View source for Atmosphere of Triton

{{Short description|Layer of gasses surrounding the moon Triton}}
{{Use dmy dates|date=May 2024}}
{{missing information|Triton's ionosphere and atmospheric chemistry|date=April 2024}}
{{AstronomicalAtmosphere
|name = Triton
|image = The Horns of Triton - Voyager 2 (43407605905).png
|caption = ''[[Voyager 2]]'' image of Triton after closest approach, showing the haze in Triton's atmosphere weakly scattering sunlight and "extending" its thin crescent
|info_refs = 
|height = ~870 km (exobase)<ref name="Krasnopolsky1993"/>
|avg_surface_pressure = ~{{convert|1.4|Pa|atm|sigfig=3|abbr=on}} (1989)<ref name="Sicardy2024"/><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"/>
|mass = 
|composition_refs = <ref name=Lellouch2010/>{{efn|These values were measured in 2010; as no occultation had observed Triton's atmospheric pressure between 1997 and 2017,<ref name="Sicardy2024"/> Lellouch and collaborators tentatively favored an assumed atmospheric pressure of 40 μbar (4 Pa). Percentages were derived by dividing the given partial pressure by an assumed atmospheric pressure of 4 Pa.<ref name=Lellouch2010/>}}
|chemical1 = [[Nitrogen]] ({{Chem|N|2}})
|quantity1 = >99%
|chemical2 = [[Methane]] ({{Chem|C|H|4}})
|quantity2 = ~0.025%
|chemical3 = [[Carbon monoxide]] ({{Chem|C|O}})
|quantity3 = ~0.06%
}}
The '''atmosphere of Triton''' is the layer of [[gas]]es surrounding [[Triton (moon)|Triton]]. Like the [[atmosphere of Titan|atmospheres of Titan]] and [[Atmosphere of Pluto|Pluto]], Triton's [[atmosphere]] is composed primarily of [[nitrogen]], with smaller amounts of [[methane]] and [[carbon monoxide]]. It hosts a layer of [[organic compound|organic]] haze extending up to 30 kilometers above its surface and a deck of thin bright clouds at about 4 kilometers in altitude.<ref name="Ohno2021"/> Due to Triton's low gravity, its atmosphere is loosely bound, extending over 800 kilometers from its surface.<ref name="Voyager"/>

Triton, along with Saturn's moon [[Titan (moon)|Titan]], is one of only two moons in the Solar System known to have significant, global atmospheres.<ref name="Wind direction"/>{{efn|Saturn's moon [[Enceladus]]; Jupiter's moons [[Atmosphere of Io|Io]],<ref name="IobookChap10"/> [[Europa (moon)#Atmosphere|Europa]],<ref name="HubbleEuropaAtmo"/> [[Ganymede (moon)#Atmosphere and ionosphere|Ganymede]],<ref name="Hall1998"/> and [[Callisto (moon)#Atmosphere and ionosphere|Callisto]];<ref name="Carlson 1999"/> and possibly Uranus's moon [[Titania (moon)#Atmosphere|Titania]]<ref name="Widemann Sicardy et al. 2009"/> all have atmospheres as well, but are too thin to drive weather or climate.}} The [[Atmospheric pressure#Surface pressure|surface pressure]] is only 14 [[microbar]]s (1.4 Pa or 0.0105[[mmHg]]), {{Fraction|70000}} of the surface pressure on [[Earth]].<ref name="Voyager"/> Similar to the atmosphere of Pluto, Triton's atmosphere is sensitive to seasonal changes; observations obtained in 1998 showed an increase in temperature, increasing the atmosphere's density.<ref name="MIT Triton"/>

