Charon is a medium-sized body, approximately 750 miles (1,207 kilometers) wide, located in the Kuiper Belt—a ring of icy debris, comets, and dwarf planets, also called trans-Neptunian objects (TNOs), at the edge of the solar system. Unlike many larger Kuiper Belt objects, Charon’s surface is not covered in volatile ices like methane, allowing scientists to gain valuable insight into the influence of sunlight on cratering these distant bodies. Furthermore, Charon is the only medium-sized TNO for which geological mapping is available. This was made possible thanks to data collected by NASA’s New Horizons spacecraft, which visited the Pluto system about a decade ago.
New models suggest that the freezing of Charon’s internal ocean could have formed deep, elongated depressions along its midsection, but this could indicate that the outer shell was thinner at some point in the moon’s history than currently predicted. The models also suggest that cryovolcanoes erupting with ice, water, and other materials are less likely in Charon’s northern hemisphere.
Charon’s icy geological features came as a shock to scientists when NASA’s New Horizons spacecraft visited the Pluto-Charon system in 2015. Previously, scientists believed Charon to be an inert ball of ice. Since then, a scientific team led by researchers from the Southwest Research Institute (SwRI) has been studying data from New Horizons, trying to understand the cause of these icy geological features.
Team member and SwRI researcher Alyssa Roden is an expert in the geophysics of icy moons, particularly those with their own oceans.
An image of Charon with markers indicating cracks on its surface that could be caused by a frozen internal ocean. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest
“A combination of geological interpretations and thermo-orbital evolution models suggests that Charon once had a liquid ocean beneath its surface, which subsequently froze,” she said. “When an internal ocean freezes, it expands, creating significant stresses within its icy shell and increasing the pressure on the water beneath. We hypothesized that this is what caused the formation of large canyons and cryovolcanic flows on Charon.” The team’s research will be published in the journal Icarus in 2023.
Roden modeled the formation of cracks in Charon’s icy shell as the ocean beneath it freezes to better understand the evolution of the moon’s surface and interior. The oceans included in the models consisted of water, ammonia, and a mixture of the two. Although ammonia can act as an antifreeze, and high concentrations may contribute to the persistence of liquid oceans, Roden found that differences in ocean composition did not significantly affect the results.
When the frozen ocean pressed on Charon’s outer shell, it caused cracks to form that ran through the entire shell. As the ocean’s volume increased, the pressure on the overlying liquid grew stronger, causing it to erupt through the cracks onto Charon’s surface.
A team of scientists searched for conditions under which cracks could have completely penetrated Charon’s icy shell, connecting surface and groundwater and facilitating ocean-sourced cryovolcanism. This revealed that current theories of Pluto’s moon’s evolution may be incorrect. These theories suggest that Charon’s icy shells were too thick to be completely fractured by the stresses associated with the freezing of the ocean.
An image of the dwarf planet Pluto (left) and its largest moon, Charon. Mark Garlick/Science Photo Library/Getty Images
Either Charon’s ice shell was less than 10 km thick when the flows formed, rather than more than 100 km as previously assumed, or the surface was not directly connected to the ocean during the eruption. If Charon’s ice shell were thin enough to completely fracture, this would imply a significantly more severe freezing of the ocean than the canyons found on Charon’s hemisphere indicate.
These canyons run along a global tectonic ridge belt that crosses Charon’s surface, dividing the northern and southern geological regions of Pluto’s moon. The model developed by the scientists suggests that the canyons may have formed in cracks in the ice sheet that do not reach its ocean, meaning they formed after fractures caused by cryovolcanism and when Charon’s shell thickened.
The idea that Charon’s cryovolcanism is driven by a frozen ocean could be confirmed if a future mission discovers additional, larger, extended features across the moon’s hemisphere. These features, not detected by New Horizons, support the hypothesis that Charon’s ocean was thicker than expected, and its crust thinner.
Ocean freezing also predicts a sequence of geological activity in which ocean-driven cryovolcanism ceases before deformation-driven tectonism sets in. A more detailed analysis of Charon’s geological record may help determine whether this scenario is realistic.
Using the James Webb Space Telescope (JWST), astronomers detected carbon dioxide and hydrogen peroxide on the frozen surface of Charon, Pluto’s largest moon, in 2024. Detecting these molecules could help scientists understand how Charon and other icy bodies on the outskirts of the solar system formed.
Charon has been extensively studied since its discovery in 1978, but previous studies were limited in the wavelengths of light they could detect. This left gaps in our understanding of the surface composition of Pluto’s moon. While scientists have found water ice, ammonia compounds, and organic compounds on Charon, carbon dioxide and hydrogen peroxide have not been detected. At least until now.
This image of the cracked and cratered surface of Pluto’s moon Charon, taken by the New Horizon mission in 2015. NASA/JPL-Caltech
A team of scientists led by Sylvia Protopapa of the Southwest Research Institute (SwRI) filled these gaps by studying Charon using the Near-Infrared Spectrograph (NIRSpec) on the Joint Westinghouse Telescope (JWST). The study was published on October 1, 2024, in the journal Nature Communications.
