Charon (moon)
On the 22nd of June 1978, astronomer James Christy examined photographic plates at the United States Naval Observatory Flagstaff Station. He noticed a slight elongation on images of Pluto that appeared periodically. This bulge had been visible on plates dating back to the 29th of April 1965. The periodicity matched Pluto's rotation period, confirming the presence of an orbiting companion. The International Astronomical Union announced this discovery to the world on the 7th of July 1978. Christy initially suggested naming the moon Oz, inspired by his wife Charlene's nickname. Colleagues proposed Persephone instead, but he insisted on Charon. This name refers to the mythological ferryman of the dead who transports souls across the river Styx. The IAU officially adopted the name in late 1985 and announced it on the 3rd of January 1986. Nearly four decades before this discovery, science fiction author Edmond Hamilton had named three moons of Pluto including Charon in his 1940 novel Calling Captain Future. A minor debate persists over pronunciation, with some astronomers using a soft sh sound while others follow classical conventions.
Charon and Pluto orbit each other every 6.387 days within the outer solar system. These two objects are gravitationally locked so that each keeps the same face toward the other at all times. This mutual tidal locking is unique among known satellite systems in our solar neighborhood. The barycenter of the Plutonian system lies outside Pluto itself because of their comparable masses. The average distance between Charon and Pluto measures about 19,640 kilometers. However, Pluto moves around this center of mass as well since it exceeds Pluto's radius. Therefore Charon orbits closer to the true center of gravity than previously thought. Only 12 percent of Pluto's mass resides in Charon yet this ratio creates an unusual dynamic. Astronomers refer to the Pluto-Charon system as a dwarf double planet due to these orbital mechanics. No other known example exists where both bodies remain tidally locked to one another except for Eris-Dysnomia and Salacia-Actaea.
Simulation work published in 2005 by Robin Canup suggested Charon formed from a collision around 4.5 billion years ago. A large Kuiper belt object struck Pluto at high velocity destroying itself and blasting off much of Pluto's outer mantle. 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 very similar density of Pluto and Charon implies parent bodies were not fully differentiated when the impact occurred. Two broad competing views on the nature of Charon's interior arose following New Horizons data. The hot start model suggests rapid accretion within approximately 3,000 years resulting from a highly disruptive giant impact scenario. This rapid time scale prevents heat from radiating away during formation leading to partial melting of outer layers. A liquid subsurface ocean forms during or soon after Charon's accretion and persists for approximately 2 billion years before freezing. In contrast the cold start model argues that a large subsurface ocean early in history is not necessary to explain surface features. According to this view Charon may have been homogeneous and more porous at formation.
Unlike Pluto's surface which consists of nitrogen and methane ices Charon appears dominated by less volatile water ice. Photometric mapping shows a latitudinal trend with a bright equatorial band and darker poles. The north polar region is dominated by a very large dark area informally dubbed Mordor Macula by the New Horizons team. These gases include nitrogen carbon monoxide and methane condensing into solid forms during winter at minus 258 degrees Celsius. When these ices are subjected to solar radiation they chemically react to form various reddish tholins. Later when the area heats up to minus 213 degrees Celsius volatiles sublimate leaving only tholins behind over millions of years. Charon has extensive grabens and scarps such as Serenity Chasma extending as an equatorial belt for at least 400 kilometers. Argo Chasma potentially reaches as deep as 7 kilometers with cliffs rivaling Verona Rupes on Miranda for height. Evidence suggests massive resurfacing events occurred perhaps prompted by partial or complete freezing of an internal ocean removing many earlier craters.
In contrast to Pluto Charon has no significant atmosphere despite speculation about an extremely thin exosphere surrounding it. There has been discussion regarding how methane sublimates briefly creating periods of exosphere formation interspersed by centuries of dormancy. Pluto does have a thin but significant atmosphere which Charon's gravity might pull toward its surface. The gas specifically nitrogen is mostly caught in the combined center of gravity before reaching Charon. Any gas that does reach Charon is held closely against the surface making amounts negligible compared to Pluto's total atmosphere. Even through stellar occultation scientists cannot confirm an existing atmosphere though this was tested in 1986 while attempting observations on Pluto. Charon acts as a protector for Pluto's atmosphere blocking solar winds that would normally collide with Pluto and damage it. Since Charon blocks these solar winds its own atmosphere diminishes instead of Pluto's. This effect provides a potential explanation for Charon's lack of atmosphere since solar winds remove gases faster than they can accumulate.
Since first blurred images appeared in 1978 researchers waited decades for clearer views of the moon. Images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. In 1994 the clearest picture showed two distinct and well-defined disks from 4.4 billion kilometers away using Hubble's Faint Object Camera. Later adaptive optics made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes. A group of amateur astronomers in Italy used a 14-inch telescope in 2008 to successfully resolve Charon in an image of Pluto. In June 2015 the New Horizons spacecraft captured consecutive images as it approached the system creating the best animation available at that date. It was the only spacecraft to visit and study Charon during July 2015 closest approach. Charon's discoverer James Christy and children of Clyde Tombaugh attended events at Johns Hopkins Applied Physics Laboratory during this historic flyby.
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Common questions
When was the moon Charon discovered by James Christy?
Astronomer James Christy discovered Charon on the 22nd of June 1978 while examining photographic plates at the United States Naval Observatory Flagstaff Station. The International Astronomical Union announced this discovery to the world on the 7th of July 1978.
How did scientists name the moon Charon after its discovery in 1978?
James Christy initially suggested naming the moon Oz but colleagues proposed Persephone before he insisted on Charon. The IAU officially adopted the name in late 1985 and announced it on the 3rd of January 1986.
What is the orbital relationship between Pluto and Charon?
Charon and Pluto orbit each other every 6.387 days within the outer solar system with a mutual tidal locking that keeps the same face toward one another. This unique dynamic places the barycenter outside Pluto itself because their masses are comparable enough to create a dwarf double planet system.
How did the moon Charon form according to scientific models from 2005?
Simulation work published in 2005 by Robin Canup suggests Charon formed from a collision around 4.5 billion years ago involving a large Kuiper belt object striking Pluto. This impact scenario explains why both bodies share very similar densities despite differences in surface composition.
Why does Charon lack a significant atmosphere compared to Pluto?
Charon has no significant atmosphere because its gravity cannot hold gases against solar winds which remove them faster than they can accumulate. Any gas reaching Charon is held closely against the surface making amounts negligible compared to Pluto's total atmosphere.
When was the first clear image of Charon captured by spacecraft or telescope?
Images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. The New Horizons spacecraft became the only mission to visit and study Charon during July 2015 closest approach creating the best animation available at that date.