View source for Charon (moon)

{{Use mdy dates|date=October 2022}}
{{distinguish|text=the minor planet [[2060 Chiron]]}}
{{short description|Largest natural satellite of Pluto}}
{{Infobox planet
| name = Charon
| image = Charon in True Color - High-Res.jpg
| background = Gainsboro
| caption = Charon as imaged by the ''[[New Horizons]]'' spacecraft, July 2015. A massive fault system involving [[Serenity Chasma]] and [[Mandjet Chasma]] crosses Charon's equator, while Charon's north pole is covered by the dark [[Mordor Macula]]
| discoverer = [[James W. Christy]]
| discovered = June 22, 1978
| alt_names = S/1978 P 1
| mpc_name = Pluto I<ref name="Nomenclature">{{cite web
 |date=November 9, 2009
 |title=Gazetteer of Planetary Nomenclature
 |publisher=IAU Working Group for Planetary System Nomenclature (WGPSN)
 |first=Jennifer |last=Blue
 |url = http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets
 |access-date=February 24, 2010}}</ref>
| pronounced = {{IPAc-en|ˈ|k|ɛər|ɒ|n|,_|-|ə|n}} {{respell|KAIR|on|,_-|ən}}<ref name=OED/> or {{IPAc-en|ˈ|ʃ|ær|ə|n}} {{respell|SHARR|ən}}<ref name=Sharon/><ref group=note name=pron>The former is the anglicized pronunciation of the {{Langx|grc|Χάρων}}, the latter is the discoverer's pronunciation.</ref>
| note = yes
| adjective = Charonian<ref group=note>From the alternative Latin oblique form {{L&S|Charon|''Charōnis''|ref}}</ref><ref>{{Cite book |url=http://link.springer.com/10.1007/978-0-387-89518-5 |title=New Horizons: Reconnaissance of the Pluto-Charon System and the Kuiper Belt |date=2009 |publisher=Springer New York |isbn=978-0-387-89517-8 |editor-last=Russell |editor-first=C. T. |location=New York, NY |pages=96 |doi=10.1007/978-0-387-89518-5 |language=en}}</ref><ref>{{Cite book |title=Theater outside Athens: drama in Greek Sicily and South Italy |date=2016 |publisher=Cambridge University Press |isbn=978-1-107-52750-8 |editor-last=Bosher |editor-first=Kathryn |location=Cambridge |pages=100, 104–105}}</ref><br>Charontian, -ean<ref group=note>From the Latin oblique form {{L&S|Charon|''Charontis''|ref}} (cf. Italian, Spanish, and Portuguese ''Caronte'')</ref><ref>{{Cite book |title=Studies in honor of Gerald E. Wade |date=1979 |publisher=J. Porrúa Turanzas |isbn=978-84-7317-086-4 |editor-last=Wade |editor-first=Gerald Edward |series=Studia humanitatis |location=Madrid |pages=125–126 |language=en, es |editor-last2=Bowman |editor-first2=Sylvia E.}}</ref><ref>{{Cite book |last=Herbert |first=William |url=https://books.google.com/books?id=mFiEy0DIFwQC |title=Attila, King of the Huns |date=1838 |publisher=Henry G. Bohn |pages=48 |language=en}}</ref><br>Charonean<ref group=note>From the Latin adjectival form {{L&S|Charon|''Charōnēus''|ref}}</ref><ref>{{Cite book |last=Kontou |first=Tatiana |url=https://books.google.com/books?id=WfKTnQAACAAJ |title=Spiritualism and Women's Writing: From the Fin de Siècle to the Neo-Victorian |date=2009 |publisher=Palgrave Macmillan |isbn=978-1-349-29915-7 |pages=60 |language=en}}</ref>
| named_after = Discoverer's wife, Charlene, and [[Charon (mythology)|Χάρων]] ''Kharōn''
| orbit_ref = &nbsp;<ref name="Buie_2006" /><!-- https://arxiv.org/pdf/astro-ph/0512491.pdf was A. J. Steffl, M. J. Mutchler, H. A. Weaver, S. A. Stern, D. D. Durda, D. Terrell, W. J. Merline, L. A. Young, E. F. Young, M. W. Buie, and J. R. Spencer, ''New Constraints on Additional Satellites of the Pluto System'', Astronomical Journal, Volume 132, Issue 2, pp. 614–619 (August 2006)-->
| epoch = {{val|2452600.5}}<br />(2002 Nov 22)
| semimajor = {{val|19595.764|0.007|0.008}} km (planetocentric)<ref name="Brozovic2024"/><br>{{val|17181.0}} km (barycentric)
| eccentricity = 0.000161<ref name="Brozovic2024"/>
| periapsis = 19 592.61 km
| apoapsis = 19 598.92 km
| period = {{val|6.387221|0.000005|0.000003|u=d}}<br>(6 d, 9 h, 17 m, 35.89 ± 0.35 s)<ref name="Brozovic2024"/>
| avg_speed = 0.21 km/s{{efn|group=note|name=calculated|Calculated on the basis of other parameters.}}
| inclination = {{val|0.080|u=°}} {{nowrap|(to Pluto's equator)}}<ref name="JPL_Pluto"/><br>{{val|119.591|0.014|u=°}} {{nowrap|(to Pluto's orbit)}}<br>{{val|112.783|0.014|u=°}} {{nowrap|(to the [[ecliptic]])}}
<!-- The latter are based on the Charonian orbital pole of α=133.046±0.014°, δ=-6.145±0.014° in [[J2000]] coordinates ([[International Celestial Reference Frame|ICRF]]) given in Buie et al. (ref name="Buie_2006")-->| asc_node = {{val|223.046|0.014|u=°}} {{nowrap|(to [[Equinox (celestial coordinates)|vernal equinox]])}}
| satellite_of = [[Pluto]]
| mean_radius = {{val|606.0|0.5|u=km}}<ref name="Stern_2015" /><ref name="Stern_2017" /> {{nowrap|(0.095 Earths, 0.51 Plutos)}}
| flattening = <0.5%<ref name="Nimmo2017"/>
| surface_area = {{val|4.6|e=6|u=km2}} {{nowrap|(0.0090 Earths)}}
| volume = {{val|9.32|.14|e=8|u=km3}} {{nowrap|(0.00086 Earths)}}
| mass = {{val|1.5897|0.0045|e=21|u=kg}}<ref name="Brozovic2024"/><br>({{val|2.66|e=-4|u=Earths}})<br>(12.2% of Pluto)
| density = {{val|1.705|0.006|u=g/cm3}}<ref name="Brozovic2024"/>
| surface_grav = {{val|0.288|ul=m/s2}}
| escape_velocity = {{val|0.59|u=km/s}}<br>0.37 mi/s
| single_temperature = {{val|-220|u=°C}} ({{val|53|ul=K}})
| rotation = [[Synchronous rotation|synchronous]]
| axial_tilt =
| albedo = 0.2 to 0.5 at a solar phase angle of 15°
| magnitude = 16.8<ref name="magnitude">{{cite web
 |title        = Classic Satellites of the Solar System
 |url          = http://www.oarval.org/ClasSaten.htm
 |website      = Observatorio ARVAL
 |date         = April 15, 2007
|access-date   = October 19, 2007
 |archive-url  = https://web.archive.org/web/20100731193653/http://www.oarval.org/ClasSaten.htm
 |archive-date = July 31, 2010
 |url-status=dead
}}</ref>
| abs_magnitude = 1<ref name="jewitt1000km">{{cite web
 | date = June 2008
 | title = The 1000 km Scale KBOs
 | publisher = Institute for Astronomy (UH)
 | first = David | last = Jewitt
 | url = http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html
 | access-date = June 13, 2008 }}</ref>
| angular_size = 55 milli-arcsec<ref>{{cite press release
 | url = http://www.eso.org/public/outreach/press-rel/pr-2006/pr-02-06.html
 | title = Measuring the Size of a Small, Frost World
 | publisher = European Southern Observatory
 | date = January 4, 2006
 | access-date = October 19, 2007
 | archive-url=https://web.archive.org/web/20060118020147/http://eso.org/outreach/press-rel/pr-2006/pr-02-06.html |archive-date=January 18, 2006}}</ref>
}}

