Mapping of the Ganymede surface reflectance from Juno/UVS data

Context . Ganymede is the only moon in the Solar System with an intrinsic magnetic field that actively interacts with the Jupiter magnetosphere. This precipitates energetic electrons that generate ultraviolet (UV) auroral emission. Aims . In sunlit auroral regions, the observed emission partly overlaps with the solar continuum reflected by the surface. An accurate modeling of the observed UV spectra therefore requires precise knowledge of the surface spectral reflectance. Methods . We analyzed Juno/UVS data acquired during the 34th perijove (PJ) flyby to constrain the Ganymede surface reflectance in the 140−205 nm range. We used the non-local thermal equilibrium radiative transfer model originally developed to simulate the auroral emission of Ganymede, which also accounts for the reflection of solar flux by the satellite surface, to fit the observed spectra in sunlit auroral regions. Results . Our results revealed that the reflectance varies strongly spatially and spectrally from 0.1% to 8% in the [140 nm; 205 nm] wavelength range. This indicates a significant surface heterogeneity. This variability likely reflects long-term interactions between the icy surface of Ganymede and precipitating energetic particles, which alter the ice structure and crystallinity and its chemical composition. In addition, the derived reflectance maps show no clear correlation with the visible surface features of Ganymede, suggesting that the UV reflectance is primarily shaped by irradiation-driven processes and not by the geological morphology. Conclusions . The resulting reflectance maps provide a critical input for future UV auroral emission modeling, particularly in preparation for observations by the Juice/UVS mission.