Charon: Pluto's Largest Moon
Pluto and its largest moon Charon as seen in natural color by NASA's New Horizons spacecraft during its historic July 2015 flyby of the dwarf planet.
Credit: NASA/JUAPL/SwRI

Among the five moons of Pluto, it is the largest, Charon, that makes the dwarf planet unique. Charon is so big that, with Pluto, the pair are considered a binary planet system. When NASA's New Horizons space probe made its historical flyby of the system in 2015, it captured a wealth of data about the amazing moon.

Although Pluto was discovered in 1930, its largest moon, Charon, wasn't spotted until 1978, by astronomer James Christy at the Naval Observatory in Washington, D.C. According to an article in Sky & Telescope magazine, Christy first thought of naming the moon after his wife, Charlene. The article states that Christy had a brainstorm and told his wife, "I could call it after you! How about Charon?” He pronounced it SHAR-on. Putting “on” at the end made it sound genuinely scientific, like electron or neutron.

However, his colleagues wanted to call the moon Persephone, after the daughter of Zeus and Demeter, who was kidnapped by Pluto and made queen of the underworld. Wanting his proposal to stand a chance, Christy did some research and discovered that Charon was the ferryman who took souls across the river Styx to the underworld. Although it would officially be pronounced "KAR-on" or "KAR-en," Christy — and others who want to honor his discovery — pronounces it "SHAR-on." 

The moon orbits its planet every 6.4 Earth days, the same amount of time it takes Pluto to rotate once. The two are tidally locked, with one face permanently turned toward the other. 

The Pluto-Charon system is considered a binary planet, the only one in the solar system. At 750 miles (1,200 kilometers) in diameter, Charon is about half as wide as Pluto. The center of mass of the two bodies lies outside the surface of the dwarf planet. 

"In terms of the dynamics of how planets form around binary star systems, Pluto is the closest example we have," Scott Kenyon, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics (CfA), told Space.com previously.

The pair likely formed at the same time, when two objects collided. Unlike most of the planets and moons orbiting the sun, the Pluto-Charon moon system is tipped on its side, and Pluto has a retrograde orbit compared to other worlds, both suggestive of a violent beginning. The proto-Pluto and proto-Charon were likely quite different, leading to two different types of worlds. The remaining debris likely formed the other four smaller moons of Pluto. 

The south pole of Charon entered polar night in 1989, and will not see sunlight again until 2107. New Horizons was able to study some of the nighttime landscape as it was very slightly illuminated by light from Pluto. The moonlight of Charon also helped scientists to study Pluto once the spacecraft left the daytime side. They gazed back on the dwarf planet in the reflected moonlight of Charon, gathering even more information about the world with the help of its companion.

NASA's New Horizons spacecraft obtained this high-resolution enhanced color view of Pluto's moon Charon just before the closest approach on July 14, 2015.
NASA's New Horizons spacecraft obtained this high-resolution enhanced color view of Pluto's moon Charon just before the closest approach on July 14, 2015.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

New Horizons revealed an unusual feature, a surprising red formation at Charon's northern pole. This reddish hue comes from Pluto's atmosphere. Pluto itself is too small to hold onto its atmosphere for its lifetime, so the nitrogen, methane and carbon monoxide leave the surface. Close-orbiting Charon captures some of the material, which is then funneled toward the surface. As the material collects, galactic cosmic rays and ultraviolet light from the sun interact to create tholins, complex compounds that form through the irradiation of simple organic compounds.

While Pluto is able to hold onto a tenuous atmosphere, Charon is not large enough. The moon is extremely cold, with polar temperatures ranging from minus 433 to minus 351 degrees Fahrenheit (minus 258 to minus 23 degrees Celsius). According to researchers, the gas that reaches Charon likely freezes directly onto the surface, skipping over the liquid phase.

The remainder of the surface of the large moon is made up of water-ice, creating a grayish-white. Charon is more heavily cratered than Pluto, suggesting that the surface is older than its companion. 

Only a single mountain stands on the moon, and it's a strange one. Nicknamed the "mountain in a moat," the mountain sits in a deep hole.

"This is a feature that has geologists stunned and stumped," Jeff Moore of NASA Ames Research Center in California, who leads New Horizons' geology, geophysics and imaging team, said in a statement.

Charon also boasts a spectacular canyon that stretches more than 1,000 miles (1,600 km) across the surface. The chasm is at least four times longer than Arizona's Grand Canyon and twice as deep in some places, according to New Horizons team members. The enormous slice likely wraps around to the unseen dark side of Charon.

"It looks like the entire crust of Charon has been split open," John Spencer, of the Southwest Research Institute in Boulder, Colorado, said in a statement.

To the south of the enormous canyon lies a smoother plain informally known as Vulcan Planum. The surface here is less cratered than the northern region. Grooves and faint ridges on the plain suggest wide-scale resurfacing that may come from cold volcanic activity known as cryovolcanism.

“The team is discussing the possibility that an internal water ocean could have frozen long ago, and the resulting volume change could have led to Charon cracking open, allowing water-based lavas to reach the surface at that time,” said Paul Schenk, a New Horizons team member from the Lunar and Planetary Institute in Houston, said in a statement.

Pluto is also thought to have once housed a liquid ocean beneath its crust.

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