'''Charon''' ({{IPAc-en|ˈ|k|ɛər|ɒ|n|,_|-|ə|n}} {{respell|KAIR|on|,_-|ən}} or {{IPAc-en|ˈ|ʃ|ær|ə|n}} {{respell|SHARR|ən}}),<ref group=note name=pron/> or '''(134340) Pluto I''', is the largest of the [[Moons of Pluto|five known natural satellites]] of the [[dwarf planet]] [[Pluto]]. It has a [[mean radius]] of {{Cvt|606|km||abbr=}}. Charon is the sixth-largest known [[trans-Neptunian object]] after Pluto, [[Eris (dwarf planet)|Eris]], [[Haumea]], [[Makemake]], and [[Gonggong (dwarf planet)|Gonggong]].<ref>{{Cite web|url=http://www.johnstonsarchive.net/astro/tnos.html#LAR|title=Trans-Neptunian objects}}</ref> It was discovered in 1978 at the [[United States Naval Observatory]] in [[Washington, D.C.]], using photographic plates taken at the [[United States Naval Observatory Flagstaff Station]] (NOFS).

With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the [[barycenter]] of the Plutonian system lies outside Pluto, and the two bodies are [[tidally locked]] to each other.<ref>{{Cite book |last=Borgia |first=Michael P. |url=https://books.google.com/books?id=3AODx2z63vEC&pg=PA201 |title=Human Vision and the Night Sky: Hot to Improve Your Observing Skills |publisher=Springer New York Springer e-books |year=2006 |isbn=978-0-387-46322-3 |series=Patrick Moore's Practical Astronomy Series |location=New York, NY |page=201}}</ref> The dwarf planet systems Pluto–Charon and Eris–[[Dysnomia (moon)|Dysnomia]] are the only known examples of mutual tidal locking in the Solar System,<ref name="Szakats2022">{{cite journal
  |display-authors = etal
  |first1 = R. |last1 = Szakáts
  |first2 = Cs. |last2 = Kiss
  |first3 = J. L. |last3 = Ortiz
  |first4 = N. |last4 = Morales
  |first5 = A. |last5 = Pál
  |first6 = T. G. |last6 = Müller
  |title      = Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry
  |journal    = Astronomy & Astrophysics
  |year       = 2023
  |volume = L3 |page = 669 |doi = 10.1051/0004-6361/202245234 |arxiv      = 2211.07987
|bibcode = 2023A&A...669L...3S |s2cid = 253522934 }}</ref> though it is likely that {{dp|Orcus}}–[[Vanth (moon)|Vanth]] is another.<ref name="Brown2023">{{cite journal
  |first1 = Michael E. |last1 = Brown
  |first2 = Bryan |last2 = Butler
  |title      = Masses and densities of dwarf planet satellites measured with ALMA
  |journal    = The Planetary Science Journal
  |date       = July 2023
  |volume     = 4
  |issue      = 10
  |id         =
  |pages      = 11
  |doi-access = free
  |doi        = 10.3847/PSJ/ace52a
  |arxiv      = 2307.04848
  |bibcode    = 2023PSJ.....4..193B
  |s2cid      = }}</ref>

