View source for Proteus (moon)

{{Short description|Large moon of Neptune}}
{{Infobox planet
| name = Proteus
| image = Proteus (Voyager 2).jpg
| caption = Processed grayscale image of Proteus from ''[[Voyager 2]]'', August 1989 (image processing date). The massive crater [[Pharos (crater)|Pharos]] occupies much of the upper right, straddling Proteus's [[terminator (solar)|terminator]]
| discoverer = ''[[Voyager 2]]'' <br /> [[Stephen P. Synnott]]
| discovered = June 16, 1989
| mpc_name = Neptune VIII
| alt_names = S/1989 N 1
| pronounced = {{IPAc-en|ˈ|p|r|oʊ|t|i|ə|s}} {{respell|PROH|tee|əs}}<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Proteus |archive-url=https://web.archive.org/web/20200322182516/https://www.lexico.com/definition/proteus |url-status=dead |archive-date=March 22, 2020 |title=Proteus |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref>
| adjectives = Protean ({{IPAc-en|ˈ|p|r|oʊ|t|i|ə|n}} {{respell|PROH|tee|ən}} or {{IPAc-en|p|r|oʊ|ˈ|t|iː|ə|n}} {{respell|proh|TEE|ən}})<ref>{{OED|Protean}}</ref>
| named_after = [[Proteus (mythology)|Πρωτεύς or Πρωτέας]], ''Prōteys or Prōteas''
| orbit_ref = <ref name="Jacobson2004" />
| epoch = 18 August 1989
| semimajor = {{val|117647|1|u=km}} (4.75&nbsp;R<sub>N</sub>)
| eccentricity = {{val|0.00053|0.00009}}
| periapsis = {{val|117584|10|u=km}}
| apoapsis = {{val|117709|10|u=km}}
| period = {{val|1.12231477|0.00000002|ul=d}}
| avg_speed = {{val|7.623|u=km/s}}
| inclination = {{val|0.524|u=°}} (to Neptune's equator) <br> {{val|0.026|0.007|u=°}} (to local [[Laplace plane]])
| satellite_of = [[Neptune]]
| dimensions = {{val|424|x|390|x|396|u=km}}<ref name="Stooke1994" /><br>{{refn | name="earlier papers" | group = lower-alpha | In other papers slightly different dimensions were reported. Thomas and Veverka in 1991 reported {{val|440|x|416|x|404|u=km}}.<ref name="Stooke1994" /><ref name="Williams2008-nssdc" /> Croft in 1992 reported {{val|430|x|424|x|410|u=km}}.<ref name="Croft1992" /> Karkoschka in 2003 reported ({{val|440|12}})&thinsp;×&thinsp;({{val|416|16}})&thinsp;×&thinsp;({{val|404|20}}) km.<ref name="Karkoschka2003" /> The difference is caused by the use of different sets of images and by the fact that the shape of Proteus is not described well by a triaxial ellipsoid.<ref name="Stooke1994" /> }}
| mean_radius = {{val|209|8|u=km}}<ref name="Croft1992" /><br>{{val|210|7|u=km}}<ref name="Karkoschka2003" />
| volume = {{val|3.4|0.4|e=7|u=km3}}<ref name="Stooke1994" />
| surface_area = 554 200 km<sup>2</sup><ref name="NASA BTN">{{cite web|url=https://solarsystem.nasa.gov/moons/neptune-moons/proteus/by-the-numbers/ |title=Proteus By The Numbers|website=solarsystem.nasa.gov/|access-date=September 4, 2020}}</ref>
| mass = ≈&thinsp;{{val|1.55|-|3.10|e=19|u=kg}}{{efn|A density of {{val|0.4|-|0.8|u=g/cm3}} was calculated. The volume was approximated as a sphere with a radius of {{val|210|7|u=km}}.<ref name="ZhangHamiltonII"/> The mass was calculated with the provided density and the assumed volume.}}<br>≈&thinsp;{{val|2.60|-|5.20|e=-6|u=[[Earth mass|Earths]]}}
| density = ≈&thinsp;{{val|0.46|-|0.91|u=g/cm3}}<ref name="ZhangHamiltonII"/>{{efn|Density obtained from the calculated mass and the provided volume of {{val|3.4|0.4|e=7|u=km3}} from Stooke (1994).<ref name="Stooke1994" />}}
| surface_grav = ≈&thinsp;{{Gr|0.0155|212}}–{{Gr|0.0310|195}} [[Acceleration|m/s<sup>2</sup>]]{{efn|Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\frac{Gm}{r^2}</math>}}
| escape_velocity = ≈&thinsp;{{V2|0.0155|212}}–{{V2|0.0310|195}} km/s{{efn|Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\sqrt{\frac{2Gm}{r}}</math>}}
| rotation = [[synchronous rotation|synchronous]]<ref name="Stooke1994" />
| axial_tilt = zero<ref name="Stooke1994" />
| albedo = 0.096<ref name="jplssd" /><ref name="Karkoschka2003" />
| magnitude = 19.7<ref name="jplssd" />
| single_temperature = ≈&thinsp;51 [[Kelvin|K]] mean (estimate)
| note = no
}}

