Charon (moon)
Charon (/ˈkɛərɒn, -ən/ KAIR-on, -ən or /ˈʃærən/ SHAIR-ən),[note 1] or (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong.[18] 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).
Not to be confused with the minor planet 2060 Chiron.Discovery
June 22, 1978
Pluto I[1]
Discoverer's wife, Charlene, and Χάρων Kharōn
S/1978 P 1
19 592.61 km
19 598.92 km
19595.764+0.007
−0.008 km (planetocentric)[10]
17181.0 km (barycentric)
0.000161[10]
6.387221+0.000005
−0.000003 d
(6 d, 9 h, 17 m, 35.89 ± 0.35 s)[10]
0.21 km/s[note 5]
223.046°±0.014° (to vernal equinox)
<0.5%[14]
4.6×106 km2 (0.0090 Earths)
(9.32±0.14)×108 km3 (0.00086 Earths)
(1.5897±0.0045)×1021 kg[10]
(2.66×10−4 Earths)
(12.2% of Pluto)
1.705±0.006 g/cm3[10]
0.288 m/s2
0.59 km/s
0.37 mi/s
0.2 to 0.5 at a solar phase angle of 15°
−220 °C (53 K)
16.8[15]
1[16]
55 milli-arcsec[17]
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.[19] The dwarf planet systems Pluto–Charon and Eris–Dysnomia are the only known examples of mutual tidal locking in the Solar System,[20] though it is likely that Orcus–Vanth is another.[21]
The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules 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,[22][23] or may have been transferred over 19,000 km (12,000 mi) from the atmosphere of Pluto to the orbiting moon.[24]
The New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km (17,000 mi) in 2015.
Name[edit]
Charon was first given the temporary designation S/1978 P 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".[28][29] Although colleagues at the Naval Observatory proposed Persephone, Christy stuck with Charon after discovering that it was serendipitously the name of an appropriate mythological figure:[28] Charon (/ˈkɛərən/;[2] Ancient Greek: Χάρων) is the ferryman of the dead, closely associated with the god Pluto. The IAU officially adopted the name in late 1985, and it was announced on January 3, 1986.[30]
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;[31]
Styx and 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 /k/ sound, and this is often followed for the moon as well.[32][33] However, Christy himself pronounced the initial ⟨ch⟩ as a /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,[note 6][34][35][36] and that is the prescribed pronunciation at NASA and of the New Horizons team.[3][note 7]
Formation[edit]
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.[37] 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 (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.[12]
Classification[edit]
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% 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.[59]
In a draft proposal for the 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.[59] 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.[60]
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).[61]