Introduction
The James Webb Space Telescope (JWST) is pushing the frontiers of planetary science by analyzing the composition of bodies far from Earth. A new study using JWST spectra investigates carbon dioxide (CO2) on eight mid-sized Saturnian satellites, from the inner moons Mimas, Enceladus, Tethys, Dione, and Rhea to the outer ones Hyperion, Iapetus, and Phoebe. The work sheds light on how CO2 is trapped and transported in cold, irradiated environments across the Saturn system.
Methods: JWST Spectroscopy of Saturn’s Moons
Researchers examined the characteristic CO2 bands near 4.26 micrometers (ν3) and 2.7 micrometers (ν1+ν3) in the infrared. The ~4.26 μm band was detected on all eight satellites, while the ~2.7 μm band appeared on all but Phoebe and the leading hemisphere of Iapetus. The wavelength shifts of these bands reveal different trapping environments and host materials for CO2, allowing a nuanced view of how the same molecule can exist in multiple states in the outer solar system.
Four Types of Trapped CO2 Across the Saturnian System
The analysis identifies four distinct CO2 trapping scenarios, which correlate with each moon’s location, surface composition, and exposure to radiation.
- Inner satellites: A primary CO2 component is trapped in amorphous ice sourced from Saturn’s E-ring. A second CO2 component appears linked to dark, trailing-hemisphere material, suggesting surfaces enriched by material exchange within the system.
- Outer satellites: CO2 is likely produced by irradiation of organics on Phoebe, and then transported to the dark leading hemisphere of Iapetus and to dark regions of Hyperion. This points to a dynamic redistribution of CO2 driven by radiation chemistry and surface transport.
- Water-ice trapping: In addition to other hosts, CO2 is observed trapped by water ice on the trailing hemisphere of Iapetus and on Hyperion, illustrating multiple trapping pathways for CO2 in extremely cold environments.
- Environmental diversity: The Saturnian system, with nearly identical solar input but varied ice exposure, organic abundance, and formation histories, provides a natural laboratory to study how CO2 can persist in different hosts and how these hosts influence the spectral signatures we observe from afar.
Implications for Formation and Surface Processes
The presence of CO2 in multiple forms underscores the importance of irradiation chemistry and surface processing in the outer solar system. The inner moons’ CO2 linked to E-ring ice suggests ongoing exchange with Saturn’s ring system, while the outer moons’ CO2 tied to organic-rich surfaces hints at irradiation-driven synthesis and subsequent migration. The detection on Phoebe and the transport to Iapetus’ leading hemisphere supports a broader narrative of material redistribution across Saturn’s realm, influencing albedo and spectral properties observed by telescopes and spacecraft alike.
Future Directions
The study highlights the need for laboratory experiments to better understand CO2 trapping mechanisms in icy and organic-rich matrices. Controlled laboratory spectra and physical models can help disentangle how different hosts, temperatures, and radiation environments alter CO2 signatures. Continued JWST observations, complemented by future missions and ground-based campaigns, will refine our picture of how CO2 behaves in the cold outer solar system and what that implies about the formation and evolution of Saturn’s moons.
Conclusion
The JWST study of CO2 on Saturn’s satellites demonstrates the complexity of trapped CO2 across a diverse set of moons. By identifying four trapping types and linking them to surface and orbital environments, the research advances our understanding of carbon chemistry in the outer solar system and sets the stage for future experiments and missions to probe these intriguing ices further.
Authors: Michael E. Brown, Samantha K. Trumbo, Matthew Belyakov, M. Ryleigh Davis, Ashma Pandaya
