Discover

'Charon' ''(kair'-ən'' or ''shair'-ən'' (), IPA , Greek ''Χάρων)'', discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. With the discovery in 2005 of two other moons of Pluto (Nix and Hydra), Charon is now also referred to as 'Pluto I'.
The ''New Horizons'' mission is scheduled to visit Charon and Pluto in July 2015.
Charon should not be confused with the similarly named Chiron, a smaller object in the outer solar system.

Contents

Discovery

Name

Physical characteristics

Orbital characteristics

Formation

Moon or dwarf planet?

References

External links

Discovery

Charon was discovered by astronomer James Christy on June 22, 1978, when he was examining highly magnified images of Pluto on photographic plates taken a couple of months before. Christy noticed that a slight bulge 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.
Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses 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 good luck that one of these intervals happened to occur so soon after Charon's discovery.
Images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. Later, the development of adaptive optics made it possible to also resolve Pluto and Charon into separate disks using ground-based telescopes.

Name

Charon was originally known by the temporary designation 'S/1978 P 1', according to the then-recently instituted convention. Christy immediately chose the name "Charon", but official adoption by the IAU would wait until late 1985 (it was announced on 3 January 1986 ^[3]). In Greek mythology, Charon was the ferryman of the dead, a figure with close ties to the god Hades (adopted by the Romans as Pluto).
Although the traditional English pronunciation of the mythological figure is with a hard ''k'' sound, Christy pronounced the ''ch'' in the moon's name as an ''sh'' sound (IPA ), after his wife Charlene (nicknamed "Char"). The ''sh'' pronunciation is now common among astronomers, in spite of the pleas to follow customary usage. In European languages other than English, speakers generally follow the pronunciation established for the mythological figure, which is the ''ch'' sound.

Physical characteristics

Charon's diameter is about 1,207 km (750 miles), just over half that of Pluto, with a surface area of 4,580,000 km². Unlike Pluto, which is covered with nitrogen and methane ices, the Charonian surface appears to be dominated by less volatile water ice, and also appears to have no atmosphere. 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 cryo-geysers.^[4]
Mutual eclipses of Pluto and Charon in the 1980s allowed astronomers to take spectra of Pluto and then the combined spectrum of the pair. By subtracting Pluto's spectrum from the total, astronomers were able to spectroscopically determine the surface composition of Charon.
Charon's volume and mass allow us to calculate its density; from this we can tell Charon is largely an icy body and contains less rock by proportion than its partner Pluto, supporting the idea Charon was created by a giant impact into Pluto's icy mantle (see Formation below.) There are two conflicting theories about Charon's internal structure: some scientists believe it to be a differentiated body like Pluto with a rocky core and an icy mantle while others believe Charon to be of uniform composition throughout. 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 cryo-geysers. The fact that the ice was still in crystalline form suggested it had been recently deposited, as solar radiation would have degraded older ice to an amorphous state after 30,000 years or so.^[4]

Orbital characteristics

Charon and Pluto revolve about each other every 6.387 days. The two objects are gravitationally locked, so each keeps the same face towards the other. The average distance between Charon and Pluto is 19,570 km (12,163 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 11.65% of the mass of Pluto.^[6] This shows it to have a density of 1.65±0.06 g/cm³, suggesting a composition of 55±5% "rock" to 45% ice, whereas Pluto is somewhat denser and about 70% "rock".

Formation

Simulation work published in 2005 by Robin Canup suggested that Charon could have formed by a giant impact around 4.5 billion years ago, much like the Earth and 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. However, such an impact should result in an icier Charon and rockier Pluto than what 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 but not violent enough to be disrupted.^[7]

Moon or dwarf planet?

The center of mass (barycenter) of the Pluto-Charon system lies outside either body. Since neither object truly rotates around the other, and they are comparable in terms of mass, it has been argued that Charon should not be considered to be a satellite of Pluto. Instead, it has been suggested that they form dual dwarf planets, following the re-classification of Pluto.
In a draft proposal for the 2006 redefinition of the term, the International Astronomical Union 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, since 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, leaving Charon's status unclear. (Charon is not in the list of dwarf planets currently recognized by the IAU.)
The moons Nix and Hydra also orbit the same barycenter, but are not large enough to be spherical, and are simply considered to be satellites of Pluto (or, under the alternative viewpoint, of the Pluto-Charon system).^[8]

References

1.
2. ''Charon’s size and an upper limit on its atmosphere from a stellar occultation'', B. Sicardy et al, , , Nature, 2006
3. IAU Circular No. 4157
4. Charon: An ice machine in the ultimate deep freeze
5. Charon: An ice machine in the ultimate deep freeze
6. http://arxiv.org/PS_cache/astro-ph/pdf/0512/0512491.pdf
7. Charon's Shadow Illuminates Its True Nature, , , Schriber, ScienceNOW, 2005
8. http://www.boulder.swri.edu/plutomoons/

External links

★ Charon Profile by NASA's Solar System Exploration

★ James W. Christy and Robert S. Harrington, "The satellite of Pluto," ''The Astronomical Journal'' '83' (1978) 1005

★ Marc W. Buie, Phases of Charon as seen from Pluto, Lowell Observatory

★ Hubble reveals new map of Pluto, BBC News, 12 September 2005

★ IAU Circular No. 3241 [1] describing the discovery

★ Measuring the Size of a Small, Frost World (ESO press release January 2006) [2]

★ M. J. Person et al.: Charon’s Radius and Density from the Combined Data Sets of the 2005 July 11 Occultation (submitted to the Astronomical Journal, 3 February 2006)