'''Proteus''' ({{IPAc-en|ˈ|p|r|oʊ|t|i|ə|s}} {{respell|PROH|tee|əs}}), also known as '''Neptune&nbsp;VIII''', is the second-largest [[Neptune|Neptunian]] [[natural satellite|moon]], and Neptune's largest [[Inner moon|inner satellite]]. Discovered by ''[[Voyager 2]]'' in 1989, it is named after [[Proteus]], the shape-changing sea god of [[Greek mythology]].<ref name="IAUC 5347" /> Proteus orbits Neptune in a nearly equatorial [[orbit]] at a distance of about 4.75 times the radius of Neptune's equator.<ref name="Jacobson2004" />

Despite being a predominantly icy body more than {{convert|400|km|mi|abbr=on}} in diameter, Proteus's shape deviates significantly from an [[ellipsoid]].<ref name="Croft1992" /> It is shaped more like an irregular [[polyhedron]] with several slightly concave facets and relief as high as {{convert|20|km|mi|abbr=on}}. Its surface is dark, neutral in color, and heavily cratered.<ref name="DumasSmithTerrile2003" /> Proteus's largest crater is [[Pharos (crater)|Pharos]], which is more than {{convert|230|km|mi|abbr=on}} in diameter. There are also a number of [[Cliff|scarps]], grooves, and [[valley]]s related to large craters.

Proteus is probably not an original body that formed with Neptune. It could have accreted later from the debris formed when the largest Neptunian satellite, [[Triton (moon)|Triton]], was captured.<ref name="Goldreich1989" />

== Discovery and naming ==
[[File:Voyager.jpg|left|thumb|upright|''Voyager 2,'' the space probe that discovered Proteus]]
Proteus was discovered from the images taken by the ''[[Voyager 2]]'' space probe two months before its Neptune flyby in August 1989. Proteus was discovered 40 years after the discovery of Neptune's moon [[Nereid (moon)|Nereid]] in 1949.<ref name="indepth"/>

Upon discovery, Proteus received the temporary [[provisional designation]] '''S/1989&nbsp;N&nbsp;1'''.<ref name="SmithSoderblom1989" /> [[Stephen P. Synnott]] and [[Bradford A. Smith]] announced its discovery on July 7, 1989, speaking only of "17 frames taken over 21 days", which gives a discovery date of sometime before June 16.<ref name="IAUC 4806" />

On 16 September 1991, S/1989&nbsp;N&nbsp;1 was named after [[Proteus]], the shape-changing sea god of [[Greek mythology]]<ref name="IAUC 5347" /> following the convention that moons of Neptune should be named after deities and creatures related to the sea.

