View source for Triton (moon)

{{Short description|Largest moon of Neptune}}
{{Distinguish|Titan (moon)}}
{{Featured article}}
{{Use mdy dates|date=August 2024}}
{{Use American English|date=October 2022}}
{{Infobox planet
| name = Triton
| note = yes
| mpc_name = Neptune I
| pronounced = {{IPAc-en|ˈ|t|r|aɪ|t|ən}} ({{respell|TRY|tən}})
| adjectives = Tritonian<!--also used for Lake Triton, which per Lewis & Short is the same root--> ({{IPAc-en|t|r|aɪ|ˈ|t|oʊ|n|i|ə|n}})<ref>Robert Graves (1945) ''Hercules, My Shipmate''</ref>
| named_after = [[Triton (mythology)|Τρίτων]] ''Trītōn''
| image = Triton-usgs23-with-frame.jpg
| caption = A black-and-white mosaic of Triton, constructed from ''Voyager 2'' imagery. Triton's massive south polar cap dominates most of the image, with cryovolcanic features such as [[Leviathan Patera]] located left of center
| background = Thistle
| discoverer = [[William Lassell]]
| discovered = October 10, 1846
| semimajor = {{val|354759|u=km|fmt=commas}}
| eccentricity = {{val|0.000016|fmt=none}}<ref name="neptuniansatfact"/>
| period = {{val|5.876854|u=d|fmt=none}}<br/>([[Retrograde and direct motion|retrograde]])<ref name="neptuniansatfact"/><ref name="NYT-20141105-DO"/>
| avg_speed = 4.39 km/s{{efn|name=calculated|Calculated on the basis of other parameters.}}
| inclination = 129.812° (to the [[ecliptic]])<br/>156.885° (to Neptune's equator)<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/><br/>129.608° (to Neptune's orbit)
| satellite_of = [[Neptune]]
| mean_radius = {{val|1353.4|0.9|u=km|fmt=commas}}<ref name="JPL-SSD-sat_phys"/> ({{Earth radius|0.2122}})
| surface_area = {{val|23018000|u=km2|fmt=commas}}<ref name="nSurfaceArea" group="lower-alpha"/>
| volume = {{val|10384000000|u=km3|fmt=commas}}<ref name="nVolume" group="lower-alpha"/>
| mass = {{val|2.1389|0.0028|e=22|u=kg}}<br/>({{val|0.00358|u=Earths|fmt=none}})<ref name="Jacobson2009-AJ"/>
| density = {{val|2.061|u=g/cm3}}<ref name="JPL-SSD-sat_phys"/>
| surface_grav = {{val|0.779|ul=m/s2}} ({{val|0.0794|u=[[G-force|''g'']]}}) (0.48 Moons)<ref name="nSurfaceGravity" group="lower-alpha"/>
| escape_velocity = {{val|1.455|u=km/s}}<ref name="nEscapeVelocity" group="lower-alpha"/>
| sidereal_day = 5 d, 21 h, 2&nbsp;min, 53 s<ref name="EncycSolSys-Triton"/>
| rotation = [[Synchronous rotation|synchronous]]
| axial_tilt = 0° (to orbit about Neptune)<ref name="AxialTilt" group="lower-alpha"/>
| albedo = 0.76<ref name="JPL-SSD-sat_phys"/>
| magnitude = 13.47<ref name="magnitude"/>
| abs_magnitude = −1.2<ref name="Fischer2006"/>
| single_temperature = {{cvt|38|K|C}}<ref name="EncycSolSys-Triton"/>
| atmosphere = yes
| surface_pressure = {{convert|1.4|Pa|atm|sigfig=3|abbr=on}} (1989)<ref name="EncycSolSys-Triton"/><br/>{{convert|1.9|Pa|atm|sigfig=3|abbr=on}} (1997)<ref name="Elliot2000"/><br/>{{convert|1.454|Pa|atm|sigfig=3|abbr=on}} (2022)<ref name="Sicardy2024"/>
| atmosphere_composition = [[nitrogen]]; [[methane]] and [[carbon monoxide]] traces<ref name="Lellouch2010"/>
}}

'''Triton''' is the largest [[natural satellite]] of the [[planet]] [[Neptune]]. It is the only moon of Neptune massive enough to be [[list of gravitationally rounded objects of the Solar System|rounded under its own gravity]] and hosts a [[atmosphere of Triton|thin, hazy atmosphere]]. Triton orbits Neptune in a [[retrograde orbit]]—revolving in the opposite direction to the parent planet's rotation—the only large moon in the [[Solar System]] to do so.<ref name="NYT-20141105-DO"/><ref name="NYT-20141018-KC">{{cite news |last=Chang |first=Kenneth |title=Dark Spots in Our Knowledge of Neptune |url=https://www.nytimes.com/2014/08/19/science/dark-spots-in-our-knowledge-of-neptune.html |date=October 18, 2014 |work= New York Times |access-date=October 21, 2014 }}</ref> Triton is thought to have once been a [[dwarf planet]] from the [[Kuiper belt]], [[gravitational capture|capture]]d into Neptune's orbit by the latter's [[gravity]].<ref name="Agnor06"/>

At {{convert|2710|km|sp=us}}<ref name="JPL-SSD-sat_phys"/> in diameter, Triton is the [[list of natural satellites#List|seventh-largest moon]] in the Solar System, the second-largest planetary moon in relation to its primary (after Earth's [[Moon]]), and larger than all of the known [[dwarf planet]]s. The mean [[density]] is {{val|2.061|u=g|up=cm3}},<ref name="JPL-SSD-sat_phys"/> reflecting a composition of approximately 30–45% [[ice|water ice]] by [[mass]],<ref name="EncycSolSys-Triton"/>{{rp|866}} with the rest being mostly rock and metal. Triton is differentiated, with a [[crust (geology)|crust]] of primarily ice atop a probable [[planetary oceanography|subsurface ocean]] of liquid water and a solid rocky-metallic [[planetary core|core]] at its center. Although Triton's orbit is nearly circular with a very low [[orbital eccentricity]] of {{val|0.000016|fmt=none}},<ref name="neptuniansatfact"/> its interior may still experience [[tidal heating]] through [[obliquity]] tides.

Triton is one of the most geologically active worlds in the Solar System, with an estimated average surface age of less than 100 million years old. Its surface is covered by [[solid nitrogen|frozen nitrogen]] and is geologically young, with very few [[impact crater]]s. Young, intricate [[cryovolcanic]] and [[tectonic]] terrains suggest a complex geological history. The atmosphere of Triton is composed primarily of nitrogen, with minor components of [[methane]] and [[carbon monoxide]]. Triton's atmosphere is relatively thin and strongly variable, with its atmospheric surface pressure varying by up to a factor of three within the past 30 years. Triton's atmosphere supports clouds of nitrogen ice crystals and a layer of organic atmospheric [[haze]]. 

