# Triton (moon) <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Triton in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Triton&embed=1" data-title="Triton in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Triton`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 day/s and watch Triton complete a lap of Neptune in about six seconds, circling against the planet's direction of spin; press l to label the bodies in this local frame; then scroll out until Triton's whole orbit, about 14 Neptune radii in radius, fits in view, and press space to pause it anywhere on the circle.* **Triton** is the largest moon of [[Neptune]] and the only large moon in the [[PORTAL_Solar_System|Solar System]] that orbits its planet backwards, against the direction of the planet's rotation.[^nasa-fs-nsat][^mckinnon2014] It is 2,707 km across, a little larger than [[Pluto]], and holds more than 99.5% of all the mass orbiting Neptune.[^nasa-fs-nsat] Its retrograde orbit means it cannot have formed around Neptune; it is thought to be a former [[Dwarf_planet|dwarf planet]] of the [[Kuiper_belt|Kuiper belt]], captured when it passed too close.[^agnor2006] Triton is made of rock and metal overlain by [[Water|water]] ice, with a surface of frozen [[Nitrogen|nitrogen]] that reflects most of the sunlight falling on it. Voyager 2 measured a surface temperature of about 38 K there, and found a thin nitrogen atmosphere, dark plumes rising 8 km, and a surface so sparsely cratered that parts of it may be only a few million years old.[^nelson1990][^soderblom1990][^schenk2007] William Lassell discovered Triton in October 1846, weeks after Neptune itself; Voyager 2's flyby of 1989 remains the only close visit, and it imaged about 40% of the surface.[^lassell1847][^mckinnon2014] The explorer at the top of this page opens Neptune's local frame with the moons at true scale. It shows Triton 354,759 km from Neptune, circling in 5.88 days in the retrograde direction, with a radius of 1,353.4 km; its orbital elements and size follow the NASA satellite fact sheet, while its starting position on the orbit is ILLUSTRATIVE.[^nasa-fs-nsat] ## Discovery and naming When John Herschel heard of Neptune's discovery in 1846, he wrote to William Lassell, a brewer by trade and a skilled amateur astronomer, urging him to look for moons. Lassell found Triton on 10 October 1846, eight days after receiving the letter.[^lassell1847][^lassell1846] He used a reflecting telescope of his own construction with a 61 cm (24-inch) metal mirror, the "two-foot" reflector later given to the Royal Observatory, Greenwich.[^rog-lassell] In the same weeks he reported what he took to be a ring around Neptune. Neptune does have rings, but they are far too faint and dark for his telescope, and the report is regarded as an instrumental artefact.[^smith1984] Lassell did not name the moon. For about a century it was simply "the satellite of Neptune", a label that sufficed until Gerard Kuiper found a second moon, Nereid, in 1949.[^usgs-names] The name Triton, after the sea god who was the son of Poseidon, Neptune's Greek counterpart, appeared in Camille Flammarion's *Astronomie populaire* of 1880, and was adopted officially only decades later.[^flammarion1880][^moore1996] Lassell did, however, successfully propose the name Hyperion for a moon of [[Saturn]] that he found in 1848.[^iau-names] Neptune's later moons were named in the same spirit, after lesser deities of the sea.[^usgs-names] Planetary moons other than Earth's [[Moon]] have not traditionally had astronomical symbols; a proposed symbol for Triton, a Greek tau combined with Neptune's trident, is not widely used.[^bala2025] ## Orbit and rotation Triton's [[Orbit|orbit]] around Neptune has a mean radius of 354,759 km, about 14.3 Neptune radii, and it goes round once every 5.877 days, at a mean speed of about 4.4 km/s (derived).[^nasa-fs-nsat] The orbit is almost perfectly circular, with an eccentricity of 0.000016, and it is retrograde: its inclination to Neptune's equator is 157°, and any inclination above 90° means motion opposite to the planet's spin.[^nasa-fs-nsat] Many small, distant irregular moons of the giant planets also orbit backwards, but they are tiny by comparison and lie much farther from their planets.[^brozovic2022][^mckinnon2014] Two tilts shape Triton's geometry. Neptune's equator is inclined about 28° to its orbit around the Sun,[^nasa-fs-n] and Triton's orbit is inclined 157° to that equator. The pole of Triton's orbit precesses around Neptune's pole with a period of about 678 years, so the angle between Triton's orbit and Neptune's orbital plane swings between about 127° and 173°.