==Composition and chemistry==
[[Nitrogen]] is the main gas in Triton's atmosphere.<ref name="The Grand Tour"/> The two other known components are [[methane]] and [[carbon monoxide]], whose abundances are a few hundredths of a percent of that of the nitrogen. Carbon monoxide, which was first detected in 2010 by the ground-based observations, is slightly more abundant than methane. The abundance of methane relative to nitrogen increased by four to five times since 1986 due to the seasonal warming observed on Triton, which passed its southern-hemisphere [[solstice]] in 2001.<ref name=Lellouch2010/> Compositionally, Triton's atmosphere strongly resembles that of Pluto's atmosphere, being almost entirely nitrogen with minor contributions from other gasses.<ref name=Lellouch_2017/><ref name=Stern_2015/>

Other possible components of the Triton's atmosphere include [[argon]] and [[neon]]. Because they were not detected in the [[ultraviolet]] part of the spectrum of Triton obtained by Voyager 2 in 1989, their abundances are unlikely to exceed a few percent.<ref name=Broadfoot1989/> In addition to the gases mentioned above, the upper atmosphere contains significant amounts of both molecular and atomic [[hydrogen]], which is produced by the [[photolysis]] of methane. This hydrogen quickly escapes into the space serving as a source of plasma in the magnetosphere of Neptune.<ref name=Broadfoot1989/>

{| class="wikitable"
|+Chemistry in Triton's atmosphere<ref name="Krasnopolsky1993"/><ref name="Kargel1995"/>{{rp|page=111}}
!Molecule
!Production
!Loss
|-
|[[Hydrogen]] ({{chem2|H2}})
|Various
|Escape
|-
|Hydrogen (H)
|Various
|Escape
|-
|[[Nitrogen]] ({{chem2|N2}})
|Cryovolcanism?<br/><chem>N + NH->N2 + H</chem><br/><chem>2N + N2->2N2</chem>
|Escape<br/><chem>N+2 + H2->N2H+ + H</chem><br/><chem>NH+ + N2->N2H+ + N</chem>
|-
|Nitrogen (N)
|<chem>H + NH->N + H2</chem><br/>
|Escape<br/><chem>N + NH->N2 + H</chem><br/><chem>2N + N2->2N2</chem><br/><chem>N+ + H2->NH+ + H</chem>
|-
|[[Methane]] ({{chem2|CH4}})
|Cryovolcanism?
|<chem>CH4 + h\nu->CH2 + 2H</chem><br/><chem>CH4 + h\nu->CH2 + H2</chem><br/><chem>CH4 + h\nu->CH + H + H2</chem>
|-
|[[Carbon monoxide]] [[Ion#Anions and cations|cations]] ({{chem2|CO+}})
|<chem>N+2 + CO->CO+ + N2</chem><br/><chem>N+ + CO->CO+ + N</chem>
|<chem>CO+ + H->CO + H+</chem><br/><chem>CO+ + e->C + O</chem><br/><chem>CO+ + H2->HCO+ + H</chem>
|-
|[[Methylene (compound)|Methylene]] ({{chem2|CH2}})
|<chem>CH4 + h\nu->CH2 + 2H</chem><br/><chem>CH4 + h\nu->CH2 + H2</chem>
|<chem>CH2 + H->CH + H2</chem>
|-
|[[Ethylene]] ({{chem2|C2H4}})
|<chem>2CH4 + h\nu->C2H4 + 1.5H2 + H</chem><br/><chem>CH + CH4->C2H4 + H</chem>
|
|-
|}

=== Interactions with Neptune ===
Triton's atmosphere interacts with Neptune through Neptune's [[magnetosphere]], with interactions complicated by Triton's retrograde orbit and Neptune's asymmetric magnetosphere. As neutral hydrogen and nitrogen escape from Triton's atmosphere, they form a large neutral cloud in orbit around Neptune called the Triton torus. The modelled rate of escape of hydrogen, both atomic and molecular, is about {{val|7|e=25}}&nbsp;particles per second; nitrogen escape rates are inferred to be 2–3&nbsp;times lower.<ref name="Decker1994"/> The density of the neutral hydrogen torus is comparable, if not greater than that of the neutral hydrogen torus maintained by Titan. The ionization of neutral particles in the Triton torus and the escape of ions from Triton's atmosphere may act as the dominant source of [[plasma (physics)|plasma]] within Neptune's magnetosphere.<ref name="Cheng1990"/>

