The JUICE 2024 close flyby of the Moon: thermal assessment from MAJIS

Abstract. We present an analysis of four mid-infrared observations of the lunar surface acquired by the MAJIS instrument during the Jupiter Icy Moons Explorer (JUICE) gravity assist in August 2024. The data span 0.49–5.56 µm at sub-kilometre spatial resolution. These data provide a rare opportunity to investigate the challenging spectral regime where reflected solar radiation and thermal emission both contribute to the measured radiance. The study explores three independent approaches to model and retrieve surface temperature and emissivity: (i) a Bayesian inversion framework, (ii) an empirical thermal correction method, and (iii) a roughness-informed thermophysical model. Rather than constituting a formal instrument validation, this paper provides a methodological consistency assessment of thermal retrieval strategies when applied to MAJIS mid-infrared data. Retrieved temperature distributions are compared with expectations from established lunar thermal behaviour, and emissivity spectra are analysed in relation to known compositional contrasts between mare and highland terrains. The analysis highlights the sensitivity of the 3–5 µm crossover regime to modelling assumptions, temperature–emissivity coupling, and surface roughness parameterization. Overall, the results demonstrate that MAJIS mid-infrared observations can be interpreted within physically consistent thermal modelling frameworks, while also revealing limitations and degeneracies inherent to this wavelength range. This workflow is directly transferable to future MAJIS observations of Jovian moons in the reflected–thermal crossover regime.