The reddish-brown cap of the north pole of Charon is composed of [[tholin]]s, [[organic compound|organic]] [[macromolecule]]s that may be essential ingredients of [[life]].  These tholins were produced from [[methane]], [[nitrogen]], and related gases which may have been released by cryovolcanic eruptions on the moon,<ref name="PHY-20220907">{{cite news |last=Yirka |first=Bob |title=A new explanation for the reddish north pole of Pluto's moon Charon |url=https://phys.org/news/2022-09-explanation-reddish-north-pole-pluto.html |date=September 7, 2022 |work=[[Phys.org]] |accessdate=September 9, 2022 }}</ref><ref name="NC-20220809">{{Cite journal |last1=Menten |first1=Stephanie M. |last2=Sori |first2=Michael M. |last3=Bramson |first3=Ali M. |display-authors=1 |date=2022-08-09 |title=Endogenically sourced volatiles on Charon and other Kuiper belt objects |journal=[[Nature Communications]] |language=en |volume=13 |issue=1 |page=4457 |bibcode=2022NatCo..13.4457M |doi=10.1038/s41467-022-31846-8 |issn=2041-1723 |pmc=9363412 |pmid=35945207}}</ref> or may have been transferred over {{convert|19,000|km|mi|abbr=on}} from the [[atmosphere of Pluto]] to the orbiting moon.<ref name="NYT-20160914">{{cite news |last1=Bromwich |first1=Jonah Engel |last2=St. Fleur |first2=Nicholas |title=Why Pluto's Moon Charon Wears a Red Cap |url=https://www.nytimes.com/2016/09/15/science/pluto-moon-charon.html |date=September 14, 2016 |work=[[The New York Times]] |access-date= September 14, 2016 }}</ref>

The ''[[New Horizons]]'' spacecraft is the only probe that has visited the Pluto system. It approached Charon to within {{convert|27000|km|mi|abbr=on}} in 2015.

==Discovery==
{{more citations needed|section|date=June 2017}}
[[File:Charon Discovery.jpg|thumb|left|Charon's discovery at the [[United States Naval Observatory Flagstaff Station|Naval Observatory Flagstaff Station]] as a time-varying bulge on the image of Pluto (seen near the top at left, but absent on the right). Negative image.]]

Charon was discovered by United States Naval Observatory astronomer [[James Christy]], using the {{convert|61|in|m |sigfig=3 |order=flip |adj=on |sp=us}} telescope at [[United States Naval Observatory Flagstaff Station]] (NOFS).<ref>{{cite web |date=December 16, 2003 |url=http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=572 |website=Solar System Exploration |title=Charon Discovery Image |publisher=NASA's Jet Propulsion Laboratory |access-date=July 10, 2013 |archive-url=https://web.archive.org/web/20130502041319/http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=572 |archive-date=May 2, 2013 |url-status=dead }}</ref> On June 22, 1978, he had been examining highly magnified images of [[Pluto]] on [[photographic plate]]s taken with the telescope two months prior. Christy noticed that a slight elongation appeared periodically. The bulge was confirmed on plates dating back to April 29, 1965.<ref>{{cite book |last=Dick |first=Steven J. |date=2013 |title=Discovery and Classification in Astronomy: Controversy and Consensus |chapter=The Pluto Affair |chapter-url=https://books.google.com/books?id=IT8oAAAAQBAJ&pg=PA9|pages=15–17 |publisher=Cambridge University Press |isbn=978-1-107-03361-0}}</ref> Subsequent observations of Pluto determined that the bulge was due to a smaller accompanying body. The periodicity of the bulge corresponded to Pluto's rotation period, which was previously known from Pluto's [[light curve]]. This indicated a [[synchronous orbit]], which strongly suggested that the bulge effect was real and not spurious. This resulted in reassessments of Pluto's size, mass, and other physical characteristics because the calculated mass and [[albedo]] of the Pluto–Charon system had previously been attributed to Pluto alone. The [[International Astronomical Union]] formally announced Christy's discovery to the world on July 7, 1978.<ref name="IAUC3241">{{cite web |url= http://www.cbat.eps.harvard.edu/iauc/03200/03241.html |title=IAUC 3241: 1978 P 1; 1978 (532) 1; 1977n |date=July 7, 1978 |website=Central Bureau for Astronomical Telegrams |access-date=July 5, 2011}}</ref>

Doubts about Charon's existence were erased when it and Pluto entered a [[Solar eclipses on Pluto|five-year period of mutual eclipses and transits]] between 1985 and 1990. This occurs when the Pluto–Charon [[Orbital plane (astronomy)|orbital plane]] is edge-on as seen from Earth, which only happens at two intervals in Pluto's 248-year orbital period. It was fortuitous that one of these intervals happened to occur soon after Charon's discovery.

==Name==
Charon was first given the temporary designation S/1978&nbsp;P&nbsp;1, after its discovery, following the then recently instituted convention. On June 24, 1978, Christy first suggested the name ''Charon'' as a scientific-sounding version of his wife Charlene's nickname, "Char".<ref name="Shilling" /><ref name="UnivToday">{{cite web |title=Charon: Pluto's Largest Moon |url= http://www.universetoday.com/41619/charon/ |website= Universe Today |date=July 14, 2015 |first=Matt |last=Williams |access-date=October 8, 2015}}</ref> Although colleagues at the [[United States Naval Observatory|Naval Observatory]] proposed ''[[Persephone]]'', Christy stuck with ''Charon'' after discovering that it was serendipitously the name of an appropriate mythological figure:<ref name="Shilling">{{cite magazine |first=Govert |last=Shilling |title=A Bump in the Night |magazine=[[Sky & Telescope]] |date=June 2008 |pages=26–27}} Prior to this, Christy had also considered naming the moon ''[[Land of Oz|Oz]]''.</ref> [[Charon (mythology)|Charon]] ({{IPAc-en|ˈ|k|ɛər|ən}};<ref name="OED">{{OED|Charon}}</ref> {{langx|grc|Χάρων}}) is the ferryman of the dead, closely associated with the god [[Pluto (mythology)|Pluto]]. The [[International Astronomical Union|IAU]] officially adopted the name in late 1985, and it was announced on January 3, 1986.<ref>{{cite web |title = IAUC 4157: CH Cyg; R Aqr; Sats of Saturn and Pluto |date = January 3, 1986 |url = http://www.cbat.eps.harvard.edu/iauc/04100/04157.html |website = Central Bureau for Astronomical Telegrams |access-date = July 5, 2011 }}</ref>

