering Record on Triton | author=Strom, Robert G.; Croft, Steven K.; Boyce, Joseph M. | doi=10.1126/science.250.4979.437 | year=1990 | journal=[[Science (journal)|Science]] | volume=250 | pages=437–39 | accessdate = | pmid=17793023 }}</ref> The largest crater observed on Triton believed to have been created by an impact is a 27&nbsp;km-diameter feature called [[Mazomba (crater)|Mazomba]].<ref name=impact/><ref>{{cite journal | title = Triton's Plumes: The Dust Devil Hypothesis | author = Ingersoll, Andrew P.; Tryka, Kimberly A. | journal = Science| volume = 250 | pages = 435–437 | doi = 10.1126/science.250.4979.435 | year = 1990 | accessdate = | pmid = 17793022 }}</ref> Although larger craters have been observed, they are generally believed to be volcanic in nature.<ref name=impact /> The few impact craters on Triton are almost all concentrated in the leading hemisphere&ndash;that facing the direction of the orbital motion—with the majority concentrated around the equator between 30° and 70° longitude,<ref name=impact /> resulting from material swept up from orbit around Neptune.<ref name="surface age">{{cite journal | journal = Icarus | volume = 192 | issue = 1 | date = December 2007 | pages = 135–49 | doi = 10.1016/j.icarus.2007.07.004 | title = On the negligible surface age of Triton | author = Schenk, Paul M.; Zahnle, Kevin | url = | accessdate = }}</ref> Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, as impacts on the leading hemisphere would be more frequent and more violent.<ref name=impact /> However, as ''Voyager'' only imaged 40% of Triton's surface, this remains uncertain. ==Observation== [[Image:Voyager 2 Neptune and Triton.jpg|thumb|200px|Neptune (top) and Triton (bottom) three days after the flyby of ''Voyager 2'']] The orbital properties of Triton had been defined with high accuracy in the 19th century, but little was known about the satellite itself until ''[[Voyager 2]]'' arrived at the end of the 20th century. The first detailed observations of Triton were not made until 1930. It was found to have a retrograde orbit, at a very high angle of inclination to the plane of Neptune's orbit.<ref name= book /> Before the arrival of ''Voyager 2'', astronomers suspected that Triton might have [[liquid nitrogen]] seas and a nitrogen/methane atmosphere with a density as much as 30% that of the Earth. Like the famous overestimates of the atmospheric density of Mars, this was completely false. As with Mars, a denser early atmosphere is postulated for the body's early history.<ref>{{cite journal | doi = 10.1016/0019-1035(92)90031-2 | title = A massive early atmosphere on Triton | author = Jonathan I. Lunine, Michael C. Nolan | journal = Icarus | issn = 0019-1035 | volume = 100 | issue = 1 | year = 1992 | date = November 1992 | pages = 221–34 | url = http://adsabs.harvard.edu/abs/1992Icar..100..221L | accessdate = 2008-02-24 }}</ref> The first attempt to measure the diameter of Triton was made by [[Gerard Kuiper]] in 1954. He obtained a value of 3,800&nbsp;km. Subsequent measurement attempts arrived at values ranging from 2,500 to 6,000&nbsp;km, or slightly smaller than our Moon, to nearly half the diameter of Earth.<ref>{{cite journal | journal = Icarus | volume = 40 | date = October 1979 | pages = 104–14 | doi = 10.1016/0019-1035(79)90057-5 | title = The diameter and reflectance of Triton | author = DP Cruikshank, A Stockton, HM Dyck, EE Becklin, W Macy | url = http://adsabs.harvard.edu/abs/1979Icar...40..104C | accessdate = 2008-02-24 }}</ref> Data from the approach of ''Voyager 2'' to Neptune on [[August 25]] [[1989]] led to a more accurate estimate of Triton's diameter (2,706&nbsp;km).