Charon << KAR ohn >> is the largest of the five moons that orbit the dwarf planet Pluto. Charon was discovered in 1978 by American astronomers James Christy and Robert Harrington at the United States Naval Observatory in Flagstaff, Arizona. The name Charon comes from Greek and Roman mythology. Charon was the ferryman who carried souls across the river to the underworld realm ruled by Pluto.
Christy proposed the name Charon to signify its location in the dark outer region of the solar system and because the first four letters were the same as that of his wife, Charlene. Most of what astronomers know about Charon comes from observations made by the U. S. National Aeronautics and Space Administration (NASA) spacecraft called New Horizons. This spacecraft made its closest flyby of Charon on July 14, 2015.
Charon has a diameter of about 750 miles (1,200 kilometers), about half the diameter of Pluto. It orbits about 12,196 miles (19,640 kilometers) from the dwarf planet. Charon is so close to Pluto in size that the two actually orbit a point above Pluto’s surface. This point between two orbiting objects is called a barycenter. Because of this, Pluto and Charon are often referred to as a binary or double planet—the only such one in our solar system. Pluto and Charon are doubly tidally locked, meaning the same sides of both bodies face each other at all times. Charon’s orbital period—the time it takes a satellite to complete one orbit—equals 6.4 Earth days.
Charon is less dense and has only about 12 percent of the mass of Pluto. Planetary scientists have determined that the moon is about 55 percent rocky material and 45 percent water ice. The northern pole is distinguished by a darker, reddish region where the surface ice is covered by a layer of material called tholins. Scientists believe tholins are made up of methane, carbon monoxide, and nitrogen that have built up at the poles over billions of years, while being chemically altered by cosmic radiation. Those molecules were captured from Pluto’s escaping upper atmosphere. Charon itself has no known atmosphere.
Charon has diverse geology. The southern hemisphere has a smoother surface with fewer impact craters than the northern hemisphere. A canyon at the equator, informally known as Serenity Chasma, extends over 1,000 miles (1,600 kilometers). Scientists think that a major resurfacing event, possibly as a result of the freezing of an internal ocean, created the canyon and covered up many impact craters in the southern hemisphere.
Scientists think that Charon, as well as Pluto’s other four moons, were formed in a collision between two large bodies billions of years ago. The similar composition of Pluto and the moons and the large tilt of the plane of their orbits to that of Pluto’s orbit support this theory.
