Charon (moon)

Charon is the largest of the five known natural satellites orbiting the dwarf planet Pluto.

With a mean radius of approximately 606 kilometers, it is a significant planetary-mass moon. Charon is also the sixth-largest known trans-Neptunian object. This means it is one of the largest objects found beyond the orbit of Neptune. It follows Pluto, Eris, Haumea, Makemake, and Gonggong in size. Because of its scale, Charon is the twelfth-largest natural satellite in the entire Solar System.

The relationship between Pluto and Charon is unique due to their mass ratio. Charon has half the diameter of Pluto but one-eighth of its mass. This high mass ratio causes a phenomenon called mutual tidal locking. In this state, both Pluto and Charon always show the same face to each other.

Most moons, like Earth's Moon, are tidally locked to their planet, but the planet can still rotate. In the Pluto-Charon system, they orbit a shared center of mass called a barycenter. Because Charon is so massive, this barycenter actually lies outside the surface of Pluto. This makes the system a dwarf double planet.

Charon's surface features a striking reddish-brown cap at its north pole.

This cap is composed of tholins. Tholins are organic macromolecules, which are large, complex molecules. These molecules may be essential ingredients for life. Scientists believe these tholins formed from methane, nitrogen, and other related gases. These gases might have been released by cryovolcanic eruptions on Charon. Alternatively, the gases may have transferred from Pluto's atmosphere to the moon.

Astronomer James Christy discovered Charon in 1978 at the United States Naval Observatory.

While examining highly magnified photographic plates of Pluto, Christy noticed a periodic elongation. This bulge appeared to move in sync with Pluto's rotation. He realized this bulge was actually a smaller body accompanying Pluto. The International Astronomical Union formally announced the discovery on July 7, 1978. Christy originally considered naming the moon Oz or Persephone. He eventually chose Charon, the ferryman of the dead in mythology, as a tribute to his wife, Charlene.

Scientists have proposed two main theories regarding how Charon formed. The first is the hot start model. This model suggests a violent collision occurred about 4.5 billion years ago. A large Kuiper belt object may have struck Pluto at high velocity. This impact would have blasted off Pluto's outer mantle, which then coalesced into Charon. This process would have generated intense heat. This heat could have created a subsurface ocean that lasted for 2 billion years.

The second theory is the cold start model. This model suggests a more gradual formation process. It argues that a large subsurface ocean was not necessary to explain Charon's surface. Under this model, Charon may have been more homogeneous and porous when it first formed. The internal structure of Charon is estimated to be 55% rock and 45% ice. This is slightly less dense than Pluto, which is about 70% rock. These differences help scientists understand the chemical history of the outer Solar System.

Our understanding of Charon changed significantly in 2015. The New Horizons spacecraft became the only probe to visit the Pluto system.

The probe approached Charon very closely, providing high-resolution data. These observations confirmed that Charon is a differentiated body. This means its internal materials, like rock and ice, have settled into distinct layers. The mission also provided updated mass measurements. These measurements showed Charon's mass is approximately 12.18% of Pluto's mass. This data helps astronomers study the orbital stability of other complex planetary systems.