== Orbit ==
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| caption = [[Hubble Space Telescope]] image of Neptune and Proteus
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Proteus orbits Neptune at a distance of approximately {{convert|117647|km|mi|abbr=on}}, nearly equal to 4.75 times the equatorial radius of the planet. The orbit of Proteus is nearly circular, having a small [[orbital eccentricity]], and is inclined by about 0.5 degrees to Neptune's equator.<ref name="Jacobson2004" /> Proteus is [[tidal locking|tidally locked]] to Neptune, and rotates [[synchronous orbit|synchronously]] with its orbital motion, which means that Proteus always presents the same face to Neptune.<ref name="Stooke1994" />

Proteus may have once been in a 1:2 [[orbital resonance]] with [[Larissa (moon)|Larissa]], where Proteus made one orbit for every two orbits made by Larissa. Due to the outward tidal migration of Proteus over time, this is no longer the case.<ref name="ZhangHamiltonI"/> Proteus may have ceased its integral orbital resonance with [[Larissa (moon)|Larissa]] several hundred million years ago.<ref name="ZhangHamiltonI"/><ref name="ZhangHamiltonII"/>

== Physical characteristics ==
[[File:Proteus_Moon_Earth.png|left|thumb|Size comparison between Proteus (lower left), the [[Moon]] (upper left) and [[Earth]]]]
Proteus is the second-largest moon of Neptune and is the largest of its regular prograde moons. It is about {{convert|420|km|mi|abbr=on}} in diameter, larger than [[Nereid (moon)|Nereid]], Neptune's third-largest moon. It was not discovered by [[Earth]]-based telescopes because Proteus orbits so close to Neptune that it is lost in the glare of reflected sunlight.<ref name="SmithSoderblom1989" />

=== Composition ===
[[File:Proteus Color.png|right|thumb|upright|False-color ''Voyager 2'' image of Proteus, with its large crater Pharos located at the right]]
The surface of Proteus is dark as it has a [[geometrical albedo]] of about 10 percent–meaning that its surface reflects 10 percent of the light it receives from the [[Sun]]. The color of its surface is neutral as the [[reflectivity]] does not change appreciably with the wavelength from violet to green.<ref name="SmithSoderblom1989" /> In the near-[[infrared]] range of around the wavelength of 2&nbsp;μm, Proteus's surface becomes less reflective, suggesting the presence of complex organic compounds such as [[hydrocarbon]]s or [[cyanide]]s. These compounds may be responsible for the low albedo of the inner Neptunian moons. While Proteus is usually thought to contain significant amounts of water ice, there is no definite spectroscopic confirmation of its presence on the surface.<ref name="DumasSmithTerrile2003" /> However, [[James Webb Space Telescope]] [[NIRCam]] [[Spectrophotometry, infrared|spectrophotometry]] of the moon has revealed a 3 μm absorption feature, indicating possible water ice or hydrated minerals on the surface of Proteus.<ref name=":0">{{Cite journal |last1=Belyakov |first1=Matthew |last2=Davis |first2=M. Ryleigh |last3=Milby |first3=Zachariah |last4=Wong |first4=Ian |last5=Brown |first5=Michael E. |date=2024-05-01 |title=JWST Spectrophotometry of the Small Satellites of Uranus and Neptune |journal=The Planetary Science Journal |volume=5 |issue=5 |pages=119 |doi=10.3847/PSJ/ad3d55 |doi-access=free |arxiv=2404.06660 |bibcode=2024PSJ.....5..119B |issn=2632-3338}}</ref> Proteus's infrared albedo measured at 1.4, 2.1, 3.0, and 4.6 microns appears to match other dark Solar System small bodies, such as [[(120216) 2004 EW95|2004 EW95]].<ref name=":0" />

=== Shape ===

The shape of Proteus is close to a sphere with a radius of about {{convert|210|km|mi|abbr=on}}, although deviations from the spherical shape are large—up to {{convert|20|km|mi|abbr=on}}; scientists believe it is about as large as a body of its density can be without being pulled into a perfect [[List of natural satellites|spherical shape]] by its own [[gravity]].<ref name="Croft1992" /> Proteus is slightly elongated in the direction of Neptune, although its overall shape is closer to an irregular [[polyhedron]] than to a [[triaxial ellipsoid]]. The surface of Proteus shows several flat or slightly concave facets measuring from 150 to 200&nbsp;km in diameter. They are probably degraded [[impact crater]]s.<ref name="Stooke1994" />
{{clear|left}}

