Cold Clues: JWST Detects Multiple Forms of CO₂ on Saturn’s Moons
In a new study led by Michael E. Brown, astronomers used the James Webb Space Telescope (JWST) to investigate the presence and behavior of carbon dioxide (CO₂) ice on eight of Saturn’s moons. Despite extremely cold temperatures where solid CO₂ shouldn't normally remain stable, the researchers found that the gas is surprisingly widespread across both the inner and outer satellites. These findings reveal not only that CO₂ exists in unexpected places, but also that it is being trapped in different ways depending on where it is on the moon's surface -- offering fresh insight into the chemistry and evolution of Saturn’s satellite system.
Background: CO₂ in the Outer Solar System
The paper begins by laying out a mystery: solid CO₂ appears across the outer solar system, including on moons and asteroids, in conditions where it should have long since evaporated. This means the CO₂ must be held in place by being trapped within other materials -- such as water ice, minerals, or organic matter. While previous missions like Cassini had detected CO₂ on Saturn’s moons, JWST’s much higher sensitivity allowed the team to analyze specific spectral features of the molecule, namely the 4.27 µm (ν₃) and 2.7 µm (ν₁ + ν₃) infrared bands, to determine how and where the gas is trapped.
Observations: Looking Through JWST’s Lens
Brown’s team collected spectra from Mimas, Enceladus, Tethys, Dione, Rhea (moons inside the orbit of Titan), and Hyperion, Iapetus, and Phoebe (moons outside Titan). They found that all moons showed evidence of the ν₃ band near 4.27 µm, and all but Phoebe and the dark side of Iapetus showed the 2.7 µm combination band. By analyzing the precise wavelengths and shapes of these features, they could identify four distinct types of CO₂ trapping environments -- some linked with water ice, others with dark organic-rich material.
The Inner Moons: Ice and Dark Material
On the inner icy moons, CO₂ appears in two distinct forms. First, there is a version trapped within amorphous water ice (a less-ordered form of ice), likely delivered from Saturn’s E-ring, which is fed by the geysers of Enceladus. This CO₂ shows a 4.272 µm absorption and a broad band at 2.708 µm. Second, a shorter wavelength feature near 4.249 µm, lacking any associated 2.7 µm absorption, is found mostly on the trailing hemispheres of Dione and Rhea, where dark, non-ice material accumulates. The source of this darker material is not fully known, but may involve fine-grained iron oxides that trap CO₂ tightly within their structure.
The Outer Moons: A Tale of Organics and Ice
The outer moons, which have more complex and darker surfaces, also show two kinds of CO₂ features. Phoebe and the dark leading hemisphere of Iapetus display strong 4.273 µm absorptions, likely due to CO₂ formed by the irradiation of organic materials. However, they do not show the 2.7 µm band, hinting that this CO₂ is not trapped in ice. Meanwhile, Hyperion and the bright trailing side of Iapetus show a sharper CO₂ signature at 4.249 µm and a narrow 2.6975 µm band -- indicating a different trapping method, potentially involving water ice in a new configuration that has yet to be recreated in lab experiments.
Isotope Clues: The Case for ¹³CO₂
The team also investigated the rare isotope ¹³CO₂, which has a slightly different wavelength signature at 4.384 µm. It was detected only on the trailing side of Iapetus, supporting the idea that this region has a distinct source or trapping history compared to Phoebe or the leading side of Iapetus. The relative strength of the ¹³CO₂ band provides hints, but not direct measurements, about carbon isotopic ratios on these distant moons.
Conclusion: A Complex Puzzle in Ice and Rock
In the final discussion, the authors reflect on how this study helps disentangle the sources and trapping mechanisms of CO₂ across the Saturnian system. Comparing these findings with those from Jupiter’s icy moons reveals both similarities and intriguing differences. For example, Europa shows spectral features similar to Iapetus and Hyperion, but lacks evidence for organics. These comparisons highlight the need for continued JWST observations and laboratory experiments to simulate the trapping conditions of CO₂ in a variety of materials. Ultimately, the study deepens our understanding of how seemingly simple molecules like CO₂ can tell complex stories about the history and chemistry of the outer solar system.
Source: Brown