The study shows that Charon’s surface retains evidence of its formation, such as carbon dioxide, as well as signs of irradiation processes, as evidenced by the presence of hydrogen peroxide. These discoveries expand on the known composition of Charon, including water ice, ammonia compounds, and organic compounds responsible for its gray-red color.
Overall, these factors make Charon an invaluable object of study, revealing much useful information. These discoveries provide valuable insights into how processes such as solar radiation and cratering shape the surface of Charon and, by extension, other medium-sized icy bodies beyond Neptune.
A graph showing Charon’s light signature, indicating the detection of hydrogen peroxide and carbon dioxide. Silvia Protopapa (SwRI), Yan Wong (STScl)
The composition of stars, planets, and moons can be determined by the light they emit or reflect from their surfaces. This is possible because elements absorb and emit light at specific wavelengths. Thus, studying the spectra of celestial bodies using a method called spectroscopy reveals the “fingerprints” of elements and chemical compounds.
Protopapa and her colleagues reached their conclusions by comparing JWST spectroscopic observations with laboratory measurements and detailed spectral models of Charon’s surface. This led them to conclude that carbon dioxide is present predominantly as a surface layer on a water-ice-rich subsurface.
The surface of Charon, as discovered by the New Horizons mission, is characterized by numerous craters surrounded by bright layers of ejecta rich in water ice and ammonia compounds. These geological features indicate that the subsurface materials were exposed by impact events, providing insight into the composition of the moon’s interior. The team interprets this as meaning that the upper layer of carbon dioxide is of internal origin and was brought to the surface by cratering. The presence of carbon dioxide was also expected, as this compound is known to be present in regions of the protoplanetary disk from which the Pluto system formed.
What the team didn’t expect was the detection of hydrogen peroxide. The presence of hydrogen peroxide on the surface of Jupiter’s moon Europa has been known since the 2000s. The unexpected presence of hydrogen peroxide on Charon led the team to suspect that the water-ice-rich surface of Pluto’s largest moon is being actively modified by ultraviolet radiation from the Sun, particles from the solar wind, and streams of charged particles from beyond the solar system called galactic cosmic rays.
Pluto’s moon Charon, as imaged by the New Horizon telescope. NASA/JPL-Caltech
Hydrogen peroxide is formed by the combination of adjacent hydroxide ion radicals, which are formed when water molecules are split by incoming ions, electrons, or photons. The team conducted new laboratory measurements to confirm the possibility of producing hydrogen peroxide even in the presence of carbon dioxide.
The team has not yet completed its study of Pluto’s largest moon. JWST will continue to study Charon, and scientists will use the data they obtain to better understand icy trans-Neptunian objects in general. Future JWST observations, focused on spectral gaps not covered by current data, could lead to new discoveries about Charon’s composition and further expansion of its chemical composition, possibly revealing other mechanisms involved.
A study due in late 2025 suggests that billions of years ago, Pluto may have captured its largest moon, Charon, in a very brief, icy “kiss.” This theory could explain how the dwarf planet was able to capture a moon roughly half its size.
A team of researchers believes that two frigid worlds located in the Kuiper Belt—a ring of icy bodies located far from the Sun at the edge of the Solar System—collided billions of years ago. Instead of annihilating each other, the two bodies merged, forming a rotating “cosmic snowman.” The bodies separated relatively quickly but remained orbitally bound, forming the Pluto/Charon system we see today. The team’s study was published on January 6, 2025, in the journal Nature Geoscience.
See also... New method for observing individual atoms confirms century-old quantum mechanical theory |
This “kiss and capture” process offers a new theory for moon capture and cosmic collisions. It could also help scientists better understand the structural strength of the cold, icy worlds in the Kuiper Belt.
It was discovered that, assuming Pluto and Charon are solid bodies, Pluto could indeed capture Charon in a giant impact. This collisional capture process is called “kiss and capture,” because Pluto and Charon briefly merge, forming a “kissing” element, before separating to form two independent bodies.
Most planetary collision scenarios are classified as “hit and run” or “touch and merge,” meaning the “kiss and grab” scenario is something entirely new.
This illustration shows Pluto and its largest moon, Charon. NASA/Robert Lea
The reason the relationship between Pluto and Charon is so puzzling for scientists is the relatively small difference in size and mass between these two icy bodies. Charon is so enormous compared to Pluto that they are effectively a double planet. It is half the size of Pluto and accounts for 12% of its mass, making it much more similar to Earth’s moon than to any other satellite in the solar system.
By comparison, our Moon is only a quarter the size of Earth, and the largest moon in the solar system, Ganymede, is about 1/28 the size of its parent planet, Jupiter.
A University of Arizona researcher, who is also a NASA postdoctoral fellow, added that capturing such a relatively large moon the “normal” way is difficult. (The “normal” method is considered to be the gravitational capture of moons such as Mars’s moons Phobos and Deimos, as well as the moons of the giant planets Jupiter and Saturn.)
This means that the prevailing theory of the formation of the Pluto-Charon system is based on the idea of collisional capture, similar to how a massive body is thought to have slammed into Earth, ejecting material that our planet captured to form our Moon.