Coincidentally, nearly four decades before Charon's discovery, science fiction author [[Edmond Hamilton]] had invented three moons of Pluto for his 1940 novel ''Calling Captain Future'' and named them Charon, Styx, and Cerberus;<ref name="Codex Regius">{{Cite book |author=Codex Regius |title=Pluto & Charon: the new horizons spacecraft at the farthest worldly shores |year=2016 |isbn=978-1-5349-6074-9 |location=Wiesbaden}}</ref>
[[Styx (moon)|Styx]] and [[Kerberos (moon)|Kerberos]] are the two smallest Plutonian moons, and were named in 2013.

There is minor debate over the preferred pronunciation of the name. The mythological figure is pronounced with a {{IPAslink|k}} sound, and this is often followed for the moon as well.<ref>{{cite web |url=http://dictionary.reference.com/browse/Charon?r=75 |title=Charon |website=Dictionary.com}}</ref><ref>{{Cite dictionary |url=http://www.lexico.com/definition/Charon |archive-url=https://web.archive.org/web/20210416150522/https://www.lexico.com/definition/charon |url-status=dead |archive-date=April 16, 2021 |title=Charon |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> However, Christy himself pronounced the initial {{angbr|ch}} as a {{IPAslink|ʃ}} sound, as he had named the moon after his wife Charlene. Many English-speaking astronomers follow the classical convention, but others follow Christy's,<ref group="note">Astronomer Mike Brown can be heard pronouncing it {{IPA|[ˈʃɛɹᵻn]}} in ordinary conversation on the KCET interview [{{cite web |title=Julia Sweeney and Michael E. Brown |work=Hammer Conversations: KCET podcast |url= http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l |year=2007 |access-date=October 1, 2008 |archive-url=https://web.archive.org/web/20081006182705/http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l |archive-date=October 6, 2008}}] at 42min 48sec.</ref><ref name="9P">Pronounced "KAIR en" or "SHAHR en" per {{cite web |title=Pluto Facts |url=http://www.nineplanets.org/pluto.html#charon |website=Nine Planets |access-date=October 3, 2008}}</ref><ref>Pronounced 'with a soft "sh" ' per {{cite web |title=Welcome to the solar system, Nix and Hydra! |work=The Planetary Society Weblog |url=http://www.planetary.org/blog/article/00000613/ |access-date=October 3, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20090210115425/http://www.planetary.org/blog/article/00000613/ |archive-date=February 10, 2009 }}</ref><ref>[[US Naval Observatory]] spokesman Jeff Chester, when interviewed on the NPR commentary {{cite web |title=Letters: Radiology Dangers, AIDS, Charon |work=Morning Edition |url= https://www.npr.org/templates/story/story.php?storyId=5162958 |date=January 19, 2006 |access-date=October 3, 2008}} (at 2min 49sec), says Christy pronounced it {{IPA|[ˈʃɛɹɒn]}} rather than classical {{IPA|[ˈkɛɹɒn]}}. In normal conversation, the second vowel is reduced to a schwa: {{IPA|/ˈkɛərən/}} in RP (ref: [[OED]]).</ref> and that is the prescribed pronunciation at NASA and of the ''New Horizons'' team.<ref name="Sharon">Pronounced "Sharon" {{IPAc-en|ˈ|ʃ|ær|ə|n}} per {{cite web |title=NASA New Horizons: The PI's Perspective—Two for the Price of One |date=June 30, 2005 |url=http://www.spaceref.com/news/viewsr.html?pid=17180 |access-date=October 3, 2008 |archive-date=October 17, 2022 |archive-url=https://web.archive.org/web/20221017141907/https://spaceref.com/status-report/nasa-new-horizons-the-pis-perspective-two-for-the-price-of-one/ |url-status=dead }} and per {{cite web |title=New Horizons Team Names Science Ops Center After Charon's Discoverer |date=October 10, 2002 |url=http://www.spaceref.com/news/viewpr.html?pid=9477 |access-date=October 3, 2008 |archive-date=October 17, 2022 |archive-url=https://web.archive.org/web/20221017141907/https://spaceref.com/press-release/new-horizons-team-names-science-ops-center-after-charons-discoverer/ |url-status=dead }}</ref><ref group="note">[[Hal Weaver]], who led the team that discovered Nix and Hydra, also pronounces it {{IPA|[ˈʃɛɹᵻn]}} on the Discovery Science Channel documentary ''Passport to Pluto'', premiered 2006-01-15.</ref>

==Orbit==
[[File:Pluto-Charon system-new.gif|thumb|left|A simulated view of the Pluto–Charon system showing that Pluto orbits a point outside itself. Also visible is the mutual [[tidal locking]] between the two bodies.]]
Charon and Pluto orbit each other every 6.387 days. The two objects are [[tidal locking|gravitationally locked]] to one another, so each keeps the same face towards the other. This is a case of mutual tidal locking, as compared to that of the Earth and the Moon, where the Moon always shows the same face to Earth, but not vice versa. The average distance between Charon and Pluto is {{convert|19570|km}}. The discovery of Charon allowed astronomers to calculate accurately the mass of the Plutonian system, and mutual [[occultation]]s revealed their sizes. However, neither indicated the two bodies' individual masses. Those could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons then revealed that Charon has approximately 12% of the mass of Pluto.<ref name="Buie_2006" />

