Charon, also known as (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. 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. 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 released from the
atmosphere of Pluto and transferred over 19,000 km (12,000 mi) to the orbiting moon.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.
== Discovery ==
Charon was discovered by United States Naval Observatory astronomer James Christy, using
the 1.55-meter (61 in) telescope at United States Naval Observatory Flagstaff Station
(NOFS), On June 22, 1978, he had been examining highly magnified images of Pluto on photographic
plates 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. The
International Astronomical Union formally announced Christy's discovery to the world
on July 7, 1978.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 ==
Author Edmond Hamilton referred to three moons of Pluto in his 1940 science fiction novel
Calling Captain Future, naming them Charon, Styx, and Cerberus.After its discovery, 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". Although colleagues at the Naval Observatory
proposed Persephone, Christy stuck with Charon after discovering that it coincidentally refers
to a Greek mythological figure: Charon ( or ; Greek Χάρων) is the ferryman of the
dead, closely associated in myth with the god Hades, whom the Romans identified with
their god Pluto. The IAU officially adopted the name in late 1985 and it was announced
on January 3, 1986.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. 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.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, and this is the prescribed
pronunciation at NASA and of the New Horizons team.
== 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. 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 about each other. The collision
would have been violent enough to boil off volatile ices like methane (CH4) 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.
== Orbit ==
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 calculate accurately 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 revealed that Charon has approximately 12% of the mass of Pluto.
== Physical characteristics ==
Charon's diameter is 1,212 kilometres (753 mi), just over half that of Pluto, and larger
than the dwarf planet Ceres, and the twelfth largest natural satellite in the solar system.
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%.
=== Interior ===
Charon's volume and mass allow calculation of its density, 1.702±0.017 g/cm3, from which
it can be determined that Charon is slightly less dense than Pluto and suggesting a composition
of 55% rock to 45% ice (± 5%), whereas Pluto is about 70% rock. 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. 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.
=== Surface ===
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.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"
by the New Horizons team. 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. In addition
to Mordor, New Horizons found evidence of extensive past geology that suggests that
Charon is probably differentiated; 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.Charon has a series
of extensive grabens or canyons, such as Serenity Chasma, which extend as an equatorial belt
for at least 1000 km. Argo Chasma potentially reaches as deep as 9 km, with steep cliffs
that may rival Verona Rupes on Miranda for the title of tallest cliff in the solar system.
==== 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.
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 from a distance of 49,000 miles (79,000 km).
== 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 Later, the development of adaptive optics made it possible to resolve
Pluto and Charon into separate disks using ground-based telescopes.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 (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.
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 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.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. 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.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).
== Gallery ==
=== Videos ===
== See also ==
List of natural satellites
== Notes ==
== References ==
== External links == Charon Profile at NASA's Solar System Exploration
site Christy, J. W; Harrington, R. S (1978). "The
satellite of Pluto". Astronomical Journal. 83: 1005. Bibcode:1978AJ.....83.1005C. doi:10.1086/112284.
Hubble reveals new map of Pluto, BBC News, September 12, 2005
Person, M. J; Elliot, J. L; Gulbis, A. A. S; Pasachoff, J. M; Babcock, B. A; Souza,
S. P; Gangestad, J (2006). "Charon's radius and density from the combined data sets of
the 2005 July 11 occultation". The Astronomical Journal. 132 (4): 1575–1580. arXiv:astro-ph/0602082.
Bibcode:2006AJ....132.1575P. doi:10.1086/507330. Cryovolcanism on Charon and other Kuiper Belt
Objects New Horizons Camera Spots Pluto’s Largest
Moon – July 10, 2013 New Horizons in the PHSF clean room at KSC,
Nov 4, 2005 40th anniversary NASA video describing the
discovery and naming of Charon (22 June 2018) NASA CGI video of Charon flyover (14 July
2017) CGI video simulation of rotating Charon by
Seán Doran (see album for more) Google Charon 3D, interactive map of the moon
"2016 Lunar & Planetary Science Conference by National Institute of Aerospace".