Charon (moon)

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Not to be confused with the centaur 2060 Chiron.
Charon
Charon in Color (HQ).jpg
Charon in approximate true color, as viewed by New Horizons on 13 July 2015
Discovery
Discovered by James W. Christy
Discovery date 22 June 1978
Designations
Pronunciation /ˈʃærən/ or /ˈkɛərən/[note 1]
Named after
 Charon
(134340) Pluto I[1]
Adjectives Charonian
Orbital characteristics [3]
Epoch 2452600.5
17536±4 km to system barycenter, 19571±4 km to the center of Pluto
Eccentricity 0.00[2]
6.3872304±0.0000011 d
(6 d, 9 h, 17 m, 36.7 ± 0.1 s)
0.21 km/s[note 2]
Inclination 0.001° (to Pluto's equator)
119.591°±0.014° (to Pluto's orbit)
112.783°±0.014° (to the ecliptic)
223.046°±0.014° (to vernal equinox)
Satellite of Pluto
Physical characteristics
Mean radius
 606±3 km[4] (0.095 Earths, 0.51 Plutos)
Flattening <1% [5][6]
4.×106 km2 (0.0090 Earths)
Volume (9.32±0.14)×108 km3 (0.00086 Earths)
Mass (1.586±0.015)×1021 kg[4]
(2.66×10−4 Earths)
(12.2% of Pluto)
Mean density
 1.702±0.021 g/cm3[4]
0.288 m/s2
0.59 km/s
0.37 mi/s
synchronous
Albedo 0.2 to 0.5 at a solar phase angle of 15°
Temperature −220 °C (53 K)
16.8[7]
1[8]
55 milli-arcsec[9]
Mosaic of best-resolution images of Charon from different angles

Charon, also called (134340) Pluto I,[1] is the largest of the five known moons of the dwarf planet Pluto. 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). It is a very large moon in comparison to its parent body, having half the diameter and one eighth the mass of Pluto. Its gravitational influence is such that the barycenter of the Pluto–Charon system lies outside Pluto.

The New Horizons spacecraft, to date the only probe to have visited the Pluto system, approached within 27,000 km (17,000 mi) of Charon.

History

Discovery

Charon's discovery at the 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)

Charon was discovered by United States Naval Observatory astronomer James Christy, using the 1.55-meter telescope at NOFS,[10] and was formally announced to the world via the International Astronomical Union on July 7, 1978.[11] On June 22, 1978, he had been examining highly magnified images of Pluto on photographic plates taken at the Lua error in Module:Convert at line 1851: attempt to index local 'en_value' (a nil value). Flagstaff telescope two months prior. Christy noticed that a slight elongation appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965.

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.

Doubts about Charon's existence were erased when it and Pluto entered a five-year period of mutual eclipses and transits between 1985 and 1990. This occurs when the Pluto–Charon 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 originally known by the temporary designation S/1978 P 1, according to 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".[12][13] Although colleagues at the Naval Observatory proposed Persephone, Christy stuck with Charon after discovering it coincidentally refers to a Greek mythological figure:[12] Charon (/ˈkɛərɒn/ or /ˈkɛərən/; Greek Χάρων) is the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans identified with their god Pluto. Official adoption of the name by the IAU waited until late 1985 and was announced on January 3, 1986.[14]

There is minor debate over the preferred pronunciation of the name. The practice of following the classical pronunciation established for the mythological ferryman Charon (IPA [ˈkɛ:rən]) is used by major English-language dictionaries, such as the Merriam-Webster and Oxford English Dictionary.[15][16] These indicate only one pronunciation of "Charon" when referring specifically to Pluto's moon: with an initial "k" sound. Speakers of many languages other than English, and many English-speaking astronomers as well, follow this pronunciation.[17]

However, Christy himself pronounced the ch as sh (IPA [ʃ]), after his wife Charlene. Because of this, as an acknowledgement of Christy and sometimes as an in-joke or shibboleth, the initial sh pronunciation is common among astronomers when speaking English,[18][17][19][20] and this is the prescribed pronunciation at NASA and of the New Horizons team.[21][22]

