A true colour image of Charon. Image © NASA/ Johns Hopkins University Applied Physics Laboratory/ Southwest Research Institute/ Alex Parker, Public domain, via Wikimedia Commons.
Carbon dioxide and hydrogen peroxide have been detected for the first time on the frozen surface of Pluto’s largest moon, Charon, using observations from the James Webb Space Telescope. The discoveries add to Charon’s known chemical inventory, previously identified by ground- and space-based observations, that includes water ice, ammonia-bearing species and the organic materials responsible for Charon’s grey and red colouration.
Dr Silvia Protopapa, of the Southwest Research Institute, led the study into Charon, the only mid-sized trans-Neptunian object, in the range of 300 to 1,000 miles in diameter, that has been geologically mapped, thanks to the SwRI-led New Horizons mission, which flew by the Pluto system in 2015.
But unlike many of the larger objects in the trans-Neptunian region, the surface of Charon is not obscured by highly volatile ices such as methane and therefore provides valuable insights into how processes like sunlight exposure and cratering affect these distant bodies.
ScienceVega spoke to Dr Protopapa about her work on Charon and her hopes for future research.
ScienceVega: What is so interesting about the moon of Charon?
SP: The moon Charon is fascinating for several reasons. It holds significance within the broader context of trans-Neptunian objects, which are objects that orbit beyond the orbit of Neptune. The Pluto-Charon system is the most extensively studied among trans-Neptunian objects, providing a crucial framework for understanding their composition, formation and evolution. Charon, in particular, unlike many other trans-Neptunian objects, is not obscured by hyper-volatiles like methane or carbon monoxide due to its size. This allows direct observation of its ‘bedrock’, offering insights into processes such as cratering and irradiation, and shedding light on surface evolution in this distant region of the Solar System.
ScienceVega: Pluto was downgraded to dwarf planet status in 2006. Does that classification impact on our understanding and categorisation of Charon?
SP: No. The concept of planet or dwarf planet is just a definition. As we learn more about these objects, we use definitions to classify and categorise them. However, these classifications do not diminish the importance of any object in advancing our understanding of specific aspects of the Solar System.
An SwRI-led team detected carbon dioxide and hydrogen peroxide spectral signatures on Pluto’s largest moon Charon using Webb telescope observations (white), which extend the wavelength coverage of previous New Horizons flyby measurements (pink). The JWST and New Horizons data are consistent in the spectral shape and the strength of the absorption bands at short wavelengths of light; the offset in the absolute amount of scattered light can largely be attributed to differences in viewing geometries. These findings offer clues about the formation and evolution of Charon, shown in the background as imaged by New Horizons. Image © S Protopapa/ SwRI/ NASA/ ESA/ CSA/ STScI/ JHUAPL.
ScienceVega: Why did you decide to further research the chemical inventory of Charon?
SP: All of these objects are very faint and far away. So what we can do from the ground is relatively limited. From Earth, we can observe these objects spectroscopically up to 2.5 microns, but beyond that, there are many more chemical elements that absorb light and show strong absorption bands, much stronger than at shorter wavelengths. One objective for the science community, something that was the focus of my PhD project, was to extend the wavelength coverage beyond 2.5 microns to better understand the chemical inventory of the surface of Charon. But this turned out to be very challenging, so we had to wait for the James Webb Space Telescope to fulfil this goal.
In general, understanding the chemical inventory is important because the more we know about the nature and physical properties of the compounds on trans-Neptunian objects, the better we can understand the formation and evolution of the outer Solar System. The JWST measurements were key to achieving this goal.
ScienceVega: Did you set out at the beginning of this research to reinforce the existing knowledge of the surface composition of Charon or were you hoping that you might find out new things about it?
SP: We didn’t have doubts about what we knew already about the composition of Charon, so the research was not about confirming necessarily the current knowledge. Instead, we wanted to see if we had missed anything because we hadn’t studied Charon with a wide enough range of light. Our main goal was to find out if there are other chemical compounds on Charon that we didn’t know about yet.
As an example, thanks to measurements from New Horizons, we observed Charon’s surface marked by several young craters surrounded by bright ejecta blankets, suggesting that material from beneath the surface had been excavated.