==Structure==
Triton's atmosphere is well structured and global.<ref name="Wind direction"/> The atmosphere extends up to 870 kilometers above the surface, where the [[exobase]] is located, and had a surface pressure of about 14 microbars as of 1989. This is only 1/70,000th of the surface pressure on [[Earth]].<ref name="Voyager"/> The surface temperature was at least {{convert|35.6|K|C|lk=in}} because Triton's [[solid 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"/> 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/> The most likely temperature was {{val|38|1|u=K}} as of 1989. In the 1990s it probably increased by about 1&nbsp;K due to the general global warming as Triton approached the peak of its southern hemisphere summer (see below).<ref name=Lellouch2010/>

[[Convection]] near Triton's surface heated by the Sun creates a [[troposphere]] (a "weather region") rising to an altitude of about 8&nbsp;km. In it temperature decreases with height reaching a minimum of about 36&nbsp;K at the [[tropopause]].<ref name=Smith1989/> There is no [[stratosphere]], defined as a layer where heating from the warmer troposphere and thermosphere is balanced by radiative cooling.<ref name="McKinnonEncyclopedia"/> Higher regions include the [[thermosphere]] (8–850&nbsp;km) and [[exosphere]] (above 850&nbsp;km).<ref name="Model"/> In the thermosphere the temperature rises reaching a constant value of about 95&nbsp;K above 300&nbsp;km.<ref name=Broadfoot1989/> The upper atmosphere continuously leaks into outer space due to the weak gravity of Triton. The loss rate is about 1{{e|25}} nitrogen molecules per second, equivalent to about 0.3&nbsp;kg/s.

==Weather and climate==
{{main|Climate of Triton}}
[[File:Triton's atmosphere.jpg|thumb|upright=0.9|right|A cloud over the limb of [[Triton (moon)|Triton]], taken by ''[[Voyager 2]]''. The bottom image crops out Triton's horizon, making the thin clouds easier to see]]
[[Nitrogen]] ice particles form [[clouds]] in the troposphere a few kilometers above the surface of Triton.<ref name="Voyager"/> Above them a haze is present extending up to 30&nbsp;km from the surface.<ref name="Nine Planets"/> It is believed to be composed largely of [[hydrocarbon]]s and [[nitrile]]s created by the action of the Sun's and stellar ultraviolet light on methane.<ref name="McKinnonEncyclopedia"/>

In 1989 Voyager 2 discovered that near the surface there are winds blowing to the east or north-east with a speed of about 5–15&nbsp;m/s.<ref name="Wind direction"/> Their direction was determined by observations of dark streaks located over the southern polar cap, which generally extend from the south-west to north-east. These winds are thought to be related to the sublimation of nitrogen ice from the southern cap as there was summer in the southern hemisphere in 1989.<ref name="Wind direction"/> The gaseous nitrogen moves northward and is deflected by the [[Coriolis force]] to the east, forming an anticyclone near the surface. The tropospheric winds are capable of moving material of over a micrometer in size thus forming the streaks.<ref name="Wind direction"/>

Eight kilometers high in the atmosphere near the tropopause, the winds change direction.<ref name="The Grand Tour"/> They now flow to the west and are driven by differences in temperature between the poles and equator.<ref name="Wind direction"/><ref name=Smith1989/> These high winds may distort Triton's atmosphere making it asymmetric. An asymmetry was actually observed during star occultations by Triton in 1990s.<ref name="Elliot1997"/>

==Observations and exploration==

===Before ''Voyager 2''===
Before ''[[Voyager 2]]'' arrived, a nitrogen and methane atmosphere with a density as much as 30% that of Earth had been suggested. This proved to be a great overestimate, similar to the early predictions of the atmospheric density of [[Mars]]. However, also like on Mars, a denser early atmosphere is postulated.<ref name="Lunine1992"/>