== Formation ==
Simulation work published in 2005 by [[Robin Canup]] suggested that Charon could have been formed by a collision around 4.5 [[1000000000 (number)|billion]] years ago, much like [[Earth]] and the [[Moon]]. In this model, a large [[Kuiper belt]] object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer [[mantle (geology)|mantle]], and Charon coalesced from the debris.<ref>{{cite journal |title=A Giant Impact Origin of Pluto–Charon |last=Canup |first=Robin |author-link=Robin Canup |journal=Science |date=January 28, 2005 |url= http://www.sciencemag.org/cgi/content/full/307/5709/546|doi=10.1126/science.1106818 |bibcode=2005Sci...307..546C |pmid=15681378 |volume=307 |issue=5709 |pages=546–50|s2cid=19558835 }}</ref> However, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon might have been two bodies that collided before going into orbit around each other. The collision would have been violent enough to boil off volatile ices like [[methane]] ({{chem|C|H|4}}) but not violent enough to have destroyed either body. The very similar density of Pluto and Charon implies that the parent bodies were not fully differentiated when the impact occurred.<ref name="Stern_2015"/>

==Physical characteristics==
{{main|Geology of Charon}}
{{see also|List of geological features on Charon}}

[[File:Charon, Earth & Moon size comparison.jpg|thumbnail|right|Size comparisons: [[Earth]], the [[Moon]], and Charon]]
Charon's diameter is {{convert|1212|km}}, just over half that of Pluto.<ref name="Stern_2015" /><ref name="Stern_2017" /> Larger than the dwarf planet [[Ceres (dwarf planet)|Ceres]], it is the twelfth-largest [[natural satellite]] in the [[Solar System]]. Charon is even similar in size to [[Uranus]]'s moons [[Umbriel]] and [[Ariel (moon)|Ariel]]. Charon's slow rotation means that there should be little flattening or tidal distortion if Charon is sufficiently massive to be in [[hydrostatic equilibrium]]. Any deviation from a perfect sphere is too small to have been detected by observations by the ''New Horizons'' mission. This is in contrast to [[Iapetus (moon)|Iapetus]], a Saturnian moon similar in size to Charon but with a pronounced [[Spheroid#Oblate spheroids|oblate]]ness dating to early in its history. The lack of such oblateness in Charon could mean that it is currently in hydrostatic equilibrium, or simply that its orbit approached its current one early in its history, when it was still warm.<ref name="Nimmo2017"/>

Based on mass updates from observations made by ''New Horizons''<ref name="Stern_2017" /> the mass ratio of Charon to Pluto is 0.1218:1. This is much larger than the Moon to the Earth: 0.0123:1. Because of the high mass ratio, the [[barycenter]] is outside of the radius of Pluto, and the Pluto–Charon system has been referred to as a dwarf [[double planet]]. With four smaller satellites in orbit about the two larger worlds, the Pluto–Charon system has been considered in studies of the orbital stability of [[circumbinary planet]]s.<ref name="Sutherland_2019" />

=== Internal structure ===
{{Multiple image
|perrow=1
|total_width=300
|image1=Charon Structures Desch and Neveu (2017).svg
|image2=Charon Structures Malamud et al. (2017).svg
|footer=
Two proposed models of Charon's interior
* A possible outcome of the ''hot start'' model, with two different levels of silicate 'fines,' or micron-sized particles<ref name="desch2017"/>
* A possible outcome of the ''cold start'' model<ref name="malamud2017"/>
}}
Charon's volume and mass allow calculation of its density, {{val|1.702|0.017|u=g/cm3}},<ref name="Stern_2017" /> from which it can be determined that Charon is slightly less dense than Pluto and suggesting a composition of 55% rock to 45% ice (±&nbsp;5%), whereas Pluto is about 70% rock. The difference is considerably lower than that of most suspected collisional satellites.{{which|date=January 2024}}

Following the ''New Horizons'' flyby, numerous discovered features on Charon's surface strongly indicated that Charon is differentiated, and may even have had a subsurface ocean early in its history. The past resurfacing observed on Charon's surface indicated that Charon's ancient subsurface ocean may have fed large-scale cryoeruptions on the surface, erasing many older features.<ref name="bagheri2022"/><ref name="desch2017"/><ref name="skyandtelescope2015"/> As a result, two broad competing views on the nature of Charon's interior arose: the so-called ''hot start'' model, where Charon's formation is rapid and involves a violent impact with Pluto, and the ''cold start'' model, where Charon's formation is more gradual and involves a less violent impact with Pluto.

According to the hot start model, Charon accreted rapidly (within ~{{val|e=4}} years) from the circumplanetary disc, resulting from a highly-disruptive giant impact scenario. This rapid time scale prevents the heat from accretion from radiating away during the formation process, leading to the partial melting of Charon's outer layers. However, Charon's crust failed to reach a melt fraction where complete differentiation occurs, leading to the crust retaining part of its silicate content upon freezing. A liquid subsurface ocean forms during or soon after Charon's accretion and persists for approximately 2 billion years before freezing, possibly driving cryovolcanic resurfacing of Vulcan Planitia. Radiogenic heat from Charon's core could then melt a second subsurface ocean composed of a [[eutectic]] water-ammonia mixture before it too freezes, possibly driving the formation of Kubrick Mons and other similar features. These freezing cycles could increase Charon's size by >20 km, leading to the formation of the complex tectonic features observed in Serenity Chasma and Oz Terra.<ref name="desch2017"/>

In contrast, the cold start model argues that a large subsurface ocean early in Charon's history is not necessary to explain Charon's surface features, and instead proposes that Charon may have been homogeneous and more porous at formation. According to the cold start model, as Charon's interior begins to warm due to radiogenic heating and heating from [[serpentinization]], a phase of contraction begins, largely driven by compaction in Charon's interior. Approximately 100-200 million years after formation, enough heat builds up to where a subsurface ocean melts, leading to rapid differentiation, further contraction, and the hydration of core rocks. Despite this melting, a pristine crust of amorphous water ice on Charon remains. After this period, differentiation continues, but the core can no longer absorb more water, and thus freezing at the base of Charon's mantle begins. This freezing drives a period of expansion until Charon's core becomes warm enough to begin compaction, starting a final period of contraction. Serenity Chasma may have formed from the expansion episode, whilst the final contraction episode may have given rise to the arcuate ridges observed in Mordor Macula.<ref name="malamud2017"/>

