MAJIS performances in the infrared during the JUICE 2024 Earth fly-by: comparisons with IASI measurements and sensitivity to trace species

Abstract. The JUpiter ICy moons Explorer spacecraft (JUICE) successfully performed a Lunar and Earth gravity assist maneuver on 19 and 20th August 2024, which provided an excellent opportunity to test its instruments before its arrival at Jupiter in 2031. Here we focus on the evaluation of the performances of the infrared channel of the Moon and Jupiter Imaging Spectrometer (MAJIS) based on Earth observations acquired over part of the Pacific Ocean at tropical latitudes. We specifically compare MAJIS observations with co-located ones from the Infrared Atmospheric Sounding Interferometer (IASI). The two instruments overlap in the spectral range 3.6 and 5.56 µm. Having removed spectra contaminated by clouds or ocean glint, we find an excellent match between the absolute radiance of the two instruments. We argue that most of the differences can be explained by natural variability of water vapour content. Once this effect is taken into account, our results suggest that the radiometric calibration of MAJIS is better than 10 %, rising to ~15 % in the range 5.25–5.56 µm. We then compare MAJIS to synthetic spectra generated with a radiative transfer algorithm, the OPerational version of the Automatized Atmospheric Absorption Atlas (4A/OP). Both the comparison of MAJIS to IASI and to synthetic spectra reveal a small spectral shift of MAJIS spectra beyond 4 µm, of the order of 4 nm. The use of 4A/OP allows us to highlight the detection and sensitivity of MAJIS spectra to several trace species: methane (CH₄), nitrous oxide (N₂O), ozone (O₃) and carbone monoxide (CO). Based on the residuals between MAJIS and synthetic spectra, we estimate the signal-to-noise ratio to lie in the range 200–300, meeting nominal expectations. These excellent performances point to promising future jovian atmospheric observations.