===''Voyager 2''===
''Voyager 2'' flew past [[Triton (moon)|Triton]] five hours after its closest approach to [[Neptune]] in mid-late August 1989.<ref name="New York Times"/> During the flyby, Voyager 2 took measurements of the atmosphere,<ref name="Triton Background"/> finding methane and nitrogen in the atmosphere.<ref name="The Grand Tour"/> Voyager 2 also captured at least two plumes erupting through the nitrogen ice of Triton, and this is the first evidence of active plumes on an icy world such as Triton. The plumes were around 100&nbsp;km long and 8&nbsp;km above the surface and produced dark shadows in the images from Voyager 2.<ref name=":0" /> Around 100 dark surface fans on the SPT are attributed to the plumes. The vapor mass flux of the plumes is estimated to be around 400&nbsp;kg/s per plume. They caused large amounts of dark substrate to be thrown through the thin nitrogen ice and then into the atmosphere. The plumes captured on Triton are similar to the plumes seen on [[Enceladus]],<ref name="Dougherty"/> and the modeled ejection speeds are more consistent with a deep source.<ref name=":1" />

It has been suggested that the observed plumes are not of eruptive origin, but instead are dust devils, suggested due to the observed plumes' length-to-height ratio.<ref name=":3"/> The proposed mechanism for the formation of dust devils is that patches of the surface without nitrogen frost would heat up more quickly than the surrounding area. Given this and Triton's low surface pressure, the atmosphere would begin to heat up due to convection, to as much as 10 K greater than the surface temperature, allowing for the creation of dust devils with wind speeds of up to 20 meters per second.<ref name=":3" /> This could explain the observed surface temperature of 38 K, and remove the need to come up with a heating mechanism for the geysers.<ref name="Kerr1990"/> However, the dust devil hypothesis is largely unconsidered today, due to questions as to why more dust devils were not observed given their proposed formation and that the observed dark streaks and fans associated with the plumes do not require dust devils to explain them,<ref name="Metzger1996"/> as well as the fact the model relied upon an erroneous temperature profile for the atmosphere. As a result, eruptive-based models for the plumes are generally favored today.<ref name=":4" />

===Later observations===
In the 1990s, observations from [[Earth]] were made of the [[occultation]] of stars by Triton's [[Limb darkening|limb]]. These observations indicated the presence of a denser atmosphere than was inferred from Voyager 2 data.<ref name="Hubblesite"/> The surface pressure in the late 1990s is thought to have increased to at least 19&nbsp;μbar<ref name="Elliot2000"/> or, possibly, even to 40&nbsp;μbar.<ref name="Elliot1998"/>
Other observations have shown an increase in temperature by 5% from 1989 to 1998.<ref name="MIT Triton"/> One of the scientists involved in investigation of Triton, [[James L. Elliot]], said:<ref name="MIT Triton"/>

<blockquote>"At least since 1989, Triton has been undergoing a period of global warming. Percentage-wise, it's a very large increase."</blockquote>

These observations indicate Triton has a warm southern-hemisphere summer season that only happens once every few hundred years, near solstices.<ref name=Lellouch2010 /> Hypotheses for this warming include the sublimation of frost on Triton's surface and a decrease in ice albedo, which would allow more heat to be absorbed.<ref name=Lellouch2010/><ref name="Scienceagogo.com"/> Another theory argues the changes in temperature are a result of the deposition of dark, red material from geological processes on the moon. Because Triton's [[bond albedo]] is among the highest within the [[Solar System|solar system]], it is sensitive to small variations in spectral albedo.<ref name="Nature"/>

====Triton Watch====
The ''Triton Watch'' program was a campaign involving astronomers to monitor changes in the atmosphere of Triton. It was launched under a grant from [[NASA]].<ref name="Triton Watch"/>

