Charon: Pluto's Faithful Companion

Scientific Summary

This article presents Charon, Pluto's largest moon, with a diameter of 1,212 km (more than half of Pluto) and a mass of about 12% of Pluto's. Its surface is mainly composed of water ice with traces of hydrated ammonia. It features spectacular landscapes, including the Serenity Chasma canyon (1,000 km long, 9 km deep) and the Mordor Macula (a reddish region of tholins). Charon and Pluto are in synchronous rotation (period of 6.387 days), and the barycenter of the system is located outside Pluto, forming a unique binary system. Its origin is likely a giant impact, and it shows signs of past tectonic activity and cryovolcanism. The New Horizons mission (2015) revealed these features.

What is Charon and what are its remarkable features?

Charon is Pluto's largest moon, discovered in 1978 by astronomer James Christy. With a diameter of about 1,212 km (more than half of Pluto's) and a mass representing 12% of Pluto's, it is one of the largest moons relative to its planet in the Solar System. Its surface is mainly composed of water ice with traces of hydrated ammonia, contrasting with Pluto, which contains more volatile ices (nitrogen, methane). Charon's geology is varied: it features the Serenity Chasma canyon (1,000 km long, 9 km deep), mountains, plains, and craters. A reddish region at the north pole, called Mordor Macula, is composed of tholins (complex organic compounds). These tholins come from a unique mechanism in the Solar System: methane escaping from Pluto's atmosphere is captured by Charon's gravity, condenses at the poles during its very long winters, and is then transformed into tholins by irradiation. Charon and Pluto are in synchronous rotation: they always face each other. Their barycenter (common center of mass) is located outside Pluto, making this duo a unique binary system in the Solar System. Charon was likely born from a giant impact between Pluto and another Kuiper Belt object, and it shows signs of past tectonic activity and cryovolcanism, with a probable subsurface ocean now frozen. The New Horizons mission (2015) revolutionized our knowledge of this fascinating moon.

Charon, Pluto's Largest Moon

Dimensions and Mass

Charon has several astonishing and unique features. Charon has a diameter of about 1,212 kilometers, slightly more than half the diameter of Pluto. It is one of the largest moons relative to the size of its planet in the Solar System. Charon's mass is about 1.586 × 10²¹ kilograms, which is approximately 12% of Pluto's mass.

This mass and diameter give Charon an average density of about 1.70 g/cm³, significantly lower than Pluto's (≈1.85 g/cm³). This difference indicates that Charon is proportionally richer in ice: its internal composition is estimated at about 55% silicate rocks and 45% ice (mainly water), compared to about 70% rocks for Pluto. This compositional difference is an important clue about Charon's formation conditions.

Composition and Surface

A Water Ice Crust

Charon's surface is primarily composed of water ice, with traces of hydrated ammonia and possibly methane ice. This composition differs from Pluto's, which contains more volatile ices such as nitrogen, methane, and carbon monoxide.

A Landscape Marked by Serenity Valley

Charon's surface is characterized by deep canyons, mountains, plains, and craters. One of the most remarkable features is the Serenity Chasma canyon, which is about 1,000 kilometers long and up to 9 kilometers deep.

The Red Mystery of Mordor Macula

Charon's north pole has a reddish region called Mordor Macula. This coloration is likely due to the presence of tholins, complex organic compounds formed by the irradiation of methane ices.

The Mechanism of Atmospheric Transfer from Pluto to Charon

The process behind the Mordor Macula is unique in the known Solar System: it involves the transfer of atmospheric material from one body to another, without the receiving body having its own atmosphere. This mechanism was detailed in 2016 by Will Grundy's team (Lowell Observatory, New Horizons mission) using data from the 2015 flyby.

Pluto's thin atmosphere, primarily composed of nitrogen and methane, continuously escapes into space. Although nitrogen is the dominant gas, methane, being lighter, escapes about 500 times faster. At a distance of about 19,600 km, Charon's gravity intercepts a small fraction of this methane flow.

The Pluto-Charon system takes 248 years to orbit the Sun, and its axis is highly tilted. This results in extreme seasons at Charon's poles: over a century of continuous night, alternating with over a century of continuous day. During these long polar winters, the surface temperature drops to just a few degrees above absolute zero (about -257°C), cold enough for the captured methane to condense and deposit directly as ice ("cold-trapping").

Once deposited, this frozen methane is exposed to solar ultraviolet radiation and cosmic rays. This irradiation breaks methane molecules and promotes their recombination into complex organic macromolecules, tholins, whose characteristic reddish-brown color gives the polar region its unique appearance. When the seasons change and the pole in question returns to light, any remaining volatile ices sublimate and return to space, but the tholins, being non-volatile, remain trapped on the surface and accumulate over successive seasonal cycles.

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This model also explains why the Mordor Macula is concentrated at the poles rather than spread across Charon's entire surface: only the polar regions reach temperatures low enough during their winter to permanently trap methane. Observations of "Pluto-shine" (sunlight reflected by Pluto) suggest that a similar deposit exists at Charon's south pole, which was in darkness during New Horizons' July 2015 flyby.

Geology

Ice Volcanoes in the Past: A Debated Hypothesis

As early as 2007, ground-based observations (Gemini telescope) detected deposits of crystalline ice and ammonia hydrates on Charon's surface. Since solar radiation normally degrades this type of ice into an amorphous form within tens of thousands of years, its presence in crystalline form suggested a recent deposit, initially attributed to past cryovolcanic activity (ejection of water, ammonia, or methane from the interior, rapidly freezing on the surface).

