The mineralogical characterisation of the lunar surface has been a central objective of planetary science since the earliest stages of lunar exploration. Understanding the composition, spatial distribution, and geological context of lunar minerals is fundamental not only for reconstructing the Moon's thermal and magmatic evolution, but also for providing insight into broader planetary differentiation processes and the crustal evolution of terrestrial bodies in the early Solar System .

Over the past two decades, advances in orbital remote sensing have revolutionised our ability to investigate the Moon's surface composition. The advent of high-resolution multi- and hyperspectral imaging has enabled detailed detection, mapping, and quantification of key mineralogical species with unprecedented spatial and spectral resolution. These developments have greatly enhanced our understanding of lunar surface processes, magmatic diversity, and regional lithological heterogeneity.

Significant progress has been made thanks to the suite of instruments flown aboard a series of landmark lunar missions. Among these are Chandrayaan-1 with the Moon Mineralogy Mapper (M³) , Kaguya (SELENE) with its Multiband Imager (MI) , the Lunar Reconnaissance Orbiter (LRO) equipped with Diviner , and earlier missions such as Clementine .

Operating in the visible/near-infrared (VNIR) and thermal infrared (TIR) spectral domains, these instruments have enabled global and regional-scale mapping of primary lunar minerals – most notably pyroxene, olivine, plagioclase, ilmenite, as well as the identification of compositionally distinctive lithologies, including exposures of Mg-suite, alkali-suite, and pure anorthosite (PAN) rocks .

The VNIR range is particularly diagnostic for lunar mineralogy, as it encompasses key electronic absorption features of ferrous iron (Fe2+) in mafic minerals . The prominent 1 and 2 µm absorption bands, associated primarily with pyroxene, provide critical constraints on mineral type, abundance, and composition . In contrast, feldspathic highland regions, dominated by Fe-poor plagioclase, exhibit high reflectance, weak or absent mafic absorptions, and a spectral continuum that clearly distinguishes them from the low-albedo, mafic-rich maria .

The lunar maria, which collectively cover approximately 17 % of the Moon's surface , display marked stratigraphic and compositional variability. This diversity is evident in the shifting positions and depths of absorption bands, variations in spectral slopes, and spatial differences in titanium and iron content . Among the most geologically and spectrally diverse regions are Mare Tranquillitatis and Mare Fecunditatis, both situated on the Moon's nearside and now observed by the JUICE mission's Moons and Jupiter Imaging Spectrometer (MAJIS) during its cruise phase (, this issue). These maria are of particular interest due to their complex volcanic histories and distinct spectral signatures .

Previous analyses using Clementine, M³ and LRO data have shown that Mare Tranquillitatis is largely composed of Fe- and Ti-rich basaltic units, while Mare Fecunditatis exhibits considerable compositional heterogeneity, including lower-Ti basalts and mixtures with highland materials . These differences point to distinct magmatic sources and eruptive chronologies, reflecting the Moon's complex volcanic evolution.

Superimposed on this intrinsic compositional diversity are the effects of space weathering – a suite of surface-altering processes driven by micrometeorite impacts, solar wind sputtering, and radiation. These processes progressively darken the lunar regolith, reduce spectral contrast, redden spectral slopes, and attenuate diagnostic absorption features, primarily through the accumulation of nanophase metallic iron (npFe0) within regolith grains .

Building upon this legacy of remote sensing, the MAJIS instrument – though primarily designed for studying the icy moons of Jupiter – has acquired valuable lunar observations during its cruise phase to the Jovian system. These spectral data offer a new opportunity to examine the Moon's surface with a modern imaging spectrometer at high spatial resolution, adding to the growing archive of high-quality hyperspectral datasets.

In this study, we present a detailed mineralogical and spectral analysis of a region encompassing Mare Tranquillitatis, Mare Fecunditatis, and the intervening highland terrains, using newly acquired MAJIS lunar observations (see Sects. , , and ). The study area includes diverse geological units, such as Upper Imbrian basalt flows and Nectarian to Imbrian-aged highland crust. This region provides a representative cross-section of nearside lunar lithologies and is well suited for examining mineralogical heterogeneity and surface alteration processes.