Triton was the first [[Neptunian moon]] to be discovered, on October 10, 1846, by English astronomer [[William Lassell]]. The 1989 [[flyby (spaceflight)|flyby]] of Triton by the ''[[Voyager 2]]'' [[spacecraft]] remains the only up-close visit to the moon as of 2025. As the [[space probe|probe]] was able to study only about 40% of the moon's surface, multiple concept missions have been developed to revisit Triton. These include a [[Discovery program|Discovery]]-class [[Trident (spacecraft)|''Trident'']] and [[New Frontiers program|New Frontiers]]-class ''Triton Ocean Worlds Surveyor'' and ''Nautilus''.<ref>{{Cite web |title=NASA Selects Four Possible Missions to Study the Secrets of the Solar System |url=https://www.jpl.nasa.gov/news/nasa-selects-four-possible-missions-to-study-the-secrets-of-the-solar-system |access-date=January 16, 2023 |website=NASA Jet Propulsion Laboratory (JPL)}}</ref><ref name="Jason Dekarske 2023"/>

== Discovery and naming ==
[[File:William Lassell.jpg|thumb|left|upright|William Lassell, the discoverer of Triton]]
Triton was discovered by British astronomer [[William Lassell]] on October 10, 1846,<ref name="LassellDiscovery"/> just 17&nbsp;days after the [[discovery of Neptune]]. When [[John Herschel]] received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell discovered Triton eight days later.<ref name="LassellDiscovery"/><ref name="Lassell refs"/><!-- 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" --> Lassell also claimed for a period{{efn|Lassell rejected his previous claim of discovery when he found that the orientation of the supposed rings changed when he rotated his telescope tube; see p. 9 of Smith & Baum, 1984.<ref name="Smithetal1984"/>}} to have discovered rings.<ref name="Smithetal1984"/> Although Neptune was later confirmed to [[Rings of Neptune|have rings]], they are so faint and dark that it is not plausible he saw them. A brewer by trade, Lassell spotted Triton with his self-built {{cvt|61|cm}} [[aperture]] metal mirror reflecting [[telescope]] (also known as the "two-foot" reflector).<ref name=":0">{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=1046|title=The Royal Observatory Greenwich – where east meets west: Telescope: The Lassell 2-foot Reflector (1847)|website=www.royalobservatorygreenwich.org|access-date=November 28, 2019}}</ref> This telescope was donated to the [[Royal Observatory, Greenwich]] in the 1880s, but was eventually dismantled.<ref name=":0"/>

Triton is named after the Greek sea god [[Triton (mythology)|Triton]] (Τρίτων), the son of [[Poseidon]] (the Greek god corresponding to the Roman [[Neptune (mythology)|Neptune]]). The name was first proposed by [[Camille Flammarion]] in his 1880 book ''Astronomie Populaire'',<ref name="Flammarion1880"/> and was officially adopted many decades later.<ref name="PMoore"/> Until the discovery of the second moon [[Nereid (moon)|Nereid]] in 1949, Triton was commonly referred to as "the satellite of Neptune". Lassell did not name his discovery; he later successfully suggested the name [[Hyperion (moon)|Hyperion]] for the eighth moon of [[Saturn]] when he discovered it.<ref name="IAU-solarsysNames"/>

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 [[tau]] (the initial of Triton) combined with Neptune's trident as the symbol of Triton ([[File:Triton 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 and rotation ==
[[File:Triton orbit & Neptune.png|thumb|The orbit of Triton (red) is [[Retrograde and direct motion|opposite in direction]] and [[orbital inclination|tilted −23°]] compared to a typical [[Moons of Neptune|moon]]'s orbit (green) in the plane of Neptune's equator.]]
Triton is unique among all large moons in the Solar System for its [[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 the irregular moons of [[Uranus]] and Neptune. However, these moons are all much more distant from their primaries and are small in comparison, with the largest of them ([[Phoebe (moon)|Phoebe]])<ref name="nLargest" group="lower-alpha"/> having only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton's orbit is associated with two tilts, the [[obliquity]] of Neptune's rotation to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's rotation, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's rotation with a period of about 678 Earth years (4.1 Neptunian years),<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/> making its Neptune-orbit-relative inclination vary between 127° and 173°. That inclination is currently 130°; Triton's orbit is now near its maximum departure from coplanarity with Neptune's.

Triton's rotation is [[tidally locked]] to be synchronous with its orbit around Neptune: it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane.<ref name="Davies1991-ControlNetwork"/> At present, Triton's rotational axis is about 40° from Neptune's [[orbital plane]], hence as Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other moves into the sunlight. Such changes were observed in 2010.<ref name="SpaceCom-TritonSeasons"/>

Triton's revolution around Neptune has become a nearly perfect circle with an [[Orbital eccentricity|eccentricity]] of almost zero. [[Viscoelastic]] damping from tides alone is not thought to be capable of [[tidal circularization|circularizing]] Triton's orbit in the time since the origin of the system, and [[Drag (physics)|gas drag]] from a [[Retrograde and prograde motion|prograde]] debris disc is likely to have played a substantial role.<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/> [[Tidal deceleration|Tidal interactions]] also cause Triton's orbit, which is already closer to Neptune than the [[Moon]] is to Earth, to gradually decay further; predictions are that 3.6&nbsp;billion years from now, Triton will pass within Neptune's [[Roche limit]].<ref name="Chyba"/> This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new [[planetary ring|ring]] system similar to that found around [[Saturn]].<ref name="Chyba"/>

== Capture ==
{{main|Capture of Triton}}
[[File:Outersolarsystem objectpositions labels comp.png|thumb|left|The [[Kuiper belt]] (green), in the Solar System's outskirts, is where Triton is thought to have originated.]]

The current understanding of moons in retrograde orbits means they cannot form in the same region of the [[solar nebula]] as the planets they orbit.  Therefore, Triton must have been captured from elsewhere in the Solar System. Astrophysicists believe it might have originated in the [[Kuiper belt]],<ref name="Agnor06"/> a ring of small icy objects extending from just inside the orbit of Neptune to about 50&nbsp;[[astronomical unit|AU]] from the Sun. Thought to be the point of origin for the majority of short-period [[comet]]s observed from Earth, the belt 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) [[orbital resonance#Pluto resonances|locked in resonant orbits]] with Neptune. Triton is only slightly larger than Pluto and is nearly identical in composition, which has led to the hypothesis that the two share a common origin.<ref name="Cruikshank2004"/>

This has been further supported in a 2024 study of the chemical composition of Pluto and Triton which suggests they originated in the same region of the outer Solar System before the latter was pulled into Neptune's orbit.<ref name="ns-20240618" />