[^jacobson2009] Because Triton's orbit is close to the planet and its precession is driven by Neptune rather than the Sun, it does not fit neatly into the usual classes of regular and irregular moons.[^brozovic2022] Triton is tidally locked: it keeps one face toward Neptune, and its equator lies almost exactly in its orbital plane.[^davies1991] Its spin axis therefore tilts with the orbit, currently about 40° from Neptune's orbital plane, so over Neptune's 165-year circuit of the Sun each of Triton's poles in turn tips toward the light. Triton has seasons, and changes in them were reported in 2010.[^space2010] [[Tide|Tides]] continue to shape the orbit. A retrograde moon raises a tidal bulge on the planet that pulls it backward, draining its orbital [[Angular_momentum|angular momentum]], so Triton is slowly spiralling inward, unlike the Moon, which is receding from Earth. A 1989 study found it could cross Neptune's Roche limit in about 3.6 billion years, where it would either hit the planet or be torn apart into a ring system like the [[Rings_of_Saturn|rings of Saturn]]; a 2025 estimate based on astrometry puts the crossing about 28 billion years away.[^chyba1989][^wang2025] ## Capture Moons that grow in a disc around a planet share the planet's direction of rotation, so a large moon on a retrograde orbit must have been captured.[^mckinnon2014] Triton is only slightly larger than Pluto and closely resembles it in [[Density|density]] and surface ices, which suggests that the two came from the same population: the icy bodies beyond Neptune that include the [[Plutino|plutinos]].[^cruikshank2004] A 2024 comparison of their chemistry supported the idea that they formed in the same region.[^ocallaghan2024] Capture is not easy. A body arriving from solar orbit enters the planet's [[Hill_sphere|Hill sphere]] faster than the planet's local escape speed, and unless it loses energy during the encounter it simply flies out again.[^mckinnon2014] Early models appealed to a collision with a moon or proto-moon already orbiting Neptune.[^mckinnon2014] Craig Agnor and Douglas Hamilton proposed in 2006 that Triton arrived as one member of a binary pair. In a close pass by Neptune the pair was pulled apart: the companion carried off extra energy and escaped, while Triton was left bound to the planet. The exchange is gentle, needs no collision, and is more likely if the companion is massive.[^agnor2006] Binaries are common among large Kuiper belt objects, which makes the scenario plausible.[^sheppard2004][^jewitt-binaries] Capture would have left Triton on a large, eccentric orbit that cut through the orbits of any original moons. This may explain why Neptune has few regular satellites and why Nereid's orbit is so eccentric.[^jacobson2009] Simulations in 2017 suggest Triton collided with at least one early moon before its orbit settled.[^rufu2017] Tides on the eccentric orbit would also have heated Triton, perhaps keeping its interior partly liquid for about a billion years; this heating ended once the orbit became circular.[^ross1990] ## Physical characteristics Triton's radius is 1,353.4 km, 5.5% of Neptune's, the largest of any giant planet's moon relative to its planet's size. Its mass, 2.14 × 10²² kg, is about 1/4,790 of Neptune's (derived), and its mean density is 2,050 kg/m³.[^nasa-fs-nsat][^nasa-fs-n] Surface gravity follows from these values as g = GM/r² ≈ 0.78 m/s², about 8% of [[Earth]]'s (derived). It is larger than Pluto, whose radius is 1,188 km, and so larger than any known [[Dwarf_planet|dwarf planet]], [[Eris_(dwarf_planet)|Eris]] included.[^nasa-fs-p][^mckinnon2014] The density implies a body of roughly 30–45% water ice by mass, the rest rock and metal.[^mckinnon2014] The surface is dominated by nitrogen ice, about 55%, with 15–35% water ice and 10–20% frozen carbon dioxide, and traces of methane and carbon monoxide.[^mckinnon2014] Triton is very bright, with a geometric albedo of 0.72, so it absorbs little of the weak sunlight at 30 AU; Voyager 2 measured a surface temperature of about 38 K, among the lowest recorded in the Solar System.[^nasa-fs-nsat][^nelson1990] Its faint reddish tint is attributed to methane processed by ultraviolet light into darker organic compounds.[^grundy2002] Triton is thought to be differentiated into a rocky metallic core, a water-ice mantle and a crust.[^mckinnon2014] [[Radioactive_decay|Radioactive decay]] in the rock may supply enough heat to keep a liquid ocean beneath the ice, as is proposed for [[Europa_(moon)|Europa]].[^hussmann2006][^nimmo2015] Radiogenic heat alone is probably too little to drive the young geology, but the moon's tilted spin produces obliquity tides that may add enough heat to keep the ice shell convecting.