Ganymede Surface UV Reflectance Derived from Juno/UVS Measurements

Ganymede, the only satellite in the Solar System possessing an intrinsic magnetic field, is continuously immersed in Jupiter’s magnetosphere. The relative motion of this conducting body through the jovian magnetic field, together with the presence of closed magnetic field regions around Ganymede, leads to the precipitation of energetic electrons along these closed field lines, producing ultraviolet auroral emissions.

In sunlit auroral regions, these emissions are superimposed on the solar radiation reflected by the surface, making an accurate knowledge of the surface spectral reflectance essential for a proper interpretation of the observed UV spectra.

We used Juno/UVS observations obtained during the 34th perijove to derive the spectral reflectance of Ganymede’s surface in the 140-205 nm range. The analysis was performed with a non-local thermodynamic equilibrium radiative transfer model initially developed for auroral emission studies, which self-consistently includes the reflection of the incident solar flux by the surface. By fitting the UVS spectra in illuminated auroral regions, we retrieved spatially resolved reflectance values.

The inferred reflectance exhibits strong spatial and spectral variability, ranging from about 0.1% to 8% over the [140-205] nm interval, revealing a highly heterogeneous surface. This variability is likely the signature of long-term irradiation by energetic particles, which modifies the physical structure, crystallinity, and chemical composition of surface ice. The resulting UV reflectance maps show no clear correlation with visible-wavelength surface morphology, indicating that irradiation-driven processes dominate over geological features in controlling the UV albedo.

These new reflectance constraints constitute a key input for future modeling of Ganymede’s ultraviolet aurora and will be particularly valuable for the interpretation of upcoming observations by the JUICE/UVS instrument.