We analyse MAJIS-derived spectral parameters – including band depth, band centre position, ratioed spectra, and spectral slopes – alongside comparative datasets from M³, LRO, Kaguya-SP, and Clementine, which facilitate the description of the geological and mineralogical context (sections , , ). Particular attention is given to the mineralogical analysis of MAJIS data, mainly focused on the shape and position of the 1 and 2 µm absorption bands, which provide robust indicators of mafic mineralogy and regolith maturity (see sections , ). Furthermore, we give a brief overview of the infrared spectral range (2.6–5.4 µm) after thermal removal, to identify potential additional absorption bands and integrate the mineralogical analysis (Sect. ). Finally, we compare our maps with those publicly available from M³ data to verify the consistency of the MAJIS results (Sect. ). By employing both calibrated radiance factor (I/F) and normalised (ratioed) spectra, we mitigate the effects of photometric distortions and instrumental artefacts, improving the accuracy and reliability of mineral detection.

On 19 August 2024, during its journey towards the Jovian system, the JUICE spacecraft performed a Moon-Earth flyby, where the MAJIS instrument acquired hyperspectral data of both bodies (, this issue). This opportunity served not only to test the instrument's performance under real observational conditions but also to enable preliminary scientific investigations. In this study, we focus exclusively on the lunar dataset, while the analysis of the Earth observations obtained during the same flyby is discussed in detail in , and , this issue.

During the lunar flyby, MAJIS acquired four hyperspectral images (“cubes”), covering an equatorial region that spans Mare Tranquillitatis, Mare Fecunditatis, and adjacent highland terrain (see Fig. ). However, due to orbital constraints of the flyby, only three of the four cubes are suitable for scientific analysis of the solar flux reflected by the lunar surface. Table  provides a summary of the key observational parameters of the lunar dataset. For clarity, we refer to the four MAJIS cubes as C1, C2, C3, and C4, corresponding to their chronological order of acquisition. All four observations were obtained at a phase angle of approximately 90°, with the incidence angle decreasing progressively from C1 to C4. Here, we focus on cubes C3 and C4, which were acquired under the best observing conditions (incidence angles < 60°). For completeness, we also considered cube C2, which entirely covers a highland region but is affected by poor solar illumination conditions and significant photometric distortions (Fig. ). Furthermore, we excluded C1 from our analysis due to the suboptimal illumination conditions.

A dedicated photometric correction was not applied in this work. The currently available MAJIS observations were acquired at phase angles around ∼ 90°, preventing the derivation of a reliable empirical phase function. Preliminary tests using a Lommel–Seeliger correction, similar to approaches adopted for M³ and lunar datasets , did not yield satisfactory results because of the limited phase-angle coverage and the unfavourable observing geometry of the dataset. A rigorous photometric analysis would require observations acquired under a broader range of illumination conditions and a dedicated investigation, which is beyond the scope of the present study. Therefore, only I/F data are considered here, and the analysis focuses primarily on the spectral behavior and relative compositional variations within the observed regions.

Additional considerations must be made regarding instrumental artefacts present in the dataset. In particular, stray light contamination affects the spectral region below 1.3 µm, with the Moon representing a worst-case scenario for this stray light contribution (, this issue). Among the four cubes, only C4 could be reliably corrected for this artefact thanks to a specific detector windowing (, this issue). The observation was deliberately planned to read a portion of the detector outside the Field of View (FOV) (namely 17 of the 64 pixels of C4 along the slit) in order to quantify the stray light (see , this issue). For further details on stray light in the VIS–NIR range, see , this issue. It should also be noted that the spectral gap between 2.22 and 2.35 µm corresponds to the transition zone between the VIS–NIR and IR channels of the MAJIS instrument. Figure  shows the average spectra of C2, C3, and C4, with stray-light correction applied only to C4.