=== Surface features ===
[[File:Proteus - August 25 1989 (31541728335).jpg|thumb|upright|left|''Voyager 2'' image of Proteus, [[Digital image processing|digitally processed]] showing surface features]]
[[File:Proteus map (Stooke).jpg|130px|thumb|upright|Map of Proteus]]

Proteus is heavily cratered, showing no sign of any geological modification.<ref name="SmithSoderblom1989" /> The largest crater, Pharos, has a diameter from {{nowrap|230 to 260 km}}.<ref name="Croft1992" /> Its depth is about 10–15&nbsp;km.<ref name="Stooke1994" /> The crater has a central dome on its floor a few kilometres high.<ref name="Stooke1994" /> Pharos is the only named surface feature on this moon. In [[Greek mythology]], it refers to the [[Lighthouse of Alexandria|island]] where Proteus reigned.<ref name="usgsPharos" /> In addition to Pharos, there are several craters 50–100&nbsp;km in diameter and many more with diameters less than 50&nbsp;km.<ref name="Stooke1994" />

Other [[landforms]] on Proteus include linear features such as [[Cliff|scarps]], valleys, and grooves. The most prominent one runs parallel to the equator to the west of Pharos. These features likely formed as a result of the giant impacts, which formed Pharos and other large craters or as a result of [[tidal force|tidal stresses]] from Neptune.<ref name="Stooke1994" /><ref name="Croft1992" />

=== Named features ===
Protean craters are named after water-related spirits, gods, goddesses (excluding Greek and Roman names). As of May 2024, there is only one named crater on this body.<ref name="usgsPharos" />

{| class="wikitable"
! [[impact crater|Crater]] !! Pronunciation !! [[Diameter]]!! Approval<br />Year !![[Eponym]] !!Ref
|-id="Pharos"
| [[Pharos (crater)|Pharos]] || {{IPAc-en|ˈ|f|ɛər|ɒ|s}} {{respell|FAIR|oss}} || align=right | 230–260 km<ref name="Croft1992" /> || align=center | 1994 || [[Lighthouse of Alexandria]] (Pharos), island where [[Proteus]] reigned || {{WGPSN|4684}}
|}

== Origin ==

Proteus, like the other inner moons of Neptune, is unlikely to be an original body that formed with it, and is more likely to have accreted from the rubble that was produced after Triton's capture. Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian moons, causing them to collide and reduce to a disc of rubble.<ref name="Goldreich1989" /> Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day moons of Neptune.<ref name="BanfieldMurray1992" />

Proteus initially had a smaller orbit around Neptune, and may have formed closer to the planet. At the time of the formation of Proteus, its orbit was estimated to be {{convert|8000|km|mi|abbr=on}} less than its current distance from Neptune.<ref name="Showalter2019"/> Over time, Proteus migrated outward from Neptune through tidal interactions. During the outward migration of Proteus, collisions and [[impact event]]s may have formed large craters and had ejected fragments of Proteus into orbit around Neptune. A collision of Proteus and another object could possibly explain the origin of Neptune's smaller moon [[Hippocamp (moon)|Hippocamp]], which orbits close to Proteus.<ref name="Showalter2019"/>

== Notes ==

{{reflist
| group = lower-alpha
}}

== References ==

{{reflist
| colwidth = 30em
| refs =

<ref name="Jacobson2004">
{{cite journal| doi = 10.1086/423037| last1 = Jacobson | first1 = R. A.| last2 = Owen | first2 = W. M. Jr. | year = 2004| pages = 1412–1417| title = The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations| journal = Astronomical Journal| volume = 128| issue = 3| bibcode = 2004AJ....128.1412J| doi-access = }}
</ref>

<ref name="Stooke1994">
{{cite journal| doi = 10.1007/BF00572198| last = Stooke| first = Philip J.| year = 1994| title = The surfaces of Larissa and Proteus| journal = Earth, Moon, and Planets| volume = 65| issue = 1| pages = 31–54| bibcode = 1994EM&P...65...31S| s2cid = 121825800}}
</ref>