This screenshot shows the Pluto/Charon system during its “snowman” phase. Robert Melikyan/Adyn Denton
In a standard “collisional capture” event, a violent collision occurs, causing the two bodies to stretch and deform like a fluid. This process provides a good explanation for the formation of the Earth/Moon system, as the intense heat released during the collision and the large mass of the bodies involved cause them to act like a fluid.
When considering the collision between Pluto and Charon, another factor must be taken into account: the structural strength of colder icy and rocky bodies. This factor has previously been ignored by researchers when considering Charon’s collisional origin.
To account for this in their simulations, the team turned to the University of Arizona’s high-performance computing cluster. When Denton and his colleagues factored the strength of these materials into their simulations, they discovered something completely unexpected. Because both bodies possess material strength, Charon did not penetrate deeply enough into Pluto to merge with it; this is not true when the bodies are liquid. Under the same impact conditions, assuming Pluto and Charon lack strength, they do merge into a single larger body, with Charon being absorbed. However, due to their strength, Pluto and Charon retain their structure during their brief merger.
Since Charon couldn’t plunge into Pluto in this scenario, it remained outside the so-called “co-rotation radius” of both bodies. As a result, it couldn’t rotate at the same speed as Pluto, meaning the two bodies couldn’t remain fused. The team hypothesizes that after their separation and the end of this icy kiss, Pluto spun Charon into a closer, higher, circular orbit, from which the moon would have drifted outward.
The “kiss” of this capture, the merger, lasts very briefly from a geological perspective, 10-15 hours, after which the two bodies separate again. Charon then begins its slow movement toward its current position.
The team believes that the initial collision occurred very early in the history of the Solar System, likely tens of millions of years after its formation, or billions of years ago.
Observations with the James Webb Space Telescope (JWST) in 2025 show that two moons, Nix and Hydra, are more similar to Charon’s interior than to other Kuiper Belt objects. The results suggest that the mid-sized moons may be composed of fragments of Charon’s interior, ejected during the violent collision that formed Pluto and Charon.
According to previous calculations, Pluto and Charon formed through a “kiss and capture” process. The pair’s progenitors entered into a close encounter, which resulted in the outer layers of proto-Charon being separated, forming a disk of icy debris. These debris later coalesced to form at least four smaller moons: Nix, Hydra, Kerberos, and Styx, according to this hypothesis.
JWST provided an opportunity to explore this idea. Earlier this year, researchers used the space telescope to classify the colors of trans-Neptunian objects (TNOs)—objects beyond the orbit of the eighth planet—in the outer solar system. However, the new study shows that Nix and Hydra don’t fit into any of these classifications.
A composite image of Pluto and Charon taken by NASA’s New Horizons spacecraft as it passed through the Pluto system. NASA/JHUAPL/SwRI
According to Brian Holler, a planetary scientist at the Space Studies Institute, this discrepancy is primarily caused by a reddish substance on the moons’ surfaces, likely containing carbon. Holler presented his new findings at the “Progress in Understanding Pluto: 10 Years After Flyby” conference, held in July in Laurel, Maryland.
In fact, Charon’s ancient interior may be falling onto its surface today. When Nix and Hydra collide with small meteorites, debris from their surfaces could be ejected into space due to their low mass and low gravity. The ejected particles could be captured by Charon’s gravity and fall onto its surface.
“We may be studying the source material of proto-Charon,” Holler said in his July presentation. Ejecta from Nix and Hydra could eventually fall to the surface, forming a dust layer several centimeters thick. Because Charon is a geologically dead world, this dust remains on the surface, forming a layer analogous to the crust and upper mantle that were ejected in the collision that created it.
When NASA’s New Horizons spacecraft flew past the Pluto system in 2015, it captured breathtaking views of Nix and Hydra. Data on the pair’s composition was obtained. Enhanced color images of Nix revealed a reddish, circular pattern. Both moons are irregularly shaped.
Pluto’s moons to scale. The largest, Charon, is shown at the bottom. NASA/JHUAPL
According to Holler, the JWST observations didn’t necessarily reveal significant details about the moons. Instead, they provided an opportunity to directly compare them with other trans-Neptunian objects. The space telescope was focused on studying Pluto and Charon. The discovery of Nix and Hydra was a completely serendipitous event.
A previous study using JWST to classify trans-Neptunian objects filled the gap between the visible and near-infrared wavelengths recorded by New Horizons. This allowed for a direct comparison of Pluto’s mid-sized moons with other outer Solar System objects.
Nix and Hydra themselves are likely largely unchanged since their formation. They are too small to have undergone active geological processes, so the only changes to their surfaces likely resulted from impacts and cosmic weathering.
By studying such systems, researchers can better understand what’s happening within the two small moons. If other trans-Neptunian objects have undergone similar collisions, resulting in the formation of their own small moons, and their compositions can be studied, this could shed even more light on these small but enigmatic Pluto satellites.
Comment on Facebook* The owner of the Meta company is recognized as extremist in Russia and is banned.See also... Complete list of HTML/Javascript event handlers with examples List of required documents for a website in the Russian Federation |
