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Charon, the largest of the three satellites of Pluto dwarf planet, around which is in synchronous rotation, at an average distance of about 19570 km, and with which it forms a kind of binary system. It was discovered by James Christy in 1978. It has a diameter of about 1200 km, the average density of 1650 kg / m3 and a mass equal to about 1.5 x 1021 kg. Christy, being the discoverer, had the right to assign a final name to the object. His choice fell on the mythological figure of Charon, the psychopomp who, in Greek mythology, carries the dead to Hades, Pluto‘s realm.
In fact, the choice of Jim Christy was based on an original combination of the mythological name of Charon, and the name of his wife, Charlene, known as “Char”. The name was officially accepted the International Astronomical Union at the end of 1985 and announced January 3, 1986. The discovery of Charon allowed the astronomers to calculate more accurately the mass and size of Pluto. Charon revolves around it in 6.387 days, a period identical to the rotation of both objects. They are therefore both in the synchronous rotation and always show the same hemisphere. While Pluto is covered with nitrogen and methane ice, the surface of Charon appears covered by less volatile water ice and seems devoid of atmosphere. Observations made in 2007 by the Gemini telescopes on patches of ammonium hydrates and water crystals on the surface did assume the presence of cryo-geysers or cryovolcanic activities.
The fact that the ice was still in the crystalline form suggested that it was recently deposited because otherwise, the solar radiation would have degraded to an amorphous state in about thirty thousand years. Looking at a series of mutual eclipses of Pluto and Charon, astronomers were able to obtain the combined spectrum of both. Then subtracting the spectrum of Pluto was possible to determine the surface composition of the satellite. The surface albedo photometric mapping shows a dependence on the latitude, with an equator lighter and darker poles. In addition, the south polar region appears darker than the northern. The US probe New Horizons also identified a long deep canyon about nine kilometers.
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