Formation

Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 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, and Charon coalesced from the debris.[23] 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 may have been two bodies that collided before going into orbit about each other. The collision would have been violent enough to boil off volatile ices like methane (CH
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.[6]

Orbit

An oblique 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 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 19,570 kilometres (12,160 mi). The discovery of Charon allowed astronomers to accurately calculate the mass of the Plutonian system, and mutual occultations revealed their sizes. However, neither indicated the two bodies' individual masses, which could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons reveal that Charon has approximately 12.18% of the mass of Pluto.[4] This shows it to have a density of 1.702 ± 0.021 g/cm3, suggesting a composition of 55 ± 5% rock to 45% ice, whereas Pluto is somewhat denser and about 70% rock.

Physical characteristics

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See also: List of geological features on Charon
The two conflicting theories about Charon's internal structure
Size comparisons: Earth, the Moon, and Charon

Charon's diameter is 1,212 kilometres (753 mi), just over half that of Pluto,[4] and larger than the dwarf planet Ceres. Charon is sufficiently massive to have collapsed into a spheroid under its own gravity. Charon's slow rotation means that there is almost no flattening. Its equatorial and polar radii differ by less than 1%[24]

Interior

Charon's volume and mass allow calculation of its density, 1.702±0.021 g/cm3,[4] from which it can be determined that Charon is slightly less dense than Pluto and thus contains a larger proportion of ice relative to rock in its interior. The difference is considerably lower than that of most suspected collisional satellites. Before New Horizons' flyby, there were two conflicting theories about Charon's internal structure: some scientists thought Charon to be a differentiated body like Pluto, with a rocky core and an icy mantle, whereas others thought it would be uniform throughout.[25] Evidence in support of the former position was found in 2007, when observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryogeysers. The fact that the ice was still in crystalline form suggested it had been deposited recently, because solar radiation would have degraded it to an amorphous state after roughly thirty thousand years.[26]

Surface

Charon in enhanced color to bring out differences in surface composition

Unlike Pluto's surface, which is composed of nitrogen and methane ices, Charon's surface appears to be dominated by the less volatile water ice. In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryogeysers and cryovolcanoes.[26][27]

Photometric mapping of Charon's surface shows a latitudinal trend in albedo, with a bright equator band and darker poles. The north polar region is dominated by a very large darker area informally dubbed "Mordor" by the New Horizons team.[28][29][30] The favored explanation for this phenomenon is that they are formed by condensation of gases that escaped from Pluto's atmosphere. In winter, the temperature is −258 °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 tholins. Later, when the area is again heated by the Sun as Charon's seasons change, the temperature at the pole rises to −213 °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.[31] Aside from Mordor, however, New Horizons imaged very few other impact craters on Charon and found a youthful surface, adding support to the above theory that Charon is geologically active and thus probably differentiated.[29] 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.[32]

Charon - youngest crater

Mountain in a moat

In a released photo by New Horizons, an unusual surface feature has captivated and baffled the scientist team of the mission. The image reveals a mountain rising out of a depression, in a zoomed in view. It's "a large mountain sitting in a moat", said Jeff Moore, of NASA’s Ames Research Center, in a statement. "This is a feature that has geologists stunned and stumped", he added. New Horizons captured the photo on Tuesday morning from a distance of 49,000 miles (79,000 km).[33][34]

Observation and exploration

Since the first blurred images of the moon (1), images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s (2). 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 circles (3). The image was taken by Hubble's Faint Object Camera (FOC) when the system was 2.6 billion miles (4.4 billion kilometers) away from Earth[35] Later, the development of adaptive optics made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes.[13]

In June 2014, 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 (4). 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.

