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Unraveling Callisto’s carbon-rich surface with JWST/NIRSpec

Presentation #401.06 in the session JWST Views of the Outer Planets and their Moons (Oral Presentation)

Published onOct 23, 2023
Unraveling Callisto’s carbon-rich surface with JWST/NIRSpec

The icy Galilean moon Callisto has an ancient surface covered by degraded craters with disaggregated rims and a blanket of dark material of unknown origin. Callisto’s apparent geologic quiescence belies its vibrant surface chemistry, sustained by CO2 and other carbon-bearing species. Observations made by Galileo’s Near Infrared Mapping Spectrometer (NIMS) identified a 4.25-μm CO2 band exhibiting a ‘bullseye’ distribution centered on Callisto’s trailing hemisphere, consistent with radiolytic generation of CO2 molecules from co-rotating plasma flowing onto Callisto’s trailing side. NIMS also detected a broad absorption band near 4.57 μm that was attributed to organic residues, possibly formed via irradiation of short-chain hydrocarbons mixed with other components. However, the low resolving power of NIMS (R ~200), numerous filter junctions between 3 and 5 μm, and typically low signal-to-noise ratios (S/N) at wavelengths >4 μm limited prior analyses. Although ground-based observations have since confirmed the presence of the 4.57-μm band, Earth’s atmosphere is opaque between ~4.2 to 4.5 μm, preventing analysis of CO2, and telluric contributions between 3.2 and 3.5 μm hamper confirmation of possible organic features.

JWST/NIRSpec (G395H, ~2.85 - 5.35 μm) observations of Callisto’s leading and trailing hemisphere in late 2022 have revealed C-bearing species on this moon in exquisite detail. These observations show that the 4.25-μm CO2 band is present across Callisto but is stronger on its trailing hemisphere, confirming prior measurements made by Galileo/NIMS. We also identified several subtle features between 3.2 and 3.5 μm that might arise from organics, as well as a broad 4.57-μm band, confirming prior identification of these spectral features. The origin of C-bearing species is unknown, and they could result from components native to Callisto’s crust and/or were delivered in dust grains from Jupiter’s irregular satellites, which likely contributed to Callisto’s ubiquitous dark material. We will present these JWST/NIRSpec data and our analyses of exhibited spectral features. We will also discuss new insights into the likely ongoing CO2 cycle operating on Callisto.

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