Revealing Callisto's carbon-rich surface and CO2 atmosphere with JWST (2401.17236v1)

Abstract: We analyzed spectral cubes of Callisto's leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25-micron absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 microns over both hemispheres, confirming the global presence of CO2 gas in Callisto's tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto's trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto's atmosphere. We detected a 4.38-micron absorption band that likely results from solid-state 13CO2. A prominent 4.57-micron absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto's leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially anti-associated. The distribution of the 4.57-micron band is more consistent with a native origin and/or accumulation of dust from Jupiter's irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide (OCS), and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface-atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA's Europa Clipper and ESA's JUICE spacecraft.

• The paper reveals JWST’s spectral evidence for solid and gaseous CO₂ on Callisto, marking the first detection of CO₂ gas on its trailing hemisphere.
• The study employs advanced NIRSpec analysis to map distinct distribution patterns of CO₂ and identify radiolytic production triggered by magnetospheric plasma.
• The research implies that sputtering, radiolysis, and geological activity drive volatile cycling, setting a foundation for future missions like Europa Clipper and JUICE.

Revealing Callisto's Carbon-Rich Surface and CO₂ Atmosphere with JWST

The paper conducted by Cartwright et al. explores the spectral characteristics of Callisto, one of Jupiter's Galilean moons, using the James Webb Space Telescope (JWST). The investigation primarily focuses on the carbon-rich surface and the tenuous atmosphere of Callisto, specifically scrutinizing the presence and distribution of carbon dioxide (CO₂) in both solid and gaseous states.

Key Findings

The paper provides a detailed spectral analysis of Callisto's leading and trailing hemispheres using JWST's Near Infrared Spectrograph (NIRSpec). The spectral cubes exhibit significant 4.25-μm absorption bands attributed to solid CO₂, with stronger signals observed at low latitudes of the trailing hemisphere. This distribution aligns with radiolytic production triggered by magnetospheric plasma interactions with water (H₂O) and carbonaceous materials. Rovibrational emission lines of CO₂ gas between 4.2 and 4.3 μm were detected across both hemispheres, indicating a global presence of CO₂ in Callisto's atmosphere. Notably, this marks the first observation of CO₂ gas over Callisto's trailing hemisphere.

Distribution Patterns and Geochemical Processes

The spatial distribution of solid-state and gaseous CO₂ suggests that processes such as sputtering, radiolysis, and geological activities play crucial roles in sustaining Callisto's atmosphere. The paper identifies a prominent absorption band at 4.57 μm, possibly resulting from cyanide (CN) bearing organics, which exhibits a stronger presence on the leading hemisphere. This band appears to be spatially anti-correlated with solid-state CO₂, suggesting distinct sources or deposition processes.

Surface-Aspects and Atmospheric Interactions

The presence of additional subtle absorption features could signify the existence of CH-bearing organics, carbonyl sulfide (OCS), and Na-bearing minerals. These findings underline the complexity of Callisto's surface chemistry and the potential diversity of its exogenic and endogenic processes. The research further highlights the necessity for laboratory simulations and refined surface-atmosphere interaction models to better comprehend carbon chemistry on icy celestial bodies.

Implications and Future Directions

The discoveries by Cartwright et al. have significant implications for understanding carbon cycling in the Jovian system. The advancement in spectral resolution provided by JWST allows for more accurate identification of compounds on Callisto and enhances our understanding of its geological and chemical evolution. The paper suggests that Callisto undergoes a cycle where CO₂ sublimes, transports, and condenses onto different terrains, implicating a potential reservoir mechanism for volatile compounds.

Looking forward, the paper suggests that future missions, such as NASA's Europa Clipper and ESA's JUICE, will benefit from these findings in preparation for further exploratory missions. Such missions might verify the laboratory-suggested compositions of Callisto's carbonaceous and sulfurous materials and assess their consistency across other icy bodies in the solar system.

Conclusion

Cartwright et al.'s findings offer a comprehensive view of Callisto's surface-atmosphere dynamics and the molecular complexities present on this ancient moon. This paper not only enriches our understanding of Callisto's geochemical processes but also provides a foundational dataset for future explorations and comparative planetology among the icy bodies of the outer solar system.