In this work, we analysed MAJIS radiance data cubes after filtering out saturated data. Thermal emission was removed, the data were converted to radiance factor (I/F), and a smoothing algorithm, with a kernel equal to 7 spectral channels, was applied. A detailed description of the MAJIS data processing workflow is provided in Appendix .

The region investigated in this study, as observed by the MAJIS instrument onboard JUICE through data cubes C2, C3, and C4 (Fig. ), encompasses geologically and compositionally diverse areas of the lunar nearside. The area includes extensive portions of two major basaltic plains – Mare Tranquillitatis and Mare Fecunditatis – as well as intervening and peripheral highland terrains, providing a representative sampling of both mare and non-mare lithologies. Geographically, the coverage extends approximately from -11 to +6° in latitude and from 21 to 61° E in longitude.

From a geological standpoint, the dominant surface unit across much of the region is classified as Upper Imbrian Mare (Im2), characterised by flat, smooth lava plains emplaced during large-scale basaltic flooding events in the Late Imbrian epoch. These deposits form the primary surface expression of both mare basins and are interpreted as relatively young, low-viscosity basaltic flows that infilled pre-existing impact basins . These empirical observations of regional mare units strongly correlate with the established physical models of lunar mare volcanism, where the morphological expression of Mare Tranquillitatis and Mare Fecunditatis can be directly tied to high-flux low-viscosity effusive regimes . By connecting the MAJIS spatial coverage to these specific volcanic dynamics – such as dike propagation, high-volume lava emplacement, and localised pyroclastic venting – the mapped units cease to be isolated features and instead provide critical constraints on the regional thermal history and volatile-driven eruption mechanisms that sustained lunar magmatism during the Moon's peak evolution. In addition to this widespread mare unit, several highland-associated geological formations are exposed, particularly within intermare regions and along mare basin margins (Fig. a). These include Imbrian Terra (It), comprising low-relief, moderately smooth surfaces with moderate to high albedo and relatively low crater densities, likely representing mixtures of local erosional debris and ejecta from nearby impact basins; Nectarian Terra (Nt), characterised by rugged topography and complex crater superposition relationships, composed largely of impact-generated megaregolith and representing some of the oldest exposed surfaces on the Moon; and undivided Imbrian basin materials (Ib), expressed as gently rolling to hilly terrains with subdued crater morphologies, plausibly associated with outer ring structures and ejecta deposits of large multi-ring impact basins. Additional minor units, including localised ejecta blankets and crater-related deposits, are scattered throughout the study area. Chronologically, the Imbrian units are dated to approximately 3.85–3.2 Ga, whereas the older Nectarian terrains likely formed between ∼ 3.92 and 3.85 Ga , attesting to a complex stratigraphic history shaped by the interplay of impact cratering and volcanic resurfacing.

Topographic information derived from the Lunar Orbiter Laser Altimeter (LOLA) onboard LRO , together with Digital Terrain Models generated from LROC Wide Angle Camera (WAC) data , provides essential context for interpreting these geological units. The surfaces of Mare Tranquillitatis and Mare Fecunditatis are generally low-lying, with elevations ranging from approximately -2.6 to -1.0 km relative to the lunar reference ellipsoid, consistent with deeply excavated impact basins subsequently infilled by dense, mafic-rich basaltic lavas. In contrast, the surrounding highlands commonly exceed +1.5 km in elevation, with some ridges and upland areas rising above +2 km. These elevation differences, locally exceeding 4 km, sharply delineate the structural boundaries of the mare basins and underscore the vertical extent of basin excavation followed by volcanic infill. Transitions between mare and highland terrains are marked by steep topographic gradients and morphologically complex surfaces, reflecting the combined effects of impact excavation, tectonic deformation, and volcanic emplacement.