=== Future exploration ===

==== Trident ====
''[[Trident (spacecraft)|Trident]]'' is a proposed NASA mission that is to further study Neptune's moon [[Triton (moon)|Triton]]. The proposed launch date of ''Trident'' is set for October 2025, arriving at the Neptune system by 2038. Triton is a probable ocean world of very high priority because of the glimpses of activity shown from the ''Voyager 2'' flyby. The origin of the activity seen from Voyager is still unclear and this makes Triton very high on the list when it comes to investigating ocean planets.<ref name="Hendrix2019"/> ''Trident'' would dramatically help in furthering knowledge of the atmosphere of Triton as well as the activity from the surface plumes captured in ''Voyager 2''. It would also help gain knowledge on the surface level of the moon and shed light on the processes that go on there.<ref name="Moran2022"/> This mission has three scientific goals it is trying to achieve, which are: if Triton has a subsurface ocean or if it has had an ocean in the past, to further understand what energy sources and sinks are at play with the resurfacing of Triton, and to investigate and study the organic constituents on Triton's surface.<ref name="Howett2020"/> To find an ocean on Triton, magnetic induction techniques will be used. The presence of the saltiness of an ocean makes it conductive, which means it is detectable to magnetic induction techniques with a spacecraft in orbit. The salinity of the ocean is mostly acquired from differentiation from volatiles in rocks on the planet and it is thought that these volatiles are sodium chloride dominant.<ref name=":1"/> To help achieve these goals, Trident would be equipped with a plasma spectrometer, a high resolution infrared spectrometer with a spectral range up to 5μm, as well as many other instruments.<ref name="Moran2022" />

==== Neptune Odyssey mission ====
The [[Neptune Odyssey]] mission concept is a flagship-class orbiter equipped with atmospheric probes that is proposed to be sent into the Neptune–Triton system. This mission would launch around 2031 and would be aboard the [[Space Launch System|SLS (Space Launch System)]] or an equivalent launch vehicle. The spacecraft would use a gravity assist from [[Jupiter]] and then cruise for 13 years to its destination in the Neptune–Triton system for its study. This mission will be trying to answer questions of: how do the interior and atmospheres of ice giants form and evolve; is Triton an ocean world; what is the cause of the plumes seen on Voyager 2; and how the geophysics of Triton can help expand the knowledge of dwarf planets such as Pluto. [1] Some measurements to be taken in this mission are: magnetic field, gravitational harmonics, spectroscopy, visible imager, ions and electrons, neutral mass spectrometry, and dust.

==See also==
* [[Climate of Triton]]
* [[Atmosphere of Pluto]]
* [[Atmosphere of Titan]]
* [[Geology of Triton]]

==References==
{{reflist|refs=

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|arxiv=1606.03293 }}</ref>

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<ref name=":0">{{cite journal |last1=Hansen |first1=Candice J. |last2=Castillo-Rogez |first2=J. |last3=Grundy |first3=W. |last4=Hofgartner |first4=J. D. |last5=Martin |first5=E. S. |last6=Mitchell |first6=K. |last7=Nimmo |first7=F. |last8=Nordheim |first8=T. A. |last9=Paty |first9=C. |last10=Quick |first10=L. C. |last11=Roberts |first11=J. H. |date=2021-07-27 |title=Triton: Fascinating Moon, Likely Ocean World, Compelling Destination! |journal=The Planetary Science Journal |language=en |volume=2 |issue=4 |pages=137 |doi=10.3847/PSJ/abffd2 |bibcode=2021PSJ.....2..137H |s2cid=236464096 |issn=2632-3338 |doi-access=free}}</ref>

<ref name=":1">{{cite book |last1=Frazier |first1=William |last2=Bearden |first2=David |last3=Mitchell |first3=Karl L |last4=Lam |first4=Try |last5=Prockter |first5=Louise |last6=Dissly |first6=Richard |title=2020 IEEE Aerospace Conference |chapter=Trident: The Path to Triton on a Discovery Budget |date=March 2020 |chapter-url=https://ieeexplore.ieee.org/abstract/document/9172502/references#references |pages=1–12 |doi=10.1109/AERO47225.2020.9172502 |isbn=978-1-7281-2734-7 |s2cid=221283146}}</ref>

