Revealing Callisto’s Carbon-rich Surface and CO₂ Atmosphere with JWST

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 μm 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 μm 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 μm absorption band that likely results from solid-state 13CO2. A prominent 4.57 μm 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 antiassociated. The distribution of the 4.57 μm 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, 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.

JWST Reveals CO Ice, Concentrated CO2 Deposits, and Evidence for Carbonates Potentially Sourced from Ariel’s Interior

“…The 2.2 μm bands detected in ground-based observations of the Uranian moons have been attributed to potentially internally derived deposits rich in NH 3 hydrates and NH 3 ice, carbonates, phyllosilicates, and/or organics (e.g., Cartwright et al 2020aCartwright et al , 2023DeColibus et al 2023). Although we did not detect evidence of NH-bearing species, phyllosilicates, or hydrocarbons in the G395M data, Ariel's 4.02 μm feature could result from a ν 1 + ν 3 combination mode expressed by CO 3 in carbonate minerals (Hexter 1958;Bishop et al 2021), similar to Callisto's 4.02 μm band (Johnson et al 2004;Cartwright et al 2024). Laboratory experiments show that although radiolytic CO 3 exhibits a 4.89 μm feature (Figure A2; Raut & Baragiola 2013), a complementary 4 μm CO 3 band was not observed in these irradiation experiments.…”

“…We report a preliminary analysis of possible 13 C enrichment of CO and CO 2 ices on Ariel in Appendix A.10. More detailed modeling of Ariel's carbon isotopes and their spectral expression at infrared wavelengths is likely required to corroborate the CO band ratios present here, to convert band ratios to number ratios, and to fully explore their significance, e.g., in terms of formation of its carbon source material (e.g., Cartwright et al 2024).…”

“…Band Measurements: We measured the band areas and depths of detected absorption features and scattering peaks using a band measurement program that defines and divides off a local linear continuum for each feature (e.g., Cartwright et al 2024). Depth and height measurements were made by averaging the reflectance values within ±0.002 to 0.003 μm of a user-defined band (B c ) or peak (P c ) center, and uncertainties were computed using standard error propagation procedures (e.g., Taylor 1997).…”

“…Specifically, the 3.33 μm and 4.90 μm bands and other possible features near 3. 01, 3.33, 4.84, 4.90, and 4.93 μm on Ariel's trailing hemisphere do not appear on many other objects where CO 2 has been detected by JWST/NIRSpec (e.g., Brown & Fraser 2023;Pinto et al 2023;Villanueva et al 2023;Bockelee-Morvan et al 2024;Cartwright et al 2024;de Pra et al 2024;Emery et al 2024;Protopapa et al 2024;Wong et al 2024; TNO examples provided in Figure A6), with the exception of Triton (Wong et al 2023). Consequently, Ariel's surface exhibits some of the most CO 2 -rich deposits in the solar system.…”

“…Such a process might explain the stronger CO 2 "triplet" band (1.9-2.1 μm) on the trailing hemispheres of the largest Uranian moons and its weakening with increasing orbital distance (i.e., strongest on Ariel, weakest on Oberon). Nevertheless, predictions of moonmagnetosphere interactions at Uranus are not well constrained, and perhaps CO 2 is native and sourced from these moons' interiors, with larger deposits on their trailing sides due to enhancement by radiolysis, similar to the multiple origin scenarios proposed for CO 2 on Jupiter's moon Callisto (e.g., Hibbitts et al 2000Hibbitts et al , 2002; J. M. Cartwright et al 2024).…”