[^nimmo2015] If the ocean exists, it has been discussed as a possible habitat.[^irwin2001] ## Atmosphere Triton has a thin atmosphere of nitrogen with traces of methane and carbon monoxide, sustained, as on Pluto, by ice sublimating from the surface.[^broadfoot1989][^lellouch2010][^cruikshank2004] Because the gas is in equilibrium with the ice, the pressure follows the [[Vapor_pressure|vapour pressure]] of nitrogen at the surface temperature. Voyager 2 measured a surface pressure of about 1.4–1.9 Pa, roughly one seventy-thousandth of Earth's (derived).[^mckinnon2014] The nitrogen ice is in its hexagonal crystal form, which puts the surface above 35.6 K, and vapour-pressure equilibrium sets an upper limit in the low 40s K.[^duxbury1993][^tryka1993] A turbulent lower layer, the troposphere, reaches about 8 km, and dark streaks left by plumes show winds able to move fine particles.[^smith1989] Above it there is no stratosphere; a thermosphere extends to about 950 km, heated to about 95 K by sunlight and by particles from Neptune's magnetosphere.[^broadfoot1989][^stevens1992] A haze of hydrocarbons and nitriles, made by sunlight acting on methane, fills much of the troposphere, and thin nitrogen clouds float 1–3 km up.[^mckinnon2014] The atmosphere changes over years. Stellar occultations observed from Earth in 1997 showed a denser atmosphere than Voyager found, and a temperature rise of about 5% between 1989 and 1998, as the southern hemisphere approached an unusually intense summer.[^savage1998][^mit1998] Later occultations showed that by 2017 the pressure had fallen back almost to its 1989 value; the cause of the rise and fall is unexplained.[^sicardy2024] ## Surface features Everything known in detail about Triton's surface comes from Voyager 2's single pass in 1989, which imaged about 40% of it. The surface is remarkably flat, with relief rarely exceeding a kilometre, and shows plains, ridges, troughs, pits and few craters.[^mckinnon2014] Crater counts give ages from about 50 million years down to about 6 million years, very young by planetary standards.[^schenk2007][^mah2019] ### Cryovolcanism Leviathan Patera, a caldera-like depression about 100 km across near the equator, sits at the centre of Cipango Planum, a volcanic plain of at least 490,000 km².[^schenk2021][^martinherrero2018] If Leviathan was the main vent, it is among the largest volcanic constructs in the Solar System. Nearby, flat-floored basins appear to be frozen lakes of cryolava, probably water with ammonia, which would make them former bodies of surface liquid.[^lpi2009][^schenk2021] ### Plumes Triton is one of the few bodies besides Earth and [[Io_(moon)|Io]] on which active eruptions have been seen.[^kargel1994] Voyager 2 saw several plumes rising about 8 km above the surface before bending into long dark streaks downwind; the best studied are named Hili and Mahilani.[^soderblom1990] One explanation is a solid-state greenhouse: sunlight passes through clear nitrogen ice, warms darker material underneath, and the gas that sublimates builds up pressure until it bursts out.[^smith1989] Voyager arrived near the peak of southern summer, when the south polar cap had been in sunlight for years, which fits this model.[^smith1989][^mckinnon2014] Carbon dioxide jets on the south polar cap of [[Mars]] are thought to work the same way.[^burnham2006] The alternative is that the plumes are cryovolcanic, fed by internal heat; their estimated output, possibly over 400 kg/s, is comparable to that of [[Enceladus]]'s plumes.[^hofgartner2022] ### Polar cap, plains and ridges A bright cap of nitrogen and methane ice covers the south polar region seen by Voyager; the north was in darkness, but a northern cap is expected.[^duxbury1993] Farther north, smooth plains such as Cipango Planum bury older terrain, and four walled plains with less than 200 m of relief appear to be filled with icy lava.[^mckinnon2014] Dark spots called maculae, typically about 100 km across and often ringed by bright halos, dot the eastern plains.[^mckinnon2014] Long double ridges with central troughs, like the lineae of Europa, may have formed by shear heating along faults stressed by tides before the orbit circularised.[^prockter2005] ### Cantaloupe terrain Much of the western hemisphere is covered by a pattern of interlocking dimples, 30–40 km across, that resembles the skin of a cantaloupe melon. It is dirty water ice and has few craters, but is thought to be the oldest surface on Triton.[^boyce1993] The dimples are too uniform in size to be impact craters; the leading explanation is diapirism, in which blobs of less dense ice rose through a denser crust.