Charon, the largest moon of Pluto

Beyond Neptune, a fascinating collection of small bodies known as Trans-Neptunian Objects (TNOs) orbits the Sun. These objects serve as time capsules, offering scientists a glimpse into the early Solar System. They are characterized by unique surface compositions, physical properties, and dynamical characteristics that hold clues to the Solar System’s origins. In addition, their surfaces are continuously exposed to space radiation and micrometeoroid impacts, which can alter their original state. Therefore, a major challenge is determining which surface compounds are pristine and which have been modified over time. Understanding this distinction is crucial for piecing together the nature of the primordial disk from which these objects formed 4.5 billion years ago. Among these TNOs, only a few have been closely investigated by a spacecraft. In 2015, NASA’s New Horizons mission flew by Pluto and its moons, the largest of which is Charon.

The mystery of carbon dioxide on Charon

The surface of Charon, as revealed by the New Horizons mission, features numerous craters surrounded by bright ejecta blankets that are rich in crystalline water ice and ammonia-bearing compounds. These geologic features suggest that materials from beneath the surface have been exposed by impact events, providing a window into the moon's subsurface composition. Despite expectations based on evidence from cometary studies suggesting the presence of carbon dioxide in the outer region of the protoplanetary disk, no carbon dioxide was detected in the data from New Horizons. This was surprising, especially given the absence of volatile ices that might otherwise obscure its presence. If carbon dioxide is presumed to exist in Charon’s interior, and subsurface material is being exposed by impact events, then why does it not appear on its surface? The only way to address this fundamental question was to observe Charon’s surface beyond the 2.5-micron wavelength limit of New Horizons' spectroscopic measurements, where several ices, including carbon dioxide, present much stronger absorption bands.

JWST observations of Charon reveal carbon dioxide on the surface

Extending the wavelength coverage of Charon's surface spectroscopy beyond 2.5 microns was a key goal of my PhD research back in 2006. This task proved to be extremely challenging due to the object’s faintness, even when using advanced facilities such as the Very Large Telescope in Paranal, Chile. Significant absorption from Earth’s atmosphere also contributed to these difficulties. While some insights were gained from these observations, it took nearly twenty years, the next-generation capabilities of the James Webb Space Telescope (JWST), and a dedicated team with a broad range of skills to fully achieve this goal.

In 2022 and 2023, we used the Near-Infrared Spectrograph on JWST to obtain four observations of Charon. These provided complete coverage of Charon’s northern hemisphere. Carbon dioxide absorption bands at approximately 2.7 and 4.3 microns are present in all of the spectra. While this finding was not shocking, the clarity of these bands was indeed extraordinary. The quality of the data enabled us to further explore the nature of the carbon dioxide. Important questions include whether the carbon dioxide exists in a pure form or is mixed with other compounds on the surface, and why it is not detected below 2.5 microns. A comprehensive investigation of these questions required integrating the remarkable JWST observations with advanced spectral modeling and innovative laboratory experiments to deliver comprehensive insights.

Our analysis reveals a layered surface structure, with pure crystalline carbon dioxide overlaying a subsurface rich in crystalline water ice and tholin-like materials (i.e., complex organic molecules). The upper layer of carbon dioxide is likely excavated from the interior through impacts. We demonstrate that the particular spectral properties of this layered structure can explain why carbon dioxide is observed only at long wavelengths. We also explored other possible contributions to the carbon dioxide inventory, including in-situ formation from radiation processing.

Hydrogen peroxide: evidence for irradiation processes

 The JWST measurements reveal another surprise: the detection of a plateau in the spectra around 3.5 microns. Through comparisons with laboratory data and JWST observations of Europa, we concluded that this spectral feature can be attributed to hydrogen peroxide. But what is the nature of this compound, and why is it present on both Charon and Europa? Its presence on Charon suggests active alteration of the water-ice-rich surface by solar ultraviolet photons, solar wind, and galactic cosmic rays. Our team conducted new laboratory measurements to confirm that it is possible to generate hydrogen peroxide even when carbon dioxide is present.

These JWST observations have added carbon dioxide and hydrogen peroxide to Charon’s compositional inventory, which was previously known to consist of water ice, ammoniated species, and tholin-like darkening constituents. Our analysis demonstrates that the surface of Charon preserves records from both formation and irradiation processes.