| 17 Feb 2026
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.
The manuscript under consideration is a relevant contribution and generally well written. In some details, the presentation is difficult to follow. I would recommend publication after revision of these shortcomings.
Comments in detail:
I would suggest a slight rephrasing of the title: "MAJIS infrared performance during the JUICE 2024 Earth flyby: comparison with IASI measurements and sensitivity to atmospheric trace species".
page 4: "We did not perform any retrievals in this paper" - a few lines below the authors state "For CO, CH4 and N2O, we .... scaled these profiles in order to obtain a best fit". These are somewhat contradictory statements.
page 5, cloud filtering: The MAJIS cloud filtering via a 4.64 um brightness T threshold appears not to be the optimal choice. Would the addition of a VIS/NIR brightness value further improve the recognition of clouds? (See Fig 3, where in the right panel more filigrane cloud structure tend to be lost.)
page 13, line 193ff: if the discrepancy is induced by differences in H2O as claimed by the authors, I would expect that the quite pronounced spectral patterns shown in Fig 10 at wavelengths beyond 4.6 um correspond to H2O spectral signatures. Adding some further simulations using 4A/OP should allow to verify this claim more explicitely.
page 15, line 239: "potential variations in meteorological conditions". This seems a persisting problem for the whole comparison, especially when it comes to the H2O profile. I wonder whether the ERA5 hourly data combined with 4A/OP runs for estimating the resulting effects on radiances would help to further substantiate the conclusions and perhaps narrow down the residual uncertainty of the MAJIS radiance calibration.
Conclusions: Here, a coincidence interval of 2 hours is mentioned, while the reader would expect (based on section 4.3 and table 1) 2.5 hours?
Further down, the conclusion discusses in length (lines 286 to end) items which have not at all been treated in the paper. Either this part should be removed or an additional section needs to be added which makes the claims of the conclusions explicit.
Data availability: I find an embargo period of about 5 years after data collection of the flyby data (this would imply data accessibility three years after publication of this study) inacceptable, roughly as distant from any contemporary FAIR principles as Jupiter is from the sun. It would appear to me that the minimum requirement from the viewpoint of a study publication would be a release of at least the subset of MAJIS spectra actually used in the form of an electronic supplement.