<ref name="Howett2020">{{Cite conference |last1=Howett |first1=Carly |last2=Procktor |first2=Louise |last3=Mitchell |first3=Karl |last4=Bearden |first4=David |last5=Smythe |first5=William |date=2020-08-04 |title=Trident: A Mission to Explore Triton, a Candidate Ocean World |work=EPSC Abstracts |volume=14 |doi=10.5194/epsc2020-138 |conference=Europlanet Science Congress 2020 |url=https://meetingorganizer.copernicus.org/EPSC2020/EPSC2020-138.html |doi-access=free }}</ref>

<ref name="Moran2022">{{Cite journal |last1=Moran |first1=Sarah E. |last2=Hörst |first2=Sarah M. |last3=He |first3=Chao |last4=Radke |first4=Michael J. |last5=Sebree |first5=Joshua A. |last6=Izenberg |first6=Noam R. |last7=Vuitton |first7=Véronique |last8=Flandinet |first8=Laurène |last9=Orthous-Daunay |first9=François-Régis |last10=Wolters |first10=Cédric |date=January 2022 |title=Triton Haze Analogs: The Role of Carbon Monoxide in Haze Formation |journal=Journal of Geophysical Research: Planets |language=en |volume=127 |issue=1 |doi=10.1029/2021JE006984 |arxiv=2112.11627 |bibcode=2022JGRE..12706984M |s2cid=245385730 |issn=2169-9097|doi-access=free }}</ref>

<ref name="Ohno2021">{{Cite journal |last1=Ohno |first1=Kazumasa |last2=Zhang |first2=Xi |last3=Tazaki |first3=Ryo |last4=Okuzumi |first4=Satoshi |date=May 2021 |title=Haze Formation on Triton |journal=The Astrophysical Journal |language=en |volume=912 |issue=1 |page=37 |doi=10.3847/1538-4357/abee82 |doi-access=free |arxiv=2012.11932 |bibcode=2021ApJ...912...37O }}</ref>

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<ref name="Scienceagogo.com">{{cite web |title=Global Warming Detected on Triton |url=http://www.scienceagogo.com/news/19980526052143data_trunc_sys.shtml |date=1998-05-28 |publisher=Scienceagogo.com |access-date=2007-12-31 |archive-url=https://web.archive.org/web/20071214235332/http://www.scienceagogo.com/news/19980526052143data_trunc_sys.shtml |archive-date=14 December 2007 |url-status=live}}</ref>

<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]] | access-date=2007-12-31| archive-url= https://web.archive.org/web/20071217130704/http://web.mit.edu/newsoffice/1998/triton.html| archive-date= 17 December 2007 | url-status= live}}</ref>

<ref name="Elliot1998">{{cite journal| title=Global warming on Triton | url=http://occult.mit.edu/_assets/documents/publications/Elliot1998Nature393.766.pdf | last=Elliot | first=J.L. |author2=Hammel, H.B. |author3=Wasserman, L.H. | journal=[[Nature (journal)|Nature]] |volume=393 |date=1998| issue=6687 |pages=765–767 |doi=10.1038/31651 |bibcode=1998Natur.393..765E | s2cid=40865426 |display-authors=etal}}</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="Triton Background">{{cite web |url=http://www.boulder.swri.edu/TritonWatch/background.html |title=Triton: Background and Science |work=Planetary Science Directorate, Boulder Office |access-date=2007-12-31 |archive-url= https://web.archive.org/web/20080119204707/http://www.boulder.swri.edu/TritonWatch/background.html |archive-date= 19 January 2008 |url-status=live}}</ref>

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}} <!--- END OF REFLIST --->

== Notes ==
{{Notelist}}

==External links==
* [http://voyager.jpl.nasa.gov/ Voyager website]

{{Triton}}
{{Atmospheres}}

[[Category:Satellite atmospheres|Triton]]
[[Category:Planetary atmospheres of the Solar System|Triton]]
[[Category:Triton (moon)]]