=== Surface ===
{{See also|Kubrick Mons}}
[[File:CharonFeatureMap Annotated.png|thumb|A map of Charon with [[International Astronomical Union|IAU]] names|center|upright=3]]
[[File:Pluto & Charon - Mountains Craters and Plains.jpg|thumb|Comparison between Pluto's [[Sputnik Planitia]] and Charon's informally named [[Vulcan Planitia]]]]
Unlike Pluto's surface, which is composed of [[nitrogen]] and [[methane]] ices, Charon's surface appears to be dominated by the less [[Volatile (astrogeology)|volatile]] water ice.

In 2007, observations by the [[Gemini Observatory]] detected patches of ammonia hydrates and water crystals on the surface of Charon that suggested the presence of active [[Geyser#Cryogeysers|cryogeysers]] and [[cryovolcano]]es. The fact that the ice was still in crystalline form suggested it may have been deposited recently, as it was expected that solar radiation would have degraded it to an [[Amorphous ice|amorphous]] state after roughly thirty thousand years.<ref name="ice"/><ref name="cook2007" /> However, following new data from the ''New Horizons'' flyby, no active cryovolcanoes or geysers were detected. Later research has also called into question the cryovolcanic origin for the crystalline water ice and ammonia features, with some researchers instead proposing that ammonia may be replenished passively from underground material.<ref name="holler2017"/>

Photometric mapping of Charon's surface shows a latitudinal trend in [[albedo]], with a bright equatorial band and darker poles. The north polar region is dominated by a very large dark area informally dubbed "[[Mordor Macula|Mordor]]" by the ''New Horizons'' team.<ref name="theweekmordor"/><ref name="NBC1"/><ref name="NYTmountains"/> The favored explanation for this feature is that it is formed by condensation of gases that escaped from [[Atmosphere of Pluto|Pluto's atmosphere]]. In winter, the temperature is −258&nbsp;°C, and these gases, which include nitrogen, carbon monoxide, and methane, condense into their solid forms; when these ices are subjected to solar radiation, they chemically react to form various reddish [[tholin]]s. Later, when the area is again heated by the Sun as Charon's seasons change, the temperature at the pole rises to −213&nbsp;°C, resulting in the volatiles sublimating and escaping Charon, leaving only the tholins behind. Over millions of years, the residual tholin builds up thick layers, obscuring the icy crust.<ref name="physorgmordor"/> In addition to Mordor, ''New Horizons'' found evidence of extensive past geology that suggests that Charon is probably differentiated;<ref name="NBC1" /> in particular, the southern hemisphere has fewer craters than the northern and is considerably less rugged, suggesting that a massive resurfacing event—perhaps prompted by the partial or complete freezing of an internal ocean—occurred at some point in the past and removed many of the earlier craters.<ref name="skyandtelescope2015"/>

Charon has a system of extensive [[graben]]s and scarps, such as [[List of geological features on Charon|Serenity Chasma]], which extend as an equatorial belt for at least {{Cvt|1000|km||abbr=|-1}}. Argo Chasma potentially reaches as deep as {{Cvt|9|km||abbr=|0}}, with cliffs that may rival [[Verona Rupes]] on [[Miranda (moon)|Miranda]] for the title of the tallest cliff in the Solar System.<ref name="nasa2017"/>

=== Hypothesized exosphere ===
[[File:PIA20375-PlutoMoon-Charon-NightSide-20150717.jpg|thumb|Charon's night side seen by ''New Horizons'']]
In contrast to Pluto, Charon has no significant atmosphere.<ref name="Stern_2015"/> There has been speculation about an extremely thin [[exosphere]] surrounding the moon contributing to the formation of dark regions such as Mordor Macula. The strong seasons experienced by Pluto and Charon could provide brief periods of exosphere formation as methane sublimates on Charon, interspersed by centuries of dormancy.<ref name="Teolis2022">{{cite journal |last1=Teolis |first1=Ben |last2=Raut |first2=Ujjwal |last3=Kammer |first3=Joshua A. |title=Extreme Exospheric Dynamics at Charon: Implications for the Red Spot |date=15 April 2022 |journal=Geophysical Research Letters |volume=49 |issue=8 |pages=e97580 |doi=10.1029/2021GL097580 |doi-access=free |bibcode=2022GeoRL..4997580T }}</ref>

Pluto does have a thin but significant atmosphere, and under some conditions{{Which|date=January 2024}} Charon's gravitation may pull some of Pluto's upper atmosphere, specifically nitrogen, from Pluto's ice formations, toward Charon's surface. The nitrogen is mostly caught in the combined center of gravity between the two bodies before reaching Charon, but any gas that does reach Charon is held closely against the surface. The gas is mostly made up of ions of nitrogen, but the amounts are negligible compared to the total of Pluto's atmosphere.<ref>{{Cite journal |last1=Tucker |first1=O. J. |last2=Johnson |first2=R. E. |last3=Young |first3=L. A. |date=January 15, 2015 |title=Gas transfer in the Pluto–Charon system: A Charon atmosphere |url=https://www.sciencedirect.com/science/article/pii/S0019103514002462 |journal=Icarus |series=Special Issue: The Pluto System |language=en |volume=246 |pages=291–297 |bibcode=2015Icar..246..291T |doi=10.1016/j.icarus.2014.05.002 |issn=0019-1035}}</ref>

