Introduction: A New Window into Outer Solar System Chemistry
The James Webb Space Telescope (JWST) is expanding our understanding of planetary surfaces far beyond Earth. A recent study focusing on the satellites of Saturn uses JWST spectra to identify solid-state CO2 trapped within diverse host materials across eight mid-sized moons. This work reveals that CO2 exists in several distinct trapping environments, even at the frigid temperatures of the outer solar system, where crystalline CO2 would normally be unstable. The research compares inner moons such as Mimas, Enceladus, Tethys, Dione, and Rhea with outer moons Hyperion, Iapetus, and Phoebe, offering a controlled laboratory-like setting where insolation is similar but ice exposure, organics, and formation histories differ widely.
Key Findings: Four Trapped CO2 Types Across Saturn’s Satellites
Using the ~4.26 μm CO2 ν3 band, detected on all eight satellites, and the ~2.7 μm ν1+ν3 band observed on all but Phoebe and the leading hemisphere of Iapetus, the team identified four distinct CO2 trapping signatures. The spectral shifts reveal nuanced relationships between CO2 and its host materials, highlighting that even in a shared solar input, local environments sculpt ice chemistry in spectacular ways.
1) Inner Satellites: Amorphous Ice Hosts and Dark Material Associations
In the inner Saturnian moons, CO2 appears to be trapped in amorphous ice supplies—likely sourced from Saturn’s E-ring. This amorphous phase can accommodate CO2 at temperatures where crystalline phases would otherwise reorganize or desorb. A second CO2 component correlates with dark material concentrated on the trailing hemispheres of Dione and Rhea. This suggests a two-component trapping system: one tied to icy grains delivered by ring material and another linked to the surface chemistry of darker, perhaps organics-rich, regions.
2) Outer Satellites: Organic Irradiation as a CO2 Origin
On the outer moons, the picture shifts. The data imply CO2 is produced by irradiation of organics on Phoebe, with the resulting CO2 or its transport products subsequently migrating to the dark leading hemisphere of Iapetus and to the dark terrains of Hyperion. This scenario underscores a dynamic coupling between surface organics, radiation processing, and lateral transport across a moon’s near-surface layers.
3) Water Ice Trapping on Iapetus and Hyperion
The observations further indicate that CO2 is also trapped by water ice on the trailing hemisphere of Iapetus and on Hyperion. This highlights a frost-like trapping mechanism where CO2 is stabilized within water ice matrices, offering a stable reservoir of volatiles in regions of complex regolith evolution.
Implications: Trapping Mechanisms, Laboratory Needs, and Solar System Context
These results emphasize that solid CO2 in the outer solar system is not a monolithic reservoir. Instead, it exists in multiple hosts and under different formation histories, from amorphous icy matrices to organic-rich coatings and water-ice inclusions. The observed diversity points to a critical need for laboratory experiments that reproduce the relevant temperature, radiation, and compositional regimes. By better understanding CO2 trapping and release processes, scientists can refine models of satellite surface evolution and the cycling of volatiles in the outer solar system.
Conclusion: A Step Forward in Planetary Spectroscopy
JWST’s spectra of eight Saturnian moons illuminate how CO2 is stored and processed in frozen environments far from the Sun. The detection of four CO2 trapping types across moons with similar solar exposure but different surfaces and histories provides a richer picture of icy chemistry and volatile transport. As laboratory studies catch up with these astronomical insights, we move closer to a comprehensive understanding of CO2 dynamics in the outer solar system—and what that means for the formation and evolution of Saturn’s complex moon system.
About the Authors
Michael E. Brown, Samantha K. Trumbo, Matthew Belyakov, M. Ryleigh Davis, Ashma Pandaya
