  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.