[^schenk1993] ### Impact craters Only 179 craters on Triton are certainly of impact origin; Miranda, a moon of [[Uranus]] with 3% of Triton's area, shows 835. The largest, Mazomba, is 27 km across.[^strom1990] The craters cluster on the leading hemisphere, which faces forward along the orbit and meets debris at higher speed. That asymmetry is stronger than impacts from heliocentric bodies would produce, and may indicate debris orbiting Neptune.[^schenk2007][^mah2019] ## Observation and exploration Triton's retrograde, steeply inclined orbit was already well determined in the 19th century, but little else was known before Voyager.[^mckinnon2014] Its size was long uncertain: Gerard Kuiper's 1954 measurement gave 3,800 km, and later estimates ranged from 2,500 to 6,000 km.[^cruikshank1979] Voyager 2's approach on 25 August 1989 fixed the diameter at 2,706 km and showed that the moon lacked the thick atmosphere and nitrogen seas some had expected.[^stone1989][^lunine1992] Voyager 2 passed Triton at a distance of about 40,000 km.[^gray1989] No other spacecraft has followed, but Triton is the prime target of several proposals. Trident, a flyby, was proposed to [[NASA]]'s Discovery Program in 2019.[^brown2019] Neptune Odyssey, a large orbiter concept, would arrive in 2049; Triton Ocean Worlds Surveyor and Nautilus are smaller New Frontiers concepts.[^rymer2020][^hansen2021][^steckel2023] A Triton Hopper lander would refine surface nitrogen into rocket propellant and hop between sites.[^oleson2015] ## Maps Global maps of Triton are mosaics of Voyager 2 images, and they are incomplete: the northern hemisphere was dark during the flyby and much of the rest was seen only at low resolution. The best coverage is of the sunlit southern hemisphere, where most named features lie.[^mckinnon2014] A 2021 study rebuilt Voyager's imagery into new topographic and geological maps of the imaged region and compared its terrains with those of Pluto and [[Charon_(moon)|Charon]].[^schenk2021] Features on the maps carry names such as Hili, a Zulu water sprite, and Mahilani, a Tongan sea spirit; names are assigned under IAU rules and listed in the USGS planetary nomenclature gazetteer, and a nomenclature report of 1990 set the first of them after the flyby.[^usgs-names][^aksnes1990] ## See also - [[Neptune]] · [[Kuiper_belt]] · [[Pluto]] - [[Dwarf_planet]] · [[Plutino]] - [[Europa_(moon)]] · [[Enceladus]] · [[Titan_(moon)]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived values: orbital speed 2π × 354,759 km / (5.877 × 86,400 s) ≈ 4.39 km/s; orbital radius 354,759 / 24,764 ≈ 14.3 Neptune radii; mass ratio 1.024 × 10²⁶ / 2.14 × 10²² ≈ 4,790; surface gravity from G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻² with the fact-sheet mass and radius; pressure ratio 101,325 Pa / 1.4 Pa ≈ 72,000. Triton's diameter is quoted as 2,706.8 km from the fact-sheet radius; the pair's 2,710 km comes from JPL's physical-parameter table. ## References [^nasa-fs-nsat]: Williams, D. R. "Neptunian Satellite Fact Sheet". NASA NSSDCA (last updated 11 March 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html (fetched 2026-09-18). [^nasa-fs-n]: Williams, D. R. "Neptune Fact Sheet". NASA NSSDCA (last updated 3 October 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html (fetched 2026-09-18). [^nasa-fs-p]: Williams, D. R. "Pluto Fact Sheet". NASA NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html (fetched 2026-09-18). [^mit1998]: Massachusetts Institute of Technology (24 June 1998). "MIT researcher finds evidence of global warming on Neptune's largest moon". http://web.mit.edu/newsoffice/1998/triton.html [^kargel1994]: Kargel, J. S. (1994). 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"Working Group for Planetary System Nomenclature". *Reports on Astronomy* 21A: 613–619. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940014368_1994014368.pdf ## External links - NASA Science: Triton. https://science.nasa.gov/neptune/moons/triton/ - NASA NSSDCA Neptunian satellite fact sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html - USGS Gazetteer of Planetary Nomenclature: Triton. https://planetarynames.wr.usgs.gov/Page/TRITON/target - JPL Photojournal: Triton. https://photojournal.jpl.nasa.gov/target/triton ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Triton_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Triton_(moon)) · pinned revision [1375101176](https://en.wikipedia.org/w/index.php?oldid=1375101176) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-064 · explorer state `?obj=Triton`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->