JWST reveals Carbon Dioxide and Hydrogen Peroxide on the stratified surface of Charon

Charon, Pluto's largest moon, stands unique as the only mid-sized Trans-Neptunian Object (TNO) — objects with a diameter ranging approximately between 500 km and 1700 km — for which geological mapping is available, thanks to measurements returned by the New Horizons mission. Unlike larger TNOs (e.g., Pluto, Eris, Makemake), Charon does not display a surface covered by super-volatiles (e.g., CH4), with the possible exception of its poles. As a result, Charon serves as an excellent candidate for retrieving valuable insights into processes such as differentiation, radiation exposure, and cratering within the Kuiper Belt. Understanding these processes is crucial to infer the interior make-up of TNOs based on remote sensing spectroscopy of their surfaces, and ultimately, unraveling how they were formed. 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, provide the first clear evidence of a layered or stratified surface. The topmost layer, just a few microns thick, presents carbon dioxide in both segregated and intimately mixed states, along with hydrogen peroxide. This is the first detection of these two species on the surface of Charon. The presence of hydrogen peroxide indicates active radiolytic/photolytic processing of the water-ice rich surface by solar ultraviolet and interplanetary medium (IPM) Lyman-α photons, solar wind ions and electrons on the sunlit hemisphere, and galactic cosmic rays (GCRs). We consider the possibilities of CO2 formation by irradiation of hydrocarbons mixed with water ice, interfacial radiolysis between carbon deposits and water ice, as well as implantation of energetic C ions from the solar wind and solar energetic particles. Considering the established connections between Charon and various celestial bodies, including Kuiper Belt objects, icy Galilean moons (see Raut et al. AGU, 2023), Uranian moons, and comets, these JWST results offer insights into potentially common compounds that may have existed within the different regions of the solar nebula where these bodies originated, as well as the possible formation of similar radiolytic and/or photolytic by-products on their surfaces.