The many spectral signatures of ice formations on the surface of Charon have led some to believe that the ice formations could supply an atmosphere, but atmosphere supplying formations have not been confirmed yet. Many scientists theorize that these ice formations could be concealed out of direct sight, either in deep craters or beneath Charon's surface. Similar{{clarify|date=August 2022}} to how Pluto transfers atmosphere to Charon, Charon's relatively low gravity, due to its low mass, causes any atmosphere that might be present to rapidly escape the surface into space.<ref>{{Cite book |last=Alan. |first=Stern, S. |url=http://worldcat.org/oclc/963785129 |title=Pluto and Charon. |date=2017 |publisher=University of Arizona Press |isbn=978-0-8165-3613-9 |oclc=963785129}}</ref> Even through stellar occultation, which is used to probe the atmosphere of stellar bodies, scientists cannot confirm an existing atmosphere; this was tested in 1986 while attempting to perform stellar occultation testing on Pluto. Charon also acts as a protector for Pluto's atmosphere, blocking the solar wind that would normally collide with Pluto and damage its atmosphere. Since Charon blocks these solar winds, its own atmosphere is diminished, instead of Pluto's. This effect is also a serious potential explanation for Charon's lack of atmosphere; when it begins to accumulate, the solar winds shut it down.{{clarify|date=August 2022}}<ref>{{Cite web |date=January 12, 2017 |title=Charon protects Pluto's atmosphere from solar wind |url=https://www.spaceflightinsider.com/missions/solar-system/charon-protects-pluto-atmosphere-solar-wind/ |access-date=May 13, 2022 |website=SpaceFlight Insider |language=en-US }}{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Although{{clarify|date=August 2022}}, it is still possible for Charon to have an atmosphere. As previously stated, Pluto transfers some of its atmospheric gas to Charon, from where it tends to escape into space. Assuming Charon's density is 1.71 g/cm<sup>3</sup>, which is the rough estimate we currently have, it would have a surface gravity of 0.6 of Pluto's. It also has a higher mean molecular weight than Pluto and a lower exobase surface temperature, so that the gases in its atmosphere would not escape as rapidly from Charon as they do from Pluto.<ref>{{Cite journal |last1=Elliot |first1=J. L. |last2=Young |first2=L. A. |year=1991 |title=Does Charon have an Atmosphere? |journal=Lunar and Planetary Science Conference |volume=22 |page=347 |bibcode=1991LPI....22..347E}}</ref>

There has been significant proof of CO<sub>2</sub> gas and H<sub>2</sub>O vapor on the surface of Charon, but these vapors are not sufficient for a viable atmosphere due to their low vapor pressures. Pluto's surface has abundant ice formations, but these are volatile, as they are made up of volatile substances like methane. These volatile ice structures cause a good deal of geological activity, keeping its atmosphere constant, while Charon's ice structures are mainly made up of water and carbon dioxide, much less volatile substances that can stay dormant and not affect the atmosphere much.<ref>{{Cite book |last1=Spohn |first1=Tilman |title=Encyclopedia of the solar system |last2=Breuer |first2=Doris |last3=Johnson |first3=Torrence V. |date=2014 |publisher=Elsevier |isbn=978-0-12-415845-0 |edition=3d |location=Amsterdam Boston |page=909-924 |chapter=Pluto |chapter-url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/charon}}</ref>

==Observation and exploration==
{{multiple image
| caption_align     = center
| header_align      = center
| align             = right
| header            = Historical observations of Charon
| width             =
| image1            = Charon Discovery.jpg
| width1            = 220
| caption1          = <small>(1)</small> Discovery;<br>1978
| image2            = Pluto-picture.jpg
| width2            = 225
| caption2          = <small>(2)</small> [[Hubble Space Telescope|HST]] – before correction;<br>1990
| image3            = Pluto and charon.jpg
| width3            = 323
| caption3          = <small>(3)</small> [[Hubble Space Telescope|HST]] – after correction;<br>1994
| image4            = First Color Animated Images show Pluto and its Moon Charon.gif
| width4            = 168
| caption4          = <small>(4)</small> 1st color animated view;<br>2015
| total_width       = 400
| perrow            = 2/2
}}
Since the first blurred images of the moon <small>(1)</small>, images showing Pluto and Charon resolved into separate disks were taken for the first time by the [[Hubble Space Telescope]] in the 1990s <small>(2)</small>. The telescope was responsible for the best, yet low-quality images of the moon. In 1994, the clearest picture of the Pluto–Charon system showed two distinct and well-defined disks <small>(3)</small>. The image was taken by Hubble's [[Faint Object Camera]] (FOC) when the system was 4.4 billion kilometers (2.6 billion miles) away from Earth<ref>{{Cite web |title=Pluto and Charon |url=http://www.spacetelescope.org/images/opo9417a/ |website=Hubble Space Telescope |date=May 16, 1994 |access-date=October 8, 2015 |archive-date=September 5, 2015 |archive-url=https://web.archive.org/web/20150905112231/http://www.spacetelescope.org/images/opo9417a/ |url-status=dead }}</ref> Later, the development of [[adaptive optics]] made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes.<ref name="UnivToday" /> Although ground-based observation is very challenging, a group of amateur astronomers in Italy used a 14-inch telescope in 2008 to successfully resolve Charon in an image of Pluto.<ref>{{Cite web |last=Atkinson |first=Nancy |date=2008-10-30 |title=Charon Imaged by Amateur Astronomers |url=https://www.universetoday.com/20351/charon-imaged-by-amateur-astronomers/ |access-date=2023-08-22 |website=Universe Today |language=en-US}}</ref>

In June 2015, the ''New Horizons'' spacecraft captured consecutive images of the Pluto–Charon system as it approached it. The images were put together in an animation. It was the best image of Charon to that date <small>(4)</small>. In July 2015, the ''New Horizons'' spacecraft made its closest approach to the Pluto system. It is the only spacecraft to date to have visited and studied Charon.  Charon's discoverer James Christy and the children of [[Clyde Tombaugh]] were guests at the [[Johns Hopkins Applied Physics Laboratory]] during the New Horizons closest approach.

