Unveiling new species on the stratified surface of Charon through JWST

Charon, Pluto's largest moon, has been the focus of extensive spectral analysis through both terrestrial observations and data collected by the New Horizons mission. These investigations have uncovered a surface dominated by crystalline water ice and ammonia‑bearing species, which appear enhanced in the bright ejecta blankets of geologically young craters, suggesting the exposure of subsurface materials. Given the limited spectral coverage of Charon's measurements to date (up to 2.5 μm), some aspects of Charon's composition remain unresolved, including 1) the detection of carbon dioxide (CO₂) on the surface of Charon, expected to be present in regions revealing exposure to subsurface materials, but never detected so far, and 2) the role of energetic radiation and UV photolysis on water ice, ammoniated species, and C-bearing species. We present JWST observations of Charon acquired as part of the Solar System Guaranteed-Time-Observations (GTO) program # 1191 (PI: Stansberry), using the Near-Infrared Spectrograph (NIRSpec) instrument on JWST. These observations longitudinally encompass the entire surface of Charon. Observations were obtained with three high-resolution gratings (G140H, G235H, G395H) and two detectors per grating (nrs1, nrs2) resulting in a spectral coverage between 1.1 and 5.1 μm with an average resolving power of R ~ 2700. These observations confirm previously identified absorption bands at 1.5, 1.65 and 2.0 μm due to crystalline water ice and a ~2.2 μm feature attributed to ammoniated species. Beyond 2.5 μm, the spectrum is dominated by prominent H₂O ice features at 3.0 μm, 3.1 μm (Fresnel peak), and 4.5 μm. The spectrum clearly displays a sharp feature at 2.7 μm and two absorptions centered around ~4.24 μm and ~4.27 μm. We attribute these features to CO₂, byproducts created by irradiation damage of CO₂-rich material, or a combination of the two. Additionally, absorption features resulting from hydrogen peroxide (H₂O₂) are seen in the spectra, possibly resulting from water-ice radiolysis. Considering the established connections between Charon and various celestial bodies, including Kuiper Belt objects, icy satellites and comets, these results offer insights into potentially common elements that may exist within the regions of the solar nebula from which these bodies originated and the possibility of shared radiolytic and/or photolytic by-products. Acknowledgments: This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. S.P. thanks the NASA grants 80NSSC19K0821 and 80NSSC19K0554 to SwRI for partial funding that supported her work.