However, the New Horizons flyby in 2015 revealed no active cryovolcanoes or geysers. Subsequent research has also questioned the cryovolcanic origin of these crystalline deposits, with some researchers proposing instead a passive renewal of ice and ammonia from the subsurface, without actual eruptions. The cryovolcanic hypothesis thus remains debated rather than established.

A Crust Marked by Tectonics

Charon shows signs of tectonic activity, with faults and rifts indicating that its crust has been fractured by internal forces. This could be the result of the contraction of its frozen subsurface ocean or the cooling and contraction of the interior.

Interaction with Pluto

A Perfect Synchronous Rotation

Charon and Pluto are in synchronous rotation, meaning they always present the same face to each other. Charon's orbital period around Pluto is 6.387 Earth days, which also corresponds to Charon's rotation period.

A Unique Binary System

Due to Charon's relatively large size compared to Pluto, the center of mass of the Pluto-Charon system (the barycenter) is located outside Pluto. This makes it a unique binary system in the Solar System.

Origin and Evolution

A Giant Impact at the Origin of Charon

It is likely that Charon formed as a result of a giant impact between Pluto and another Kuiper Belt object. The debris from this collision would have accreted to form Charon.

A Subsurface Ocean Now Gone

Charon may have had a subsurface ocean of liquid water, maintained by heat generated by the decay of radioactive elements. Over time, this ocean would have frozen, contributing to Charon's current geological features.

Atmosphere

An Almost Non-Existent Atmosphere

Unlike Pluto, Charon does not have a significant atmosphere. Any initially present atmosphere would likely have been too weak to persist under Charon's conditions of extreme cold and low gravity.

References

FAQ: Everything You Need to Know About Charon

What is Charon and how was it discovered?

Charon is the largest moon of Pluto. It was discovered in 1978 by American astronomer James Christy, who was analyzing photographs of Pluto taken at the United States Naval Observatory. He noticed a small bulge on the edge of Pluto, which turned out to be a moon. Charon was named after the ferryman of the underworld in Greek mythology.

How big is Charon compared to Pluto?

Charon is exceptionally large relative to Pluto. Its diameter of about 1,212 km is more than half that of Pluto (2,377 km). Its mass is about 12% of Pluto's. This proportion is unique in the Solar System, making the Pluto-Charon system a binary system: the center of mass (barycenter) is located outside Pluto.

What is Charon's surface composed of?

Charon's surface is primarily composed of water ice, with traces of hydrated ammonia and possibly methane ice. This composition differs from Pluto's, which contains more volatile ices (nitrogen, methane, carbon monoxide). The surface features deep canyons (Serenity Chasma, up to 9 km deep), mountains, plains, and craters. At the north pole, the Mordor Macula is a reddish region due to the presence of tholins, complex organic compounds formed by the irradiation of methane ices.

What is the Pluto-Charon binary system?

The Pluto-Charon system is a binary system because the barycenter (common center of mass) is located outside Pluto, about 960 km from its surface. This means that both Pluto and Charon orbit around this common point in a unique gravitational dance. Additionally, they are in synchronous rotation: they always present the same face to each other, which is also the case for Pluto relative to Charon. Their common orbital period is 6.387 Earth days.

What is the likely origin of Charon?

The most likely hypothesis for Charon's formation is a giant impact between Pluto and another Kuiper Belt object of comparable size. This collision would have ejected debris that later accreted to form Charon. This scenario is similar to the formation of Earth's Moon. The differences in composition between Pluto and Charon (Charon being richer in water ice, Pluto richer in volatiles) support this hypothesis.

Does Charon have geological activity?

Yes, Charon shows signs of past geological activity:
• Tectonic activity: Faults, rifts, and canyons (Serenity Chasma) indicate that the crust has been fractured, likely due to internal contraction (cooling and freezing of a subsurface ocean).
• Evidence of cryovolcanism: Cryovolcanoes (ice volcanoes) may have ejected water, ammonia, or methane that rapidly froze in the cold space.
• Past subsurface ocean: Charon may have hosted a liquid water ocean maintained by radioactive heat, now frozen, which contributed to its current landscape.

Where exactly does the methane that forms Mordor Macula come from?

Charon does not have its own atmosphere capable of producing tholins. The methane comes from Pluto's atmosphere: a small fraction of the gas escaping from it is captured by Charon's gravity, about 19,600 km away, with methane escaping about 500 times faster than nitrogen, which is the dominant gas in Pluto's atmosphere. This methane then condenses at Charon's poles during extremely long winters, exceeding a century on Earth due to the slow revolution of the Pluto-Charon pair around the Sun (248 years) and its tilt. Ultraviolet and cosmic radiation then transforms this frozen methane into reddish tholins, which remain trapped on the surface even when the surrounding volatile ices sublimate. This phenomenon of one body being "painted" by another's atmosphere is unique in the known Solar System.

What did the New Horizons mission discover about Charon?

The New Horizons probe (NASA) flew by Pluto and Charon in 2015, revolutionizing our knowledge of Charon. It revealed:
• The detailed topography of Charon, with canyons (Serenity Chasma) up to 9 km deep and mountains.
• The surface composition, dominated by water ice, with traces of ammonia.
• The Mordor Macula, a reddish region at the north pole, composed of tholins (complex organic compounds).
• Signs of tectonics and past cryovolcanism.
• The absence of a significant atmosphere on Charon, unlike Pluto.
• Precise information about the binary system and orbital dynamics.