<ref name="Williams2008-nssdc">
{{cite web
| last = Williams
| first = Dr. David R.
| date = 2008-01-22
| title = Neptunian Satellite Fact Sheet
| publisher = [[NASA]] (National Space Science Data Center)
| url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html
| access-date = 2008-12-12
}}
</ref>

<ref name="Croft1992">
{{Cite journal | year = 1992 | bibcode = 1992Icar...99..402C | pages = 402–408 | volume = 99 | first1 = S. | doi = 10.1016/0019-1035(92)90156-2 | title = Proteus: Geology, shape, and catastrophic destruction | last1 = Croft | journal = Icarus | issue = 2}}
</ref>

<ref name="jplssd">
{{cite web
| title = Planetary Satellite Physical Parameters
| date = 2010-10-18
| publisher = [[JPL]] (Solar System Dynamics)
| url = http://ssd.jpl.nasa.gov/?sat_phys_par
| access-date = 2011-10-11
}}
</ref>

<ref name="Karkoschka2003">
{{cite journal| doi = 10.1016/S0019-1035(03)00002-2| last = Karkoschka| first = Erich| year = 2003| title = Sizes, shapes, and albedos of the inner satellites of Neptune| journal = Icarus| volume = 162| issue = 2| pages = 400–407| bibcode = 2003Icar..162..400K}}
</ref>

<ref name="SmithSoderblom1989">
{{cite journal| doi = 10.1126/science.246.4936.1422| last1 = Smith| first1 = B. A.| last2 = Soderblom| first2 = L. A.| last3 = Banfield| first3 = D.| last4 = Barnet| first4 = C.| last5 = Basilevsky| first5 = A. T.| last6 = Beebe| first6 = R. F.| last7 = Bollinger| first7 = K.| last8 = Boyce| first8 = J. M.| last9 = Brahic| first9 = A.| year = 1989| title = Voyager 2 at Neptune: Imaging Science Results| journal = Science| volume = 246| issue = 4936| pages = 1422–1449| bibcode = 1989Sci...246.1422S| pmid = 17755997| s2cid = 45403579| ref = {{sfnRef|Smith Soderblom et al.|1989}}| url = https://zenodo.org/record/1230992}}
</ref>

<ref name="IAUC 4806">
{{cite journal
| last = Green
| first = Daniel W. E.
| title = 1989 N 1
| date = July 7, 1989
| journal = IAU Circular
| volume = 4806
| url = http://www.cbat.eps.harvard.edu/iauc/04800/04806.html
| access-date = 2011-10-24
}}
</ref>

<ref name="IAUC 5347">
{{cite journal
| last = Marsden
| first = Brian G.
| title = Satellites of Saturn and Neptune
| date = September 16, 1991
| journal = IAU Circular
| volume = 5347
| url = http://www.cbat.eps.harvard.edu/iauc/05300/05347.html
| access-date = 2011-10-24
}}
</ref>

<ref name="DumasSmithTerrile2003">
{{cite journal| last1 = Dumas| first1 = Christophe| last2 = Smith| first2 = Bradford A.| last3 = Terrile| first3 = Richard J.| year = 2003| title = Hubble Space Telescope NICMOS Multiband Photometry of Proteus and Puck| journal = [[The Astronomical Journal]]| volume = 126| issue = 2| pages = 1080–1085| doi = 10.1086/375909| bibcode = 2003AJ....126.1080D| ref = {{sfnRef|Dumas Smith et al.|2003}}| doi-access = }}
</ref>

<ref name="usgsPharos">
{{cite web
| title = Planetary Names: Crater, craters: Pharos on Proteus
| publisher = USGS Astrogeology
| work = Gazetteer of Planetary Nomenclature
| url = https://planetarynames.wr.usgs.gov/Feature/4684?__fsk=-1708907535
| access-date = 24 June 2021
}}
</ref>