<ref>{{cite journal | title = The Voyager 2 Encounter with the Neptunian System | author = EC Stone, ED Miner | journal = Science | issn = 0036-8075 | volume = 246 | year = 1989 | date = [[December 15]] [[1989]] | pages = 1417–21 | url = http://adsabs.harvard.edu/abs/1989Sci...246.1417S | accessdate = 2008-02-24 | doi = 10.1126/science.246.4936.1417 | pmid = 17755996 }} And the following 12 articles pp. 1422–1501.</ref> In the 1990s, various observations from Earth were made of the limb of Triton using the occultation of nearby stars, which indicated the presence of an atmosphere and an exotic surface. The observations suggest that the atmosphere is denser than the Voyager 2 measurements had indicated.<ref name="Hubblesite" /> ==See also== {{portal|Solar System|Solar system.jpg}} * [[List of geological features on Triton]] * [[Neptune in fiction#Neptune's moons in fiction|Neptune's moons in fiction]] ==Notes== <div class="references-small"> <ol type="a"> <li>{{Note_label|A|a|none}} Surface area derived from the radius ''r'': ''4*pi*r<sup>2</sup>''. <li>{{Note_label|B|b|none}} Volume ''v'' derived from the radius ''r'': ''4/3*pi*r<sup>3</sup>''. <li>{{Note_label|C|c|none}} Mass ''m'' derived from the density ''d'' and the volume ''v'': ''m=d*v''. <li>{{Note_label|D|d|none}} Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''g'' and the radius ''r'': ''g*m/r<sup>2</sup> ''. <li>{{Note_label|E|e|none}} Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''g'' and the radius ''r'': ''sqrt((2*g*m)/r).</li> <li>{{Note_label|F|f|none}} Mass of Triton: 2.14{{e|22}} kg. Combined mass of 12 other known moons of Neptune: 7.53{{e|19}} kg, or 0.35 percent. The mass of the rings is negligible.</li> <li>{{Note_label|G|g|none}} Triton Mass:&nbsp;2.1{{e|22}}<br /> [[Titania (moon)|Titania]]&nbsp;3.5{{e|21}} [[Rhea (moon)|Rhea]]&nbsp;2.3{{e|21}} [[Oberon (moon)|Oberon]]&nbsp;3.0{{e|21}} [[Iapetus (moon)|Iapetus]]&nbsp;1.8{{e|21}} [[Charon (moon)|Charon]]&nbsp;1.5{{e|21}} [[Umbriel (moon)|Umbriel]]&nbsp;1.2{{e|21}} [[Ariel (moon)|Ariel]]&nbsp;1.3{{e|21}} [[Dione (moon)|Dione]]&nbsp;1.0{{e|21}} [[Tethys (moon)|Tethys]]&nbsp;0.6{{e|21}} [[Enceladus (moon)|Enceladus]]&nbsp;0.1{{e|21}} [[Miranda (moon)|Miranda]]&nbsp;0.06{{e|21}} [[Proteus (moon)|Proteus]]&nbsp;0.05{{e|21}} [[Mimas (moon)|Mimas]]&nbsp;0.04{{e|21}}<br /> Sum of remaining moons (est.)&nbsp;0.08616{{e|21}}<br /> Sum of mass:&nbsp;16.53{{e|21}} = 1.7{{e|22}}<br /> See: [[List of moons by diameter]]</li> <li>{{Note_label|H|h|none}} Largest [[irregular moon]]s: Saturn's [[Phoebe (moon)|Phoebe]] (210km), Uranus' [[Sycorax (moon)|Sycorax]] (150km), and Jupiter's [[Himalia (moon)|Himalia]] (85km)</li> </ol> </div> ==References== {{reflist|3}} ==External links== {{commonscat}} * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Triton Triton Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration] {{Moons of Neptune|state=uncollapsed}} {{Neptune}} {{Solar System moons (compact)}} <!--Interwiki--> {{featured article}} <!--Categories--> [[Category:Neptune's moons]] {{Link FA|pt}} <!--Other languages--> [[frp:Triton (satèlite)]] [[bg:Тритон (спътник)]] [[ca:Tritó (satèl·lit)]] [[cs:Triton (měsíc)]] [[co:Tritone]] [[cy:Triton (lloeren)]] [[da:Triton (måne)]] [[de:Triton (Mond)]] [[et:Triton (kuu)]] [[es:Tritón (luna)]] [[eo:Tritono]] [[eu:Triton]] [[fr:Triton (lune)]] [[zh-classical:海衛一]] [[ko:트리톤 (위성)]] [[hr:Triton (mjesec)]] [[io:Triton]] [[it:Tritone (astronomia)]] [[he:טריטון (ירח)]] [[ht:Triton]] [[la:Triton (satelles)]] [[lv:Tritons (pavadonis)]] [[lt:Tritonas (palydovas)]] [[hu:Triton (hold)]] [[nl:Triton (maan)]] [[ja:トリトン (衛星)]] [[no:Triton (måne)]] [[nn:Neptunmånen Triton]] [[pl:Tryton (księżyc)]] [[pt:Tritão (satélite)]] [[ro:Triton (satelit)]] [[ru:Тритон (спутник)]] [[simple:Triton (moon)]] [[sk:Triton (mesiac)]] [[sl:Triton (luna)]] [[fi:Triton (kuu)]] [[sv:Triton (måne)]] [[th:ไทรทัน]] [[vi:Triton (vệ tinh)]] [[tr:Triton (uydu)]] [[uk:Тритон (супутник)]] [[zh:海卫一]]