Studying prior data on the two bodies, the team found that both have a large amount of nitrogen and trace amounts of methane and carbon monoxide, which could have accumulated in the outer regions of the young nebula "For some reason, Triton was then ejected from this region and ensnared by Neptune". "They had to have formed beyond the water-ice line," says Mandt, referring to the distance from the sun where water would freeze into ice or snow, which is why Triton and Pluto have similar amounts of certain key elements. "One possibility is that the giant planets moved closer to the sun early in the first 100 million years or so of the Solar System, which may have disrupted the orbits of some bodies like Triton", says Mandt.<ref name="ns-20240618">{{Cite news |last=O'Callaghan |first=Jonathan |date=June 18, 2024 |title=Pluto and the largest moon of Neptune might be siblings |url=https://www.newscientist.com/article/2436032-pluto-and-the-largest-moon-of-neptune-might-be-siblings/ |access-date=June 27, 2024 |work=[[New Scientist]] |url-access=subscription}}</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 [[giant planet]]s. Triton's initially eccentric orbit would have intersected the orbits of irregular moons and [[Perturbation (astronomy)|disrupted]] those of smaller regular moons, dispersing them through [[gravitation]]al interactions.<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/>

Triton's eccentric post-capture orbit would have also resulted in [[tidal heating]] of its interior, which could have kept Triton fluid for a billion years; this inference is supported by evidence of differentiation in Triton's interior. This source of internal heat disappeared following tidal locking and circularization of the orbit.<ref name="Ross1990"/>

Two types of mechanisms have been proposed for Triton's capture. To be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape.<ref name="EncycSolSys-Triton"/> An early model 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="EncycSolSys-Triton"/> A more recent hypothesis suggests that, before its capture, Triton was part of a binary system. When this binary encountered Neptune, it interacted in such a way that the binary dissociated, with one portion of the binary expelled, and the other, Triton, becoming bound to Neptune. This event is more likely for more massive companions.<ref name="Agnor06"/> This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects.<ref name="IOPorg-KuiperObjectBinaries"/><ref name="Jewitt2005"/> The event was brief but gentle, saving Triton from collisional disruption. Events like this may have been common during the formation of Neptune, or later when it [[Planetary migration|migrated outward]].<ref name="Agnor06"/>

However, simulations in 2017 showed that after Triton's capture, and before its orbital eccentricity decreased, it probably did collide with at least one other moon, and caused collisions between other moons.<ref>{{cite journal|last1=Raluca Rufu and [[Robin Canup]]|title=Triton's evolution with a primordial Neptunian satellite system|journal=The Astronomical Journal|volume=154|issue=5|page=208|arxiv=1711.01581|date=November 5, 2017|doi=10.3847/1538-3881/aa9184|pmid=31019331|pmc=6476549|bibcode=2017AJ....154..208R |doi-access=free }}</ref><ref>{{cite journal|title=Triton crashed into Neptune's moons|journal=New Scientist|date=November 18, 2017|volume=236|issue=3152|page=16|doi=10.1016/S0262-4079(17)32247-9|bibcode=2017NewSc.236...16.|url=https://www.newscientist.com/article/mg23631521-900-neptunes-other-moons-were-normal-until-triton-crashed-the-party|url-access=subscription}}</ref>

== Physical characteristics ==
{{multiple image
 |direction = vertical
 |align = right
 |width = 238
 |image1=Masa_de_triton.svg
 |image2=Triton, Earth & Moon size comparison.jpg
 |caption1=Triton dominates the Neptunian moon system, with over 99.5% of its total mass. This imbalance may reflect the elimination of many of Neptune's original satellites following Triton's capture.<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/>
 |caption2=Triton (''lower left'') compared to the Moon (''upper left'') and Earth (''right''), to scale
}}

Triton is the seventh-largest moon and [[List of Solar System objects by size|sixteenth-largest object]] in the Solar System and is modestly larger than the [[dwarf planet]]s [[Pluto]] and [[Eris (dwarf planet)|Eris]]. It is the largest known object believed to have originated in the [[Kuiper Belt]]. It is also the largest retrograde moon in the Solar System. It accounts for more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and fifteen other known moons,<ref name="nMassTriton" group="lower-alpha"/> and is also more massive than all known moons in the Solar System smaller than itself combined.<ref name="nMassOthers" group="lower-alpha"/> Also, with a diameter 5.5% that of Neptune, it is the largest moon of a gas giant relative to its planet in terms of diameter, although Titan is bigger relative to Saturn in terms of mass (the ratio of Triton's mass to that of Neptune is approximately 1:4788). It has a radius, density (2.061 g/cm<sup>3</sup>), temperature, and chemical composition similar to that of [[Pluto]].<ref name="voyager"/>

Triton's surface is covered with a transparent layer of [[annealing (metallurgy)|annealed]] [[solid nitrogen|frozen nitrogen]]. Only 40% of Triton's surface has been observed and studied, but it may be entirely covered in such a thin sheet of nitrogen ice. Triton's surface consists of 55% nitrogen ice with other ices mixed in. [[Water]] ice comprises 15–35% and frozen [[carbon dioxide]] ([[dry ice]]) the remaining 10–20%. Trace ices include 0.1% [[methane]] and 0.05% [[carbon monoxide]].<ref name="EncycSolSys-Triton"/>{{rp|868}} There could also be [[ammonia]] ice on the surface, as there are indications of ammonia [[dihydrate]] in the [[lithosphere]].<ref name="ammonia"/> Triton's mean density implies that it probably consists of about 30–45% [[ice|water ice]] (including relatively small amounts of volatile ices), with the remainder being rocky material.<ref name="EncycSolSys-Triton"/> Triton's surface area is 23&nbsp;million&nbsp;km<sup>2</sup>, which is 4.5% of [[Earth]], or 15.5% of Earth's land area. Triton has an unusually high [[albedo]], reflecting 60–95% of the sunlight that reaches it, and it has changed only slightly since the first observations. By comparison, the Moon reflects only 11%.<ref name="Medkeff2002-LunarAlbedo"/> This high albedo causes Triton to reflect a lot of whatever little sunlight there is instead of absorbing it,<ref>{{cite web | url=https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/12%3A_Rings_Moons_and_Pluto/12.03%3A_Titan_and_Triton | title=12.3: Titan and Triton | date=October 7, 2016 }}</ref><ref>{{cite web | url=https://sos.noaa.gov/catalog/datasets/triton-neptunes-moon/ | title=Triton: Neptune's Moon | date=January 2010 }}</ref> causing it to have the coldest recorded temperature in the Solar System at {{cvt|38|K|C|0}}.<ref>{{cite web | url=https://science.nasa.gov/neptune/moons/triton/#hds-sidebar-nav-1 | title=Triton – NASA Science | access-date=January 7, 2024 | archive-date=January 7, 2024 | archive-url=https://web.archive.org/web/20240107182347/https://science.nasa.gov/neptune/moons/triton/#hds-sidebar-nav-1 }}</ref><ref>{{cite journal |author=Nelson, R.M. |display-authors=4 |author2=Smythe, W.D. |author3=Wallis, B.D. |author4=Horn, L.J. |author5=Lane, A.L. |author6=Mayo, M.J. |title=Temperature and Thermal Emissivity of the Surface of Neptune's Satellite Triton |journal=Science |date=1990 |volume=250 |issue=4979 |pages=429–431 |doi=10.1126/science.250.4979.429 |pmid=17793020 |bibcode=1990Sci...250..429N|s2cid=20022185 }}</ref> Triton's reddish color is thought to be the result of methane ice, which is converted to [[tholin]]s under exposure to [[ultraviolet]] radiation.<ref name="EncycSolSys-Triton"/><ref name="Grundy"/>