<ref name="Goldreich1989">
{{cite journal
| last1 = Goldreich
| first1 = P.
| last2 = Murray | first2 = N.
| last3 = Longaretti | first3 = P. Y.
| last4 = Banfield | first4 = D.
| title = Neptune's story
| journal = Science
| volume = 245
| issue = 4917
| date = 1989
| pages = 500–504
| bibcode = 1989Sci...245..500G
| doi = 10.1126/science.245.4917.500
| pmid = 17750259
| s2cid = 34095237
}}
</ref>

<ref name="BanfieldMurray1992">
{{cite journal| doi = 10.1016/0019-1035(92)90155-Z| last1 = Banfield| first1 = Don| last2 = Murray| first2 = Norm| date=October 1992 | title = A dynamical history of the inner Neptunian satellites| journal = Icarus| volume = 99| issue = 2| pages = 390–401| bibcode = 1992Icar...99..390B}}
</ref>

<ref name="indepth">{{cite web|title=Proteus In Depth|url=https://solarsystem.nasa.gov/moons/neptune-moons/proteus/in-depth/|publisher=NASA Solar System Exporation|access-date=12 March 2019|archive-url=https://web.archive.org/web/20230921191706/https://solarsystem.nasa.gov/moons/neptune-moons/proteus/in-depth/|archive-date=2023-09-21}}</ref>

<ref name="Showalter2019">{{cite journal|last1= Showalter|first1=M. R.|last2=de Pater|first2= I.|last3= Lissauer|first3=J. J.|last4= French|first4=R. S.|url=https://www.spacetelescope.org/static/archives/releases/science_papers/heic1904/heic1904a.pdf|title= The seventh inner moon of Neptune|journal= Nature|volume= 566|issue= 7744|year= 2019|pages= 350–353|doi= 10.1038/s41586-019-0909-9|pmid= 30787452|pmc= 6424524|bibcode= 2019Natur.566..350S}}</ref>

<ref name="ZhangHamiltonI">{{cite journal |last1=Zhang |first1=K. |last2=Hamilton |first2=D. P. |year=2007 |title=Orbital resonances in the inner Neptunian system: I. The 2:1 Proteus–Larissa mean-motion resonance |journal=[[Icarus (journal)|Icarus]] |volume=188 |issue=2 |pages=386–399 |bibcode=2007Icar..188..386Z |doi=10.1016/j.icarus.2006.12.002}}</ref>

<ref name="ZhangHamiltonII">{{cite journal |last1=Zhang |first1=K. |last2=Hamilton |first2=D. P. |year=2008 |title=Orbital resonances in the inner Neptunian system: II. Resonant history of Proteus, Larissa, Galatea, and Despina |journal=[[Icarus (journal)|Icarus]] |volume=193 |issue=1 |pages=267–282 |bibcode=2008Icar..193..267Z |doi=10.1016/j.icarus.2007.08.024}}</ref>
}}

== External links ==
{{Commons category|Proteus (moon)}}
* [https://web.archive.org/web/20200217213118/https://solarsystem.nasa.gov/moons/neptune-moons/proteus/in-depth/ Proteus In Depth] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* [http://www.nineplanets.org/proteus.html Proteus page] at ''The Nine Planets''
* [http://www.solarviews.com/eng/proteus.htm Proteus, A Moon Of Neptune] on ''Views of the Solar System''
* [https://web.archive.org/web/20071004222910/http://www.strykfoto.org/proteus.htm Ted Stryk's Proteus Page]
* [https://web.archive.org/web/20110628211148/http://www.dtm.ciw.edu/users/sheppard/satellites/nepsatdata.html Neptune's Known Satellites] (by [[Scott S. Sheppard]])

{{Moons of Neptune}}
{{Neptune}}
{{Solar System moons (compact)}}

{{DEFAULTSORT:Proteus (Moon)}}
[[Category:Proteus (moon)| ]]
[[Category:Moons of Neptune]]
[[Category:Astronomical objects discovered in 1989|19890616]]
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[[Category:Moons with a prograde orbit]]