Charon

## General Information

Charon is the biggest of Pluto's five known natural satellites. Pluto is a dwarf planet. Its average radius is 606 kilometers. With Pluto, Eris, Haumea, Makemake, and Gonggong as the five biggest known trans-Neptunian objects, Charon is the sixth largest. Using photographic plates obtained at the United States Naval Observatory Flagstaff Station, it was found in 1978 at the United States Naval Observatory in Washington, DC.

Compared to its parent planet, Pluto's moon Charon is much larger, having half the diameter and one eighth the mass. Due to the magnitude of its gravitational pull, Pluto and the other body are tidally locked to one another, with the barycenter of the Plutonian system located outside Pluto.

Tholins are organic macromolecules that are thought to be necessary components of life, and they make up the reddish-brown cap at Charon's north pole. These tholins were created from methane, nitrogen, and associated gasses that may have been carried over 19,000 kilometers from Pluto's atmosphere to the moon it orbits, or they may have been released during cryovolcanic eruptions on the moon.

### History

#### Discovery

Astronomer James Christy of the US Naval Observatory made the discovery of Charon. He had been studying much enlarged photos of Pluto on film plates acquired with the telescope two months earlier on June 22, 1978. Every now and again, Christy detected a small elongation. Validation of the bulge was done on plates dated April 29, 1965. On July 7, 1978, the International Astronomical Union publicly declared Christy's finding to the world.

Pluto was seen again, and the bulge was found to be caused by a tiny companion planet. From Pluto's light curve, it was previously established that the bulge's periodicity correlated with the planet's rotation period. This pointed to a synchronous orbit, indicating that the bulge effect was most likely genuine rather than a hoax. Pluto's size, mass, and other physical attributes have to be reevaluated as a result, as Pluto had previously been solely linked to the system's predicted mass and albedo.

#### Formation

Robin Canup's simulation study from 2005 revealed that Charon may have originated similarly to Earth and the Moon—roughly 4.5 billion years ago—by a collision. According to this concept, Pluto was struck by a massive object from the Kuiper belt at a high speed, which caused it to explode and blow off much of its outer mantle. Charon then formed from the debris. Nonetheless, an impact of this kind ought to produce a rockier Pluto and an icier Charon than what astronomers have observed. Pluto and Charon are currently believed to have been two entities that collided before entering an orbit around one another. Methane (CH4) and other volatile ices would have boiled off from the impact, but neither body would have been completely annihilated.

#### Observations

For the first time since the initial hazy lunar photos, the Hubble Space Telescope captured photographs in the 1990s that showed Pluto and Charon resolved into distinct disks. The greatest, but grainy, photographs of the moon came via the telescope. The Pluto-Charon system was best shown in 1994, when two distinct and well-defined disks were visible. Pluto and Charon might later be resolved into distinct disks with ground-based telescopes thanks to the advancement of adaptive optics. A team of amateur astronomers in Italy managed to successfully detect Charon in a picture of Pluto in 2008, despite the difficulty of ground-based observation.

The New Horizons spacecraft neared the Pluto-Charon system in June 2015 and took a series of pictures of it. An animation was created by combining the photographs. At that time, it was the best photograph of Charon. The New Horizons spacecraft approached the Pluto system at its closest point in July 2015. It is the sole spacecraft that has investigated and visited Charon thus far. James Christy, the discoverer of Charon, and Clyde Tombaugh's kids were there at the Johns Hopkins Applied Physics Laboratory as New Horizons made its closest approach.

### Properties

With a diameter of 1,212 kilometers, Charon is slightly larger than Pluto. It is the twelfth biggest natural satellite in the Solar System, bigger even than the dwarf planet Ceres. Charon's size is comparable to that of Umbriel and Ariel, the moons of Uranus. If Charon is big enough to be in hydrostatic equilibrium, its sluggish rotation should result in minimal flattening or tidal distortion. Any departure from a perfect sphere is too slight for the New Horizons mission's observations to have picked up on. On the other hand, Iapetus is a Saturnian moon that is about the same size as Charon but has a noticeable oblateness that dates back to its early existence.

Charon and Pluto have a mass ratio of 0.1218:1 according to mass updates from New Horizons observations. This is 0.0123:1, far bigger than the Moon to the Earth. Pluto's radius does not include the barycenter because to the high mass ratio; hence, the Pluto-Charon system is sometimes referred to as a dwarf double planet. The Pluto–Charon system has been taken into consideration in research on the orbital stability of circumbinary planets as it contains four smaller satellites around the two bigger worlds.

#### Structure

Based on its volume and mass, Charon's density of 1.702±0.017 g/cm3 can be calculated. This indicates that Charon is somewhat less dense than Pluto, with a composition of around 55% rock and 45% ice (±5%), compared to Pluto's roughly 70% rock composition. When compared to the majority of likely collisional satellites, the difference is noticeably smaller. There were two competing ideas on the interior composition of Charon prior to New Horizons' flyby: one group of scientists believed it would be uniform throughout, while the other believed it would be a differentiated body similar to Pluto, with a rocky core and an ice mantle.

#### Surface

The less volatile water ice seems to predominate on Charon's surface, in contrast to Pluto's surface, which is made up of nitrogen and methane ices. The existence of active cryogeysers and cryovolcanoes was proposed in 2007 by the Gemini Observatory's observations of areas of ammonia hydrates and water crystals on Charon's surface.

The surface of Charon has been photometrically mapped, revealing a latitudinal pattern in albedo with darker poles and a brilliant equatorial band. The vast, gloomy region that the New Horizons crew has affectionately named "Mordor" dominates the north polar region. The most popular explanation for this feature is that gasses that escaped from Pluto's atmosphere condensed to form it. At −258 °C in the winter, these gases—nitrogen, carbon monoxide, and methane—condense into solid forms. When these ices are exposed to sunlight, they undergo a chemical reaction that produces a variety of reddish tholins. Later, as Charon's seasons change and the region becomes heated by the Sun once more, the temperature near the pole climbs to −213 °C, which causes the volatiles to sublimate. The frozen crust becomes obscured over millions of years as massive layers of leftover tholin accumulate. Apart from Mordor, New Horizons also discovered extensive evidence of past geology that implies that Charon is likely differentiated; specifically, the southern hemisphere is significantly less rugged and has fewer craters than the northern, indicating that a massive resurfacing event occurred at some point in the past, possibly due to the partial or complete freezing of an internal ocean, which removed many of the earlier craters.

#### Orbit

When Charon and Pluto began a five-year cycle of mutual eclipses and transits between 1985 and 1990, all doubts regarding Charon's existence were dispelled. This happens just twice in Pluto's 248-year orbital cycle, when the Pluto–Charon orbital plane is edge-on as viewed from Earth. It was fortunate that one of these pauses fell on the heels of Charon's discovery.

The orbital period of Charon and Pluto is 6.387 days. Due to their mutual gravitational attraction, the two objects maintain their identical faces toward one another. Unlike the Moon and Earth, which never switch faces but always reveal the same face to Earth, this situation involves reciprocal tidal locking. Pluto and Charon are separated by an average of 19,570 kilometers (12,160 miles). Based on specifics found in the orbits of the outer moons, Charon is around 12% of Pluto's mass.

#### Classifications

The Pluto-Charon system's barycenter, or center of mass, is located outside of both bodies. It has been suggested that Charon and Pluto should be regarded as members of a binary planet since neither planet really circles the other and since Charon is 12.2% more massive than Pluto. As of right now, the International Astronomical Union considers Charon to be nothing more than Pluto's satellite; nevertheless, that classification of Charon as a dwarf planet may change in the future.