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 [[Volatile (astrogeology)|volatile]] in the Solar System, comprises Triton's mantle, enveloping a core of rock and metal. There is enough rock in Triton's interior for [[radioactive decay]] to maintain a liquid [[subsurface ocean]] to this day, similar to what is thought to exist beneath the surface of [[Europa (moon)|Europa]] and several other icy outer Solar System worlds.<ref name="EncycSolSys-Triton"/><ref name="Hussman2006"/><ref name='Sci Am 2017'>{{cite web |url=https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ |title=Overlooked Ocean Worlds Fill the Outer Solar System |first=John |last=Wenz |work=Scientific American |date=October 4, 2017}}</ref><ref name="Nimmo2015">{{cite journal |last1=Nimmo |first1=Francis |title=Powering Triton's recent geological activity by obliquity tides: Implications for Pluto geology |journal=Icarus |date=January 15, 2015 |volume=246 |pages=2–10 |doi= 10.1016/j.icarus.2014.01.044 |bibcode=2015Icar..246....2N |s2cid=40342189 |url=https://escholarship.org/content/qt99s8t6zm/qt99s8t6zm.pdf?t=nnm476 }}</ref> This is not thought to be adequate to power convection in Triton's icy crust. However, the strong [[obliquity]] [[Tidal Heating|tides]] are believed to generate enough additional heat to accomplish this and produce the observed signs of recent surface geological activity.<ref name="Nimmo2015"/> The black material ejected is suspected to contain [[organic compound]]s,<ref name="Sci Am 2017"/> and if liquid water is present on Triton, it has been speculated that this could make it [[Planetary habitability|habitable]] for some form of life.<ref name='Sci Am 2017'/><ref name="Irwin2001-Plausibility"/><ref name="space.com">{{cite web | url=http://www.space.com/17470-neptune-moon-triton-subsurface-ocean.html | title=Does Neptune's moon Triton have a subsurface ocean? | work=Space.com | date=September 6, 2012 | access-date=September 18, 2015 | author=Doyle, Amanda}}</ref>

== Atmosphere ==
{{Main|Atmosphere of Triton|Climate of Triton}}
[[File:The_Horns_of_Triton_-_Voyager_2_(43407605905).png|thumb|left|Departing image of Triton, showing its hazy atmosphere illuminated by sunlight and "extending" its crescent]]

Triton has a tenuous but well-structured and global [[nitrogen]] atmosphere,<ref name="Wind direction"/> with trace amounts of carbon monoxide and small amounts of methane near its surface.<ref name="nature2"/><ref name="grand"/><ref name="Lellouch2010"/> Like [[Pluto]]'s atmosphere, the atmosphere of Triton is thought to result from the evaporation of nitrogen from its surface.<ref name="Cruikshank2004"/> Its surface temperature is at least {{cvt|35.6|K|C}} because Triton's 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"/> An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere.<ref name="Tryka1993-Determination"/> This is colder than Pluto's average equilibrium temperature of {{cvt|44|K|C}}. Triton's surface atmospheric pressure is only about {{cvt|1.4–1.9|Pa|mbar|lk=on}}.<ref name="EncycSolSys-Triton"/>
[[File:Tritoncloud.jpg|thumb|Clouds observed above Triton's limb by ''Voyager 2''.]]
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 over a micrometer in size.<ref name="SmithSoderblom1989"/> Unlike other atmospheres, Triton's lacks a [[stratosphere]] and instead has a [[thermosphere]] from altitudes of 8 to 950&nbsp;km and an exosphere above that.<ref name="EncycSolSys-Triton"/> The temperature of Triton's upper atmosphere, at {{val|95|5|u=K}}, is higher than that at its surface, due to heat absorbed from solar radiation and Neptune's [[magnetosphere]].<ref name="nature2"/><ref name="Stevens1992-thermosphere"/> A haze permeates most of Triton's troposphere, thought to be composed largely of [[hydrocarbon]]s and [[nitrile]]s created by the action of sunlight on methane. Triton's atmosphere also has clouds of condensed nitrogen that lie between 1 and 3&nbsp;km from its surface.<ref name="EncycSolSys-Triton"/>

In 1997, observations from [[Earth]] were made of Triton's limb as it [[occultation|passed in front of stars]]. These observations indicated a denser atmosphere than was deduced from ''[[Voyager 2]]'' data.<ref name="Hubblesite"/> Other observations have shown an increase in temperature by 5% from 1989 to 1998.<ref name="MIT Triton"/> These observations indicated Triton was approaching an unusually warm southern hemisphere summer season that happens only once every few hundred years. Hypotheses 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"/> Another hypothesis argues that temperature changes are a result of the deposition of dark, red material from geological processes. Because Triton's [[Bond albedo]] is among the highest in the [[Solar System]], it is sensitive to small variations in spectral albedo.<ref name="Nature"/> Based on the increase in atmospheric pressure between 1989 and 1997, it is estimated that by 2010 Triton's atmospheric pressure may have increased to as much as 4 Pa.<ref name="Lellouch2010"/> By 2017, however, Triton's atmospheric surface pressure had nearly returned to ''Voyager 2'' levels; the cause for the rapid spike in atmospheric pressure between 1989 and 2017 remains unexplained.<ref name="Sicardy2024"/>

== Surface features ==
{{main|Geology of Triton}}
[[File:Geology of Triton.jpg|thumb|Interpretative [[geomorphology|geomorphological]] map of Triton]]
All detailed knowledge of the surface of Triton was acquired from a distance of 40,000&nbsp;km by the ''Voyager 2'' spacecraft during a single encounter in 1989.<ref name="Gray1989"/> The 40% of Triton's surface imaged by ''Voyager 2'' revealed blocky outcrops, ridges, troughs, furrows, hollows, plateaus, icy plains, and a few craters. Triton is relatively flat; its observed topography never varies beyond a kilometer.<ref name="EncycSolSys-Triton"/> The [[impact crater]]s observed are concentrated almost entirely in Triton's [[leading hemisphere]].<ref name = "Mah2019">{{cite journal |last1= Mah|first1= J.|last2= Brasser|first2= R.|title= The origin of the cratering asymmetry on Triton|journal= Monthly Notices of the Royal Astronomical Society|volume= 486|pages= 836–842|date= 2019|doi= 10.1093/mnras/stz851|doi-access= free|arxiv= 1904.08073|s2cid= 118682572}}</ref> Analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from an estimated 50&nbsp;million years old to just an estimated 6&nbsp;million years old.<ref name="Schenk2007"/> Fifty-five percent of Triton's surface is covered with frozen nitrogen, with water ice comprising 15–35% and [[dry ice|frozen CO<sub>2</sub>]] forming the remaining 10–20%.<ref>{{cite news |last=Williams |first=Matt |url=https://www.universetoday.com/56042/triton/ |title=Neptune's Moon Triton |work=Universe Today |date=July 28, 2015 |access-date=September 26, 2017 }}</ref> The surface also has deposits of [[tholin]]s, a dark, tarry slurry of various organic chemical compounds.<ref name="LPI Oleson">{{cite conference |last1=Oleson |first1=Steven R. |last2=Landis |first2=Geoffrey |title=Triton Hopper: Exploring Neptune's Captured Kuiper Belt Object |url=https://www.hou.usra.edu/meetings/V2050/pdf/8145.pdf |conference=Planetary Science Vision 2050 Workshop 2017 }}</ref>