== Classification ==

The [[centre of mass#Barycenter|center of mass]] (barycenter) of the Pluto–Charon system lies outside either body. Because neither object truly orbits the other, and Charon has 12.2% of the mass of Pluto, it has been argued that Charon should be considered to be part of a [[binary planet]] with Pluto. The [[International Astronomical Union]] (IAU) states that Charon is a satellite of Pluto, but the idea that Charon might be classified as a dwarf planet in its own right may be considered at a later date.<ref name="IAU_DP"/>

In a draft proposal for the [[IAU definition of planet|2006 redefinition]] of the term, the IAU proposed that a planet is defined as a body that orbits the Sun that is large enough for gravitational forces to render the object (nearly) spherical. Under this proposal, Charon would have been classified as a planet, because the draft explicitly defined a planetary satellite as one in which the barycenter lies within the major body. In the final definition, Pluto was reclassified as a [[dwarf planet]], but a formal definition of a planetary satellite was not decided upon. Charon is not in the list of dwarf planets currently recognized by the IAU.<ref name="IAU_DP"/> Had the draft proposal been accepted, even the [[Moon]] would hypothetically be classified as a planet in billions of years when the [[tidal acceleration]] that is gradually moving the Moon away from Earth takes it far enough away that the center of mass of the system no longer lies within Earth.<ref name="moonplanet" />

The other moons of Pluto{{snd}} [[Nix (moon)|Nix]], [[Hydra (moon)|Hydra]], [[Kerberos (moon)|Kerberos]], and [[Styx (moon)|Styx]]{{snd}} orbit the same barycenter but they are not large enough to be spherical and they are simply considered to be satellites of Pluto (or of Pluto–Charon).<ref name="NixHydra"/>
== See also ==
* [[List of natural satellites]]

==Notes==
{{reflist
|group = note
}}

{{notelist}}

==References==
{{reflist
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<ref name="Brozovic2024">{{cite journal |last1=Brozović |first1=Marina |last2=Jacobson |first2=Robert A. |title=Post-new-horizons Orbits and Masses for the Satellites of Pluto |date=8 May 2024 |journal=The Astronomical Journal |volume=167 |issue=256 |page=256 |doi=10.3847/1538-3881/ad39f0 |doi-access=free |bibcode=2024AJ....167..256B }}</ref>

}}

==External links==
{{Commons category|Charon (moon)}}
* [https://web.archive.org/web/20141008221417/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Plu_Charon Charon Profile] at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* {{cite journal  |bibcode=1978AJ.....83.1005C |title=The satellite of Pluto |journal=Astronomical Journal |volume=83 |pages=1005 |last1=Christy |first1=J. W |last2=Harrington |first2=R. S |s2cid=120501620 |year=1978 |doi=10.1086/112284}}
* [http://news.bbc.co.uk/2/hi/science/nature/4230918.stm Hubble reveals new map of Pluto], [[BBC]] News, September 12, 2005
* {{Cite journal |arxiv=astro-ph/0602082 |last1=Person |first1=M. J |title=Charon's radius and density from the combined data sets of the 2005 July 11 occultation |journal=The Astronomical Journal |volume=132 |issue=4 |pages=1575–1580 |last2=Elliot |first2=J. L |last3=Gulbis |first3=A. A. S |last4=Pasachoff |first4=J. M |last5=Babcock |first5=B. A |last6=Souza |first6=S. P |last7=Gangestad |first7=J |year=2006 |doi=10.1086/507330|bibcode=2006AJ....132.1575P |s2cid=6169239 }}
* [https://web.archive.org/web/20090320174750/http://spacescience.arc.nasa.gov/agu/abstracts/Deschetal.pdf Cryovolcanism on Charon and other Kuiper Belt Objects]
* [https://web.archive.org/web/20131208200701/http://pluto.jhuapl.edu/news_center/news/20130710.php ''New Horizons'' Camera Spots Pluto's Largest Moon – July 10, 2013]
* [https://www.nasa.gov/feature/charon-at-40-four-decades-of-discovery-on-pluto-s-largest-moon 40th anniversary NASA video] {{Webarchive|url=https://web.archive.org/web/20220511150127/https://www.nasa.gov/feature/charon-at-40-four-decades-of-discovery-on-pluto-s-largest-moon/ |date=May 11, 2022 }} describing the discovery and naming of Charon (June 22, 2018)
* NASA CGI [http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20170714-2 video] of Charon flyover (July 14, 2017)
* [https://www.flickr.com/photos/136797589@N04/36101560426/ CGI video] simulation of rotating Charon by Seán Doran (see [https://www.flickr.com/photos/136797589@N04/albums/72157686474817595 album] for more)
* [https://www.google.com/maps/space/charon/@23.0752291,9.3670842,8033611m/data=!3m1!1e3 Google Charon 3D], interactive map of the moon
* {{cite web |url=http://livestream.com/viewnow/LPSC2016 |title=2016 Lunar & Planetary Science Conference by National Institute of Aerospace |access-date=April 6, 2016 |archive-date=June 23, 2018 |archive-url=https://web.archive.org/web/20180623004727/https://livestream.com/viewnow/LPSC2016 |url-status=dead }}
* [https://gravitysimulator.org/solar-system/the-plutonian-system Interactive 3D gravity simulation of Pluto and Charon in addition to Pluto's four other moons Styx, Kerberos, Hydra and Nix] {{Webarchive|url=https://web.archive.org/web/20200611221454/https://gravitysimulator.org/solar-system/the-plutonian-system/ |date=June 11, 2020 }}

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[[Category:Charon (moon)| ]]
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