Context. We present the first spectroscopic observations of Ganymede by the James Webb Space Telescope undertaken in August 2022 as part of the proposal "ERS observations of the Jovian system as a demonstration of JWST's capabilities for Solar System science". Aims. We aimed to investigate the composition and thermal properties of the surface, and to study the relationships of ice and non-water-ice materials and their distribution. Methods. NIRSpec IFU (2.9-5.3 μm) and MIRI MRS (4.9-28.5 μm) observations were performed on both the leading and trailing hemispheres of Ganymede, with a spectral resolution of ∼2700 and a spatial sampling of 0.1 to 0.17″ (while the Ganymede size was ∼1.68″). We characterized the spectral signatures and their spatial distribution on the surface. The distribution of brightness temperatures was analyzed with standard thermophysical modeling including surface roughness. Results. Reflectance spectra show signatures of water ice, CO₂, and H₂O₂. An absorption feature at 5.9 μm, with a shoulder at 6.5 μm, is revealed, and is tentatively assigned to sulfuric acid hydrates. The CO₂ 4.26-μm 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 boreal region of the leading hemisphere, the CO₂ band is dominated by a high wavelength component at ∼4.27 μm, consistent with CO₂ trapped in amorphous water ice. At equatorial latitudes (and especially on dark terrains), the observed band is broader and shifted toward the blue, suggesting CO₂ adsorbed on non-icy materials, such as minerals or salts. Maps of the H₂O Fresnel peak area correlate with Bond albedo maps and follow the distribution of water ice inferred from H₂O absorption bands. Amorphous ice is detected in the ice-rich polar regions, and is especially abundant on the northern polar cap of the leading hemisphere. Leading and trailing polar regions exhibit different H₂O, CO₂, and H₂O₂ spectral properties. However, 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 nanometer- and micrometer-scale texture, of diurnal or geographic origin, is observed in both hemispheres. Ice frost is tentatively observed on the morning limb of the trailing hemisphere, which 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- and Mg-sulfate salts, sulfuric acid hydrates, and possibly phyllosilicates mixed with fine-grained opaque minerals, with a highly porous texture. Latitude and local time variations of the brightness temperatures indicate a rough surface with mean slope angles of 15° - 25° and a low thermal inertia Γ = 20-40 J m^-2 s^-0.5 K^-1, consistent with a porous surface, with no obvious difference between the leading and trailing sides.

Pluto's fly-by by the New Horizons spacecraft in July 2015 has revealed a dark reddish equatorial region, named Cthulhu, covered by a dark, non-icy material whose origin and composition have yet to be determined. It has been suggested that this material could form from the sedimentation of photochemical aerosols, originating from dissociation and ionisation processes in Pluto's high atmosphere (similarly to aerosols forming Titan's haze). This hypothesis is here further investigated by comparing New Horizons spectra collected both in the visible and the near-infrared to laboratory reflectance measurements of analogues of Pluto's aerosols (Pluto tholins). These aerosols were synthesised in conditions mimicking Pluto's atmosphere, and their optical and reflectance properties were determined, before being used in Hapke models. In particular, the single scattering albedo and phase function of Pluto tholins were retrieved through Hapke model inversion, performed from laboratory reflectance spectra collected under various geometries. From reconstructed reflectance spectra and direct comparison with New Horizons data, some of these tholins are shown to reproduce the photometric level (i.e. reflectance continuum) reasonably well in the near-infrared. Nevertheless, a misfit of the red visible slope still remains and tholins absorption bands present in the modelled spectra are absent in those collected by the New Horizons instruments. Several hypotheses are considered to explain the absence of these absorption features in LEISA data, namely high porosity effects or GCR irradiation. The formation of highly porous structures, which is currently our preferred scenario, could be promoted by either sublimation of ices initially mixed with the aerosols, or gentle deposition under Pluto's weak gravity.