=== Cryovolcanism ===
{{Further|Cryovolcano}}

One of the largest [[cryovolcanic]] features found on Triton is [[Leviathan Patera]],<ref>{{Cite journal | doi=10.1016/j.pss.2018.03.010| title=Heat flow in Triton: Implications for heat sources powering recent geologic activity| year=2018| last1=Martin-Herrero| first1=Alvaro| last2=Romeo| first2=Ignacio| last3=Ruiz| first3=Javier| journal=Planetary and Space Science| volume=160| pages=19–25| bibcode=2018P&SS..160...19M| s2cid=125508759}}</ref> a caldera-like feature roughly 100&nbsp;km in diameter seen near the equator. Surrounding this caldera is a massive cryovolcanic plain, Cipango Planum, which is at least 490,000 km<sup>2</sup> in area; assuming Leviathan Patera is the primary vent, Leviathan Patera is one of the largest volcanic or cryovolcanic constructs in the Solar System.<ref name="schenk2021">{{Cite journal |doi=10.3390/rs13173476 |title=Triton: Topography and Geology of a Probable Ocean World with Comparison to Pluto and Charon |date=September 2021 |last1=Schenk | first1=Paul |last2=Beddingfield |first2=Chloe |last3=Bertrand |first3=Tanguy |display-authors=et al. |journal=Remote Sensing | volume=13 |issue=17 |page=3476 |doi-access=free |bibcode=2021RemS...13.3476S |hdl=10150/661940 |hdl-access=free }}</ref> This feature is also connected to two enormous cryolava lakes seen northwest of the caldera. Because the cryolava on Triton is believed to be primarily water ice with some ammonia, these lakes would qualify as stable bodies of surface liquid water while they were molten. This is the first place such bodies have been found apart from Earth, and Triton is the only icy body known to feature cryolava lakes,<ref>{{cite web |title=Triton's Volcanic Plains |date=August 25, 2009 |url=https://science.nasa.gov/resource/tritons-volcanic-plains |publisher=Lunar and Planetary Institute}}</ref> although similar cryomagmatic extrusions can be seen on [[Ariel (moon)|Ariel]], [[Ganymede (moon)|Ganymede]], [[Charon (moon)|Charon]], and [[Titan (moon)|Titan]].<ref>{{cite web |last1=Schenk |first1=Paul |last2=Prockter |first2=Louise |title=Candidate Cryovolcanic Features in the Outer Solar System |url=https://www.hou.usra.edu/meetings/cryovolcanism2018/pdf/2035.pdf |publisher=Lunar and Planetary Institute}}</ref>

=== Plumes ===
{{main|Geology of Triton#South polar plumes}}
The ''[[Voyager 2]]'' probe in 1989 observed a handful of [[geyser]]-like eruptions of nitrogen gas or water and [[Entrainment (physical geography)|entrained]] dust from beneath the surface of Triton in plumes up to 8&nbsp;km high.<ref name="voyager"/><ref name="Soderblom2"/> Triton is thus one of the few bodies in the Solar System on which active eruptions of some sort have been observed.<ref name="Kargel1994-Cryovolcanism"/> The best-observed examples are the [[Hili (plume)|Hili plume]] and [[Mahilani (plume)|Mahilani plume]] (named after a [[Zulu mythology|Zulu]] [[Tikoloshe|water sprite]] and a [[Tonga]]n sea spirit, respectively).<ref name="USGS-planetarynames-Hili-Mahilani"/>

The precise mechanism behind Triton's plumes is debated;<ref name=":1">{{Cite journal |last1=Hofgartner |first1=Jason D. |last2=Birch |first2=Samuel P. D. |last3=Castillo |first3=Julie |last4=Grundy |first4=Will M. |last5=Hansen |first5=Candice J. |last6=Hayes |first6=Alexander G. |last7=Howett |first7=Carly J. A. |last8=Hurford |first8=Terry A. |last9=Martin |first9=Emily S. |last10=Mitchell |first10=Karl L. |last11=Nordheim |first11=Tom A. |last12=Poston |first12=Michael J. |last13=Prockter |first13=Louise M. |last14=Quick |first14=Lynnae C. |last15=Schenk |first15=Paul |date=March 15, 2022 |title=Hypotheses for Triton's plumes: New analyses and future remote sensing tests |url=https://www.sciencedirect.com/science/article/pii/S0019103521004796 |journal=Icarus |volume=375 |article-number=114835 |doi=10.1016/j.icarus.2021.114835 |issn=0019-1035|arxiv=2112.04627 |bibcode=2022Icar..37514835H }}</ref> one hypothesis is that Triton's plumes are driven by solar heating underneath a transparent or [[translucent]] layer of nitrogen ice, creating a sort of "solid [[greenhouse effect]]". As solar radiation warms the darker material beneath, this causes a rapid increase in pressure as the nitrogen begins to [[sublimation (phase transition)|sublimate]] until enough pressure accumulates for it to erupt through the translucent layer. This model is largely supported by the observation that Triton was near peak southern summer at the time of ''Voyager 2''{{'}}s flyby, ensuring its southern polar cap was receiving prolonged sunlight.<ref name="EncycSolSys-Triton" /><ref name="SmithSoderblom1989" /> [[carbon dioxide|CO<sub>2</sub>]] [[geysers on Mars]] are thought to erupt from its [[Climate of Mars#Polar caps|south polar cap]] each spring in the same way.<ref name="THEMIS" />

The significant geological activity on Triton has led to alternative proposals that the plumes may be cryovolcanic in nature, rather than driven by solar radiation. A cryovolcanic origin better explains the estimated output of Triton's plumes, which possibly exceeds {{Convert|400|kg/s|lb/s}}. This is similar to that which is estimated for Enceladus's cryovolcanic plumes at {{Convert|200|kg/s|lb/s|abbr=on}}. If Triton's plumes are cryovolcanically driven, it remains to be explained why they predominantly appear over its southern polar cap.<ref name=":1" /> Triton's high surface heat flux may directly melt or vaporize nitrogen ice at the base of its polar caps, creating 'hot spots' which break through the ice or move to the ice caps' margins, before erupting explosively.<ref name=":1" />

Though only observed up close once by the ''Voyager 2'' spacecraft, it is estimated that a plume eruption on Triton may last up to a year. 

