  JWST NIRSpec IFU (2.9-5.3 mu) and MIRI MRS (4.9-28.5 mu) observations were
performed on both the leading and trailing hemispheres of Ganymede with a
spectral resolution of ~2700. Reflectance spectra show signatures of water ice,
CO2 and H2O2. An absorption feature at 5.9 mu is revealed and is tentatively
assigned to sulfuric acid hydrates. The CO2 4.26-mu band shows latitudinal and
longitudinal variations in depth, shape and position over the two hemispheres,
unveiling different CO2 physical states. In the ice-rich polar regions, which
are the most exposed to Jupiter's plasma irradiation, the CO2 band is
redshifted with respect to other terrains. In the leading northern polar cap,
the CO2 band is dominated by a high wavelength component at ~4.27 mu,
consistent with CO2 trapped in amorphous water ice. At equatorial latitudes
(and especially on dark terrains) the observed band is broader and shifted
towards the blue, suggesting CO2 adsorbed on non-icy materials. Amorphous ice
is detected in the ice-rich polar regions, and is especially abundant on the
leading northern polar cap. In both hemispheres the north polar cap ice appears
to be more processed than the south polar cap. A longitudinal modification of
the H2O ice molecular structure and/or nano/micrometre-scale texture, of
diurnal or geographic origin, is observed in both hemispheres. Ice frost is
observed on the morning limb of the trailing hemisphere, possibly formed during
the night from the recondensation of water subliming from the warmer
subsurface. Reflectance spectra of the dark terrains are compatible with the
presence of Na-/Mg-sulfate salts, sulfuric acid hydrates, and possibly
phyllosilicates mixed with fine-grained opaque minerals, having an highly
porous texture. Mid-IR brightness temperatures indicate a rough surface and a
very low thermal inertia of 20-40 J m-2 s-0.5 K-1, consistent with a porous
surface.