From a compositional perspective, the region exhibits pronounced heterogeneity, as revealed by multispectral and hyperspectral observations acquired by multiple lunar missions (e.g., ). Clementine UVVIS colour-ratio composites highlight key spectral trends, notably the presence of high-titanium basalts, mapped as dark blue tones, particularly within Mare Tranquillitatis and covering a substantial fraction of the MAJIS C4 cube (Fig. b). These Ti-rich units are consistent with volcanic flows derived from Fe- and Ti-enriched magmatic sources (Fig. c and d). In contrast, mature, glass-rich highland soils in the southeastern portion of C4 display enhanced spectral reddening and subdued absorption features, indicative of extensive space weathering and high agglutinate abundance . However, it should be noted that the spectral effects of lunar space weathering are generally expected to approach saturation on timescales of the order of ∼ 1 Gyr. Consequently, variations in spectral maturity observed across the study area should not necessarily be interpreted as directly proportional to absolute surface age beyond this timescale, but rather as indicators of relative exposure conditions, regolith evolution, and the degree of recent surface refreshing processes .

Cube C3 primarily encompasses Mare Fecunditatis (Fig. ), which reveals greater compositional diversity relative to Mare Tranquillitatis, with generally lower TiO2 contents suggesting more evolved basaltic compositions (Fig. d) . The higher density of fresh impact craters in this region excavates less-weathered subsurface materials, thereby enhancing spectral heterogeneity and locally exposing mafic mineral assemblages (Fig. e–j).

These interpretations are strongly supported by elemental abundance maps derived from the CLASS X-ray spectrometer onboard Chandrayaan-2 , which indicate elevated Fe and Mg abundances across both mare regions, consistent with the dominance of mafic silicates such as pyroxene and olivine, and a corresponding depletion in Al relative to feldspathic highlands. High-resolution mineralogical maps obtained from the Kaguya/SELENE Multiband Imager further corroborate these trends, revealing widespread clinopyroxene and subordinate orthopyroxene associated with basaltic lava flows, together with low plagioclase abundances, particularly in Mare Tranquillitatis and Mare Fecunditatis, consistent with a mafic-rich, feldspar-poor composition (; Figs. g–j).

Additional mineralogical constraints are provided by the Moon Mineralogy Mapper (M³), whose spectral parameters indicate that 2 µm absorption band centres in both mare regions fall within the expected range for Ca- and Fe-rich clinopyroxenes, confirming the mafic nature of these basalts, while the spatial distribution of 1 µm band depths highlights enhanced pyroxene abundances within the maria and minimal absorption in the surrounding highlands, where plagioclase-dominated assemblages prevail (Fig. e and f). Taken together, these multi-mission datasets – encompassing imaging spectroscopy (MAJIS, M³, Kaguya MI), multispectral imaging (Clementine), elemental mapping (Chandrayaan-2/CLASS), and topographic measurements (LOLA, LROC-WAC) – provide a coherent and internally consistent framework for interpreting the geological, mineralogical, and geochemical variability of the Mare Tranquillitatis–Mare Fecunditatis region. The convergence of spectral, elemental, and morphological evidence underscores the fundamental roles of volcanic emplacement, impact gardening, and space weathering in shaping the present-day lunar surface and highlights the value of integrated, multi-instrument analyses for reconstructing the Moon's geological evolution.

In this section, we describe the spectral indices and ratioing technique applied to study the MAJIS cubes C2, C3, and C4.

In this section, we focus primarily on cubes C3 and C4, which predominantly cover basaltic mare terrains, with a minor portion of highlands included in C4. Both C3 and C4 are dominated by dark basaltic surfaces but exhibit considerable spectral variability. This variability is largely driven by the presence of small, fresh impact craters, such as Maskelyne G in C4 and the ejecta blankets of Messier and Messier A in C3 (Figs.  and ). Highland areas, where present, appear spectrally brighter, as shown in the I/F maps at 0.55 µm (Fig. ).

Cube C3 also displays spectral variability – albeit less pronounced – within Mare Fecunditatis, where these prominent ejecta blankets exhibit especially strong absorption bands. On the other hand, a darker region between Ibn Battuta and Lindbergh craters shows higher spectral slopes, suggesting possible compositional heterogeneity within the same mare unit. Cube C