We also knew that carbon dioxide was present in the protoplanetary disc where trans-Neptunian objects formed. This raised an important question before the JWST mission: why haven’t we detected carbon dioxide on Charon’s surface, even though there is evidence of material being excavated from the subsurface?
We knew that at wavelengths longer than 2.5 microns, there are much stronger absorption bands of carbon dioxide. We were hopeful that these would be detectable – and they were.
Discovered by the Hubble Space Telescope in 2005, Pluto’s moons Nix and Hydra are roughly 5,000 times fainter than Pluto and are about two to three times farther from Pluto than its large moon, Charon. The discovery of Nix and Hydra confirmed that the Kuiper Belt, a swarm of icy bodies encircling the solar system beyond Neptune, was more complex and dynamic than astronomers once thought. Pluto resides inside the Kuiper Belt. The moons’ orbits are in the same plane as the orbit of the much larger satellite Charon (discovered in 1978). This likely means the moons were not captured, but instead were born, along with Charon, in what is commonly theorised to have been a titanic collision between two Pluto-sized objects over 4 billion years ago. Image © NASA Hubble, CC BY 2.0 < https://creativecommons.org/licenses/by/2.0 >, via Wikimedia Commons.
ScienceVega: What has the James Webb Space Telescope brought to modern astronomy in in your field of research?
SP: Everything is relatively new, and many observations are still being processed and may not have been published yet. However, we have already made significant discoveries, including the detection of carbon dioxide on Europa, one of the moons of Jupiter.
This is important because we believe the carbon dioxide on Europa’s surface originates from its interior. Understanding this is crucial for gaining insights into the composition of the ocean beneath the moon’s icy surface. Additionally, evidence suggests that the nature of the carbon dioxide – whether pure or mixed with other materials – differs between Europa and Charon due to their distinct environments.
Another significant discovery was the detection of carbon dioxide distributed throughout the trans- Neptunian region, in objects beyond Neptune’s orbit. In the field of astrophysics, notable findings include potential evidence of growing supermassive black holes.
ScienceVega: With so many bodies in the Universe, how do you go about choosing what to investigate? Why did you choose Charon?
SP: In the case of Charon, pursuing this research felt like a natural next step, building on the work I did during my PhD. As a co-investigator of the New Horizons mission, I studied the Pluto-Charon system extensively. For Pluto-Charon, there were clear unresolved questions, and our team saw an immediate opportunity to explore them further.
The Pluto-Charon system observations are part of a larger collaboration aimed at understanding the composition of trans-Neptunian objects. This effort leverages the extended wavelength coverage and the high accuracy and signal-to-noise ratio of JWST measurements to gain deeper insights into the composition of these distant, faint objects. Too faint to be observed from Earth, these objects are now being studied in unprecedented detail, thanks to JWST, revolutionising our understanding of them.
The ultimate goal, not just for Pluto-Charon but for other trans-Neptunian objects as well, is to uncover what these objects are made of and to better understand the origin and evolution of the Solar System.
ScienceVega: Do you plan on continuing to study Charon or are there other worlds that you would be interested in taking a look at?
SP: I am definitely interested in continuing studies of Charon. While we’ve addressed several questions and made exciting new discoveries, many questions remain unanswered. I hope to build on this work with additional JWST observations if possible, though competition for telescope time is extremely high.
That said, my research extends beyond Pluto-Charon. It also includes comets, asteroids and other objects throughout the Solar System, exploring the diverse worlds within it.
However my research does not involve only Pluto-Charon; it also involves comets and asteroids as well as several other objects in the Solar System, in several other worlds.
A composite of enhanced colour images of Pluto (lower right) and Charon (upper left), taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015. This image highlights the striking differences between Pluto and Charon. The colour and brightness of both Pluto and Charon have been processed identically to allow direct comparison of their surface properties, and to highlight the similarity between Charon’s polar red terrain and Pluto’s equatorial red terrain. Pluto and Charon are shown with approximately correct relative sizes, but their true separation is not to scale. The image combines blue, red and infrared images taken by the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC). Image © NASA/ Johns Hopkins University Applied Physics Laboratory/ Southwest Research Institute, Public domain, via Wikimedia Commons.
