Charon (moon) - WikiHQ

Charon ( or ), 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. The dwarf planet systems Pluto–Charon and Eris–Dysnomia and the dwarf planet candidate system Salacia–Actaea are the only known examples of mutual tidal locking in the Solar System, though it is likely that –Vanth is another.

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, or may have been transferred over from the atmosphere of Pluto to the orbiting moon.

The New Horizons spacecraft, the only probe to visit the Pluto system, approached Charon to within in 2015.

Discovery

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 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. 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.

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

thumb|right|[[Psyche (mythology)|Psyche with Charon in his lunate boat, the basis of the symbol 16px|alt=⯕]]

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 Oz, then 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 was serendipitously the name of an appropriate mythological figure: Charon (; ) 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.

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;

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 sound, and this is often followed for the moon as well. However, Christy himself pronounced the initial as a sound, as he had named the moon after his wife Charlene. Many English-speaking astronomers follow the classical convention, but others follow Christy's, and that is the prescribed pronunciation at NASA and of the New Horizons team.

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a symbol for Charon (16px|⯕) that combines the high orb of Pluto's bident symbol with a crescent, suggesting both Charon as a moon and the mythological Charon's boat crossing the river Styx. This symbol is not widely used, but it coincidentally matches a Pluto symbol used in Uranian astrology.

Orbit

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 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 about . However, Pluto moves with respect to the barycenter of the system as well, as the barycenter's distance from the center of Pluto is larger than the radius of Pluto. Therefore, Charon is not as far from the true center of the Pluto–Charon system, at only .

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. 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. 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 () 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.

Physical characteristics

thumbnail|right|Size comparisons: [[Earth, the Moon, and Charon]]

Charon's diameter is , just over half that of Pluto.

Pluto does have a thin but significant atmosphere, which Charon's gravitation might pull toward Charon's surface. The gas, specifically 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.

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. Charon's relatively low gravity, due to its low mass, causes any atmosphere that might be present to rapidly escape the surface into space. 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 potential explanation for Charon's lack of atmosphere; the solar winds remove gases faster than they can accumulate. It is still possible for Charon to have an atmosphere, as 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/cm3, 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.

There has been significant proof of CO2 gas and H2O 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.

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 disks (3). 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 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% 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.

External links

Charon Profile at NASA's Solar System Exploration site
Hubble reveals new map of Pluto, BBC News, September 12, 2005
Cryovolcanism on Charon and other Kuiper Belt Objects
New Horizons Camera Spots Pluto's Largest Moon – July 10, 2013
40th anniversary NASA video describing the discovery and naming of Charon (June 22, 2018)
NASA CGI video of Charon flyover (July 14, 2017)
CGI video simulation of rotating Charon by Seán Doran (see album for more)
Google Charon 3D, interactive map of the moon
Interactive 3D gravity simulation of Pluto and Charon in addition to Pluto's four other moons Styx, Kerberos, Hydra and Nix