<gallery widths="200px" heights="200px">
File:Leviathan Patera Volcanic Dome.gif|Close up of the volcanic province of [[Leviathan Patera]], the caldera in the center of the image. [[Kraken Catena]] and [[Set Catena]] extend radially from the caldera to the right and upper-right of the image, while [[Ruach Planitia]] is seen to the upper left. Just off-screen to the lower left is a fault zone aligned radially with the caldera, indicating a close connection between the tectonics and volcanology of this geologic unit.
File:Voyager 2 Triton 14bg r90ccw colorized.jpg|Dark streaks across Triton's south polar cap surface, thought to be dust deposits left by eruptions of [[nitrogen]] geysers.
File:Cryolava-lake-triton.jpg|Two large [[cryolava]] lakes on Triton, seen west of [[Leviathan Patera]]. Combined, they are nearly the size of [[Kraken Mare]] on [[Titan (moon)|Titan]]. These features are unusually crater free, indicating they are young and were recently molten.
</gallery>

=== Polar cap, plains and ridges ===
[[File:Triton (moon).jpg|thumb|Triton's bright south polar cap above a region of cantaloupe terrain]]
Triton's south polar region 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, but it is thought that Triton must also have a north polar ice cap.<ref name="Duxburyetal1993"/>

The high plains found on Triton's eastern hemisphere, such as Cipango Planum, 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="EncycSolSys-Triton"/>

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;m. They are thought to have formed from the eruption of icy lava.<ref name="EncycSolSys-Triton"/> The plains near Triton's eastern limb are dotted with black spots, the ''[[Macula (planetary geology)|maculae]]''. Some maculae are simple dark spots with diffuse boundaries, and others comprise a dark central patch surrounded by a white halo with sharp boundaries. <!-- Typical diameter of maculae is about 100&nbsp;km and width of halo is between 20 and 30&nbsp;km. Some speculate the maculae are outliers of the south polar cap, which retreats in summer. --> The maculae typically have diameters of about 100&nbsp;km and widths of the halos of between 20 and 30&nbsp;km.<ref name="EncycSolSys-Triton"/>

There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles.<ref name="Elliot1998-warming"/> Many also appear to be tectonic and may result from an extension or [[strike-slip fault]]ing.<ref name="linea"/> There are long double ridges of ice with central troughs bearing a strong resemblance to [[Europa (moon)#Lineae|Europan lineae]] (although they have a larger scale<ref name="Prockter"/>), and which may have a similar origin,<ref name="EncycSolSys-Triton"/> possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized.<ref name="Prockter"/> These faults with parallel ridges expelled from the interior cross complex terrain with valleys in the equatorial region. The ridges and furrows, or ''[[Sulcus (geology)|sulci]],'' such as [[Yasu Sulci]], [[Ho Sulci]], and [[Lo Sulci]],<ref name="Aksnes1990-Nomenclature"/> are thought 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 ===
[[File:PIA01537 Triton Faults.jpg|thumb|Cantaloupe terrain viewed from 130,000&nbsp;km by ''[[Voyager 2]]'', with crosscutting [[Europa (moon)|Europa]]-like double ridges. Slidr Sulci (vertical) and Tano Sulci form the prominent "X".]]
Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because it resembles the skin of a [[cantaloupe]] melon. Although it has few craters, it is thought that this is the oldest terrain on Triton.<ref name="cantaloupe"/> It probably covers much of Triton's western half.<ref name="EncycSolSys-Triton"/>

Cantaloupe terrain, which is mostly dirty water ice, is known to exist only on Triton. It contains depressions {{nowrap|30–40&nbsp;km}} in diameter.<ref name="cantaloupe"/> The depressions (''cavi'') are probably not impact craters because they are all of the similar size and have smooth curves. The leading hypothesis for their formation is [[diapir]]ism, the rising of "lumps" of less dense material through a stratum of denser material.<ref name="EncycSolSys-Triton"/><ref name="Diapirism"/> Alternative hypotheses include formation by collapses, or by flooding caused by [[cryovolcanism]].<ref name="cantaloupe"/>

=== Impact craters ===
[[File:PIA01538 Complex Geologic History of Triton.jpg|thumb|Tuonela Planitia (left) and Ruach Planitia (center) are two of Triton's [[cryovolcanic]] "walled plains". The paucity of craters is evidence of extensive, relatively recent, 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]]'s moon [[Miranda (moon)|Miranda]], which has only three percent of Triton's [[surface area]].<ref name="impact"/> The largest crater observed on Triton thought to have been created by an impact is a {{convert|27|km|mi|adj=mid|sp=us|-diameter}} feature called [[Mazomba (crater)|Mazomba]].<ref name="impact"/><ref name="Ingersoll1990-plumes"/> Although larger craters have been observed, they are generally thought to be volcanic.<ref name="impact"/>

The few impact craters on Triton are almost all concentrated in the leading hemisphere—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="Schenk2007"/> Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, due to impacts on the leading hemisphere being more frequent and more violent.<ref name="impact"/> ''Voyager 2'' imaged only 40% of Triton's surface, so this remains uncertain. However, the observed cratering asymmetry exceeds what can be explained based on the impactor populations, and implies a younger surface age for the crater-free regions (≤ 6&nbsp;million years old) than for the cratered regions (≤ 50&nbsp;million years old).<ref name = "Mah2019"/>

== Observation and exploration ==
[[File:PIA23874-NeptuneMoonTriton-TridentMission-20200616.jpg|thumb|left|NASA illustration detailing the studies of the proposed Trident mission]]
[[File:Voyager 2 Neptune and Triton.jpg|thumb|Neptune (top) and Triton (bottom) three days after flyby of ''Voyager 2'']]
The orbital properties of Triton were already determined with high accuracy in the 19th century. It was found to have a retrograde orbit, at a very high angle of inclination to the plane of Neptune's orbit. The first detailed observations of Triton were not made until 1930. Little was known about the satellite until ''[[Voyager 2]]'' flew by in 1989.<ref name="EncycSolSys-Triton"/>

Before the [[Planetary flyby|flyby]] 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 Earth. Like the famous overestimates of the [[Atmosphere of Mars|atmospheric density of Mars]], this proved incorrect. As with Mars, a denser atmosphere is postulated for its early history.<ref name="Lunine1992-massive"/>

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 from slightly smaller than the Moon (3,474.2&nbsp;km) to nearly half the diameter of Earth.<ref name="Cruikshank1979-diameterreflectance"/> 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 name="Stone1989-Voyager 2-Neptune"/>

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. Observations in late 1997 suggest that Triton is heating up and the atmosphere has become significantly denser since ''Voyager 2'' flew past in 1989.<ref name="Hubblesite"/>

