Mapping of the Ganymede surface reflectance from Juno/UVS data

¹ Aix-Marseille Université, CNRS, CNES, Institut Origines, LAM, Marseille, France
² Laboratory of Atmospheric and Planetary Physics, STAR Institute, University of Liège, Belgium
³ Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA
⁴ University of Bern, Faculty of Science, Physics Institute, Space Research & Planetary Sciences (WP), Switzerland
⁵ Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA
⁶ NASA Langley Research Center, Hampton, Va, USA
⁷ Science Systems and Applications Inc., Hampton, VA, USA
⁸ Physique des Interactions Ioniques et Moléculaires, CNRS et Aix-Marseille Université, France
⁹ Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
¹⁰ LATMOS/CNRS, Sorbonne Université, UVSQ, Paris, France
¹¹ The University of Edinburgh, School of GeoSciences, Edinburgh, UK
¹² Centre for Exoplanet Science, University of Edinburgh, Edinburgh, UK
¹³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁴ Univ. Grenoble Alpes, CSUG, 38000 Grenoble, France
¹⁵ IRAP, CNRS-Université Paul Sabatier, Toulouse Cedex 4, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 September 2025
Accepted: 5 December 2025

Abstract

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.