Timeline of Charon observations
(1) Discovery;
1978
(2) HST - before correction;
1990
(3) HST - after correction;
1994
(4) 1st color animated view;
2015

Classification as a moon or dwarf planet

Pluto and Charon, to scale. Photo taken by New Horizons on approach

The 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% the mass of Pluto, it has been argued that Charon should be considered to be part of a binary system with Pluto. The International Astronomical Union (IAU) states that Charon is considered to be just a satellite of Pluto, but the idea that Charon might be classified a dwarf planet in its own right may be considered at a later date.[36]

In a draft proposal for the 2006 redefinition of the term, the IAU proposed that a planet be 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 the formal definition of a planetary satellite was not decided upon. Charon is not in the list of dwarf planets currently recognized by the IAU.[37] Had the draft proposal been accepted, even the Moon would 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.[38]

The other moons of Pluto, Nix, Hydra, Kerberos and Styx, 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).[39]

In popular culture

The Mass Effect video game franchise is built around the premise of a series of relays that allow faster-than-light travel around the Milky Way without time dilation. In the series, Charon is discovered in 2149 not to be a moon or a dwarf planet, but to be a chunk of ice containing an intricate piece of technology built by an ancient civilization that allows this travel to occur, leading to the discovery of other planetary systems and enabling their colonization. Operating the Charon Relay gradually circularized Pluto's orbit.

The Jonathan Coulton song "I'm Your Moon" from the Thing a Week Four album is a love song sung to Pluto by Charon.[40]

Gallery
Global map of Charon 
Identically processed enhanced-color views of Pluto and Charon 
High resolution enhanced-color mosaic of Charon 
Pluto and Charon as viewed by New Horizons
(color; July 11, 2015). 
Pluto and Charon as viewed by New Horizons
(false-color; July 13, 2015). 

Videos

Charon - Flyover video animation (00:20)
(released October 1, 2015).

See also

Notes

  1. ↑ In US dictionary transcription, US dict: shăr′·ən, kār′·ən, the latter per the anglicized pronunciation of the Greek: Χάρων.
  2. ↑ Calculated on the basis of other parameters.

References

  1. ↑ 1.0 1.1 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.. a, i, e per JPL (site updated 2008 Aug 25)
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 The Pluto system: Initial results from its exploration by New Horizons Stern et. al. October 16, 2015 Table 1
  5. ↑ Charon Geology and imaging Our derived radius of Charon is 606 ± 3 km, similar to ground-based measurements (29); we also determined that Charon is not detectably oblate, with an upper bound on polar flattening of 1%
  6. ↑ 6.0 6.1 http://www.sciencemag.org/content/350/6258/aad1815.full
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ 12.0 12.1 Govert Shilling, "A Bump in the Night" in Sky & Telescope (June 2008), pp. 26–27. Prior to this, Christy had considered naming the moon Oz.
  13. ↑ 13.0 13.1 Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Charon – 5 dictionary results
  16. ↑ Charon at Oxford English Dictionary
  17. ↑ 17.0 17.1 Pronounced "KAIR en" or "SHAHR en" per Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Astronomer Mike Brown can be heard pronouncing it [ˈʃɛɹɪn] in ordinary conversation on the KCET interview Lua error in package.lua at line 80: module 'strict' not found. at 42min 48sec. Being a long-time resident of California, he does not distinguish the /ær/ vowel of the name Sharon and the /ɛər/ vowel of the classical pronunciation of Charon.
  19. ↑ Pronounced 'with a soft "sh" ' per Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ US Naval Observatory spokesman Jeff Chester, when interviewed on the NPR commentary Lua error in package.lua at line 80: module 'strict' not found. (at 2min 49sec), says Christy pronounced it [ˈʃɛɹɒn] rather than classical [ˈkɛɹɒn]. In normal conversation, the second vowel is reduced to a schwa: /ˈkɛərən/ in RP (ref: OED).
  21. ↑ Pronounced "Sharon" /ˈʃærən/ per Lua error in package.lua at line 80: module 'strict' not found. and per Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Hal Weaver, who led the team that discovered Nix and Hydra, also pronounces it [ˈʃɛɹɪn] (/ˈʃærən/ with a General American accent) on the Discovery Science Channel documentary Passport to Pluto, premiered 2006-01-15.
  23. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  25. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  26. ↑ 26.0 26.1 Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  28. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ 29.0 29.1 Lua error in package.lua at line 80: module 'strict' not found.
  30. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  31. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  32. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  33. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  34. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  35. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  36. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  37. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  38. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  39. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  40. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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