[[Neptune Orbiter|New concepts for missions to the Neptune system]] to be conducted in the 2010s were proposed by [[NASA]] scientists on numerous occasions over the last decades. All of them identified Triton as being a prime target and a separate Triton lander comparable to the [[Huygens (spacecraft)|''Huygens'' probe]] for [[Titan (moon)|Titan]] was frequently included in those plans. No efforts aimed at Neptune and Triton went beyond the proposal phase and NASA's funding for missions to the outer Solar System is currently focused on the Jupiter and Saturn systems.<ref name="NASAgov-428154"/> A proposed lander mission to Triton, called ''[[Triton Hopper]]'', would mine nitrogen ice from the surface of Triton and process it to be used as a propellant for a small rocket, enabling it to fly or 'hop' across the surface.<ref>{{cite magazine |last=Ferreira |first=Becky |date=August 28, 2015 |title=Why We Should Use This Jumping Robot to Explore Neptune |url=https://www.vice.com/en/article/neptune-or-bust/ |magazine=[[Vice Motherboard]] |access-date=March 20, 2019}}</ref><ref name='Oleson 2015'>{{cite web |url=https://www.nasa.gov/feature/triton-hopper-exploring-neptunes-captured-kuiper-belt-object/ |title=Triton Hopper: Exploring Neptune's Captured Kuiper Belt Object |date=May 7, 2015 |first=Steven |last=Oleson |publisher=NASA Glenn Research Center |access-date=February 11, 2017 }}</ref> Another concept, involving a flyby, was formally proposed in 2019 as part of NASA's [[Discovery Program]] under the name ''[[Trident (spacecraft)|Trident]]''.<ref name="NYT-Trident">{{cite news |last=Brown |first=David W. |date=March 19, 2019 |title=Neptune's Moon Triton Is Destination of Proposed NASA Mission |url=https://www.nytimes.com/2019/03/19/science/triton-neptune-nasa-trident.html |work=[[The New York Times]] |access-date=March 20, 2019}}</ref> [[Neptune Odyssey]] is a mission concept for a Neptune orbiter with a focus on Triton being studied beginning April 2021 as a possible [[large strategic science mission]] by NASA that would launch in 2033 and arrive at the Neptune system in 2049.<ref name="Rymer">{{cite web |author1=Abigail Rymer |author2=Brenda Clyde |author3=Kirby Runyon |title=Neptune Odyssey: Mission to the Neptune-Triton System |url=https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Neptune%20Odyssey.pdf |access-date=April 18, 2021 |date=August 2020 |archive-date=December 15, 2020 |archive-url=https://web.archive.org/web/20201215003151/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Neptune%20Odyssey.pdf }}</ref> Two lower-cost mission concepts were subsequently developed for the [[New Frontiers program]]: the first the following June and the second in 2023. The first is ''Triton Ocean World Surveyor'', which would launch in 2031 and arrive in 2047,<ref name="TOWS">{{Cite web|url=https://science.nasa.gov/wp-content/uploads/2023/10/t-ows-triton-ocean-worlds-surveyor.pdf|date=June 7, 2021|title=Triton Ocean Worlds Surveyor concept study |website=NASA|last1=Hansen-Koharcheck|first1=Candice|last2=Fielhauer|first2=Karl}}</ref> and the second is ''Nautilus'', which would launch August 2042 and arrive in April 2057.<ref name="Steckel Conrad Dekarske Dolan 2023">{{cite journal | last1=Steckel | first1=Amanda | last2=Conrad | first2=Jack William | last3=Dekarske | first3=Jason | last4=Dolan | first4=Sydney | last5=Downey | first5=Brynna Grace | last6=Felton | first6=Ryan | last7=Hanson | first7=Lavender Elle | last8=Giesche | first8=Alena | last9=Horvath | first9=Tyler | last10=Maxwell | first10=Rachel | last11=Shumway | first11=Andrew O | last12=Siddique | first12=Anamika | last13=Strom | first13=Caleb | last14=Teece | first14=Bronwyn | last15=Todd | first15=Jessica | last16=Trinh | first16=Kevin T | last17=Velez | first17=Michael A | last18=Walter | first18=Callum Andrew | last19=Lowes | first19=Leslie L | last20=Hudson | first20=Troy | last21=Scully | first21=Jennifer E. C. | title=The Science Case for Nautilus: A Multi-Flyby Mission Concept to Triton | journal=AGU - Agu23 | publisher=AGU | date=December 12, 2023 | issue=3089 | bibcode=2023AGUFM.P23D3089S | url=https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1425806 | access-date=January 11, 2024}}</ref><ref name="Jason Dekarske 2023">{{cite web | title=Planetary Science Summer School · Jason Dekarske | website=Jason Dekarske | date=December 19, 2023 | url=https://www.jasondekarske.com/research/psss/ | access-date=January 25, 2024}}</ref>

==Maps==
{|align="center"
|-
|{{Annotated image | image = PIA18668 Map of Triton.jpg | image-width =  400
| height      =  204  <!-- to crop the lower part of the image -->
| float       = none | annotations=   <!-- this parameter must be there, empty or not! -->
| caption = Enhanced-color map; leading hemisphere is on right}}
|{{Annotated image | image = Triton polar maps.jpg | image-width =  400
| height      =  196  <!-- to crop the lower part of the image -->
| float       = none | annotations=   <!-- this parameter must be there, empty or not! -->
| caption = Enhanced-color polar maps; south is right}}
|}

== See also ==
{{portal|Solar System|Outer space|Astronomy}}
{{div col|colwidth=20em}}
* [[List of natural satellites]]
* [[Geology of Triton]]
* [[Neptune in fiction#Triton|Neptune in fiction § Triton]]
* ''[[Triton Hopper]]'', a proposed lander to Triton
* [[Extraterrestrial sky#The sky of Triton|Triton's sky]]
* [[Shensuo]], a proposed mission that would flyby Triton
{{div col end}}

== Notes ==
{{Reflist| group = caption}}

{{Reflist
| group = lower-alpha
| refs =

<ref name="nSurfaceArea">
Surface area derived from the radius ''r'': <math>4 \pi r^2</math>.
</ref>

<ref name="nVolume">
Volume ''v'' derived from the radius ''r'': <math>\frac{4}{3}\pi r^3</math>.
</ref>

<ref name="nSurfaceGravity">
Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'':  <math>\frac{Gm}{r^2}</math>.
</ref>

<ref name="nEscapeVelocity">
Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\sqrt{2Gm/r}</math>.
</ref>

<ref name="nMassTriton">
Mass of Triton: 2.14{{e|22}}&nbsp;kg. Combined mass of 12 other known moons of Neptune: 7.53{{e|19}}&nbsp;kg, or 0.35%. The mass of the rings is negligible.
</ref>

<ref name="AxialTilt">
With respect to Triton's orbit about Neptune.
</ref>

<ref name="nMassOthers">
The masses of other spherical moons are: [[Titania (moon)|Titania]]—3.5{{e|21}}, [[Oberon (moon)|Oberon]]—3.1{{e|21}}, [[Rhea (moon)|Rhea]]—2.3{{e|21}}, [[Iapetus (moon)|Iapetus]]—1.8{{e|21}}, [[Charon (moon)|Charon]]—1.6{{e|21}}, [[Ariel (moon)|Ariel]]—1.2{{e|21}}, [[Umbriel]]—1.3{{e|21}}, [[Dione (moon)|Dione]]—1.1{{e|21}}, [[Tethys (moon)|Tethys]]—0.6{{e|21}}, [[Enceladus]]—0.11{{e|21}}, [[Miranda (moon)|Miranda]]—0.06{{e|21}}, [[Mimas]]—0.04{{e|21}}. The total mass of remaining moons is about 0.12{{e|21}}. So, the total mass of all moons smaller than Triton is about 1.68{{e|22}}. (See [[List of Solar System objects by size]])
</ref>

<ref name="nLargest">
Largest [[irregular moon]]s: Saturn's [[Phoebe (moon)|Phoebe]] (210&nbsp;km), Uranus's [[Sycorax (moon)|Sycorax]] (160&nbsp;km), and Jupiter's [[Himalia (moon)|Himalia]] (140&nbsp;km)
</ref>

}}

== References ==
{{Reflist
| colwidth = 30em
| refs =

<ref name="neptuniansatfact">{{cite web
 |title       = Neptunian Satellite Fact Sheet
 |publisher   = NASA
 |author      = Williams, David R.
 |date        = November 23, 2006
|url         = http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html
 |access-date  = January 18, 2008
|archive-url  = https://web.archive.org/web/20111020174353/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html
 |archive-date = October 20, 2011
}}</ref>

<ref name="JPL-SSD-Neptune">
{{cite web
 |author      = Jacobson, R. A. – AJ
 |date        = April 3, 2009
|title       = Planetary Satellite Mean Orbital Parameters
 |work        = JPL satellite ephemeris
 |publisher   = [[JPL]] (Solar System Dynamics)
 |url         = http://ssd.jpl.nasa.gov/?sat_elem
 |access-date  = October 26, 2011
|archive-url  = https://web.archive.org/web/20111014234918/http://ssd.jpl.nasa.gov/?sat_elem
 |archive-date = October 14, 2011
}}

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<ref name="Jacobson2009-AJ">
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| journal = Reports on Astronomy
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<ref name="cantaloupe">
{{cite journal
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| 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
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|date=March 1993
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<ref name="Diapirism">
{{cite journal
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| title = Diapirism on Triton: A record of crustal layering and instability
| journal = [[Geology (journal)|Geology]]
| volume = 21
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| date = April 1993
| doi = 10.1130/0091-7613(1993)021<0299:DOTARO>2.3.CO;2
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}}
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<ref name="impact">
{{cite journal
| title = The Impact Cratering Record on Triton
| author = Strom, Robert G.
| author2 = Croft, Steven K.
| author3 = Boyce, Joseph M.
| doi = 10.1126/science.250.4979.437
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<ref name="Ingersoll1990-plumes">
{{cite journal
| title = Triton's Plumes: The Dust Devil Hypothesis
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<ref name="Lunine1992-massive">
{{cite journal
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| journal = Icarus
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| issue = 1
|date=November 1992
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<ref name="Cruikshank1979-diameterreflectance">
{{Cite journal | doi = 10.1016/0019-1035(79)90057-5| title = The diameter and reflectance of Triton| journal = Icarus| volume = 40| issue = 1| pages = 104–114| year = 1979| last1 = Cruikshank | first1 = D. P. | last2 = Stockton | first2 = A. | last3 = Dyck | first3 = H. M. | last4 = Becklin | first4 = E. E. | last5 = Macy | first5 = W.| bibcode = 1979Icar...40..104C}}
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<ref name="Stone1989-Voyager 2-Neptune">
{{cite journal
| title = The Voyager 2 Encounter with the Neptunian System
| author = Stone, EC |author2=Miner, ED
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| date = December 15, 1989
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<ref name="NASAgov-428154">{{cite web |url=http://www.nasa.gov/pdf/428154main_Planetary_Science.pdf |title=USA.gov: The U.S. Government's Official Web Portal |publisher=Nasa.gov |date=September 27, 2013 |access-date=October 10, 2013 |archive-date=October 25, 2012 |archive-url=https://web.archive.org/web/20121025152957/http://www.nasa.gov/pdf/428154main_Planetary_Science.pdf }}</ref>

<ref name="NYT-20141105-DO">{{cite news |last=Overbye |first=Dennis |author-link=dennis Overbye |title=Bound for Pluto, Carrying Memories of Triton |url=https://www.nytimes.com/2014/11/05/science/bound-for-pluto-carrying-memories-of-triton.html |date=November 5, 2014 |work=[[New York Times]] |access-date=November 5, 2014 }}</ref>

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{{cite journal
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{{cite journal
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<ref name="Wind direction">{{cite journal|last=Ingersoll|first=Andrew P.|date=1990|title=Dynamics of Triton's atmosphere|journal= Nature|volume=344|issue=6264 |pages=315–317|doi=10.1038/344315a0|bibcode = 1990Natur.344..315I |s2cid=4250378 }}</ref>

<ref name="Elliot2000">{{cite journal|first1=J.L.|last1=Elliot|last2=Strobel|first2=D.F.|first3=X.|last3=Zhu|title=The Thermal Structure of Triton's Middle Atmosphere |journal=Icarus |volume=143 |issue=2 |pages=425–428 |doi=10.1006/icar.1999.6312 |url=http://occult.mit.edu/_assets/documents/publications/Elliot2000Icarus143.425.pdf |date=2000 |bibcode=2000Icar..143..425E |display-authors=etal}}</ref>

<ref name="Sicardy2024">{{cite journal |last1=Sicardy |first1=B. |last2=Tej |first2=A. |last3=Gomez-Júnior |first3=A. R. |display-authors=et al. |title=Constraints on the evolution of the Triton atmosphere from occultations: 1989-2022 |journal=Astronomy & Astrophysics |date=February 2024 |volume=682 |page=8 |doi=10.1051/0004-6361/202348756  |arxiv=2402.02476 |bibcode=2024A&A...682L..24S |url=https://www.aanda.org/10.1051/0004-6361/202348756/pdf }}</ref>

}}

== External links ==
{{Commons category}}
* [https://web.archive.org/web/20161012075648/http://solarsystem.nasa.gov/planets/triton Triton profile] at NASA's Solar System Exploration site
* {{YouTube|h82GNysAH_w|''Voyager 2 Encounters Neptune and Triton'' (1989)}}
* [http://nineplanets.org/triton.html Triton page] at ''The Nine Planets''
* [http://solarviews.com/eng/triton.htm Triton page] (including [http://www.solarviews.com/eng/trimap.htm labelled Triton map]) at ''Views of the Solar System''
* [http://www.lpi.usra.edu/icy_moons/neptune/triton/ Triton map] from Paul Schenk, Lunar and Planetary Institute
* [http://photojournal.jpl.nasa.gov/target/Triton Triton images] from the NASA/JPL Photojournal
* [https://planetarynames.wr.usgs.gov/Page/TRITON/target Triton nomenclature] from the USGS Planetary Nomenclature website

{{Triton|state=uncollapsed}}
{{Moons of Neptune}}
{{Neptune}}
{{Solar System moons (compact)}}
{{Dwarf planets}}
{{Authority control}}
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[[Category:Triton (moon)| ]]
[[Category:Irregular satellites]]
[[Category:Former dwarf planets]]
[[Category:Astronomical objects discovered in 1846|18461010]]
[[Category:Objects observed by stellar occultation|M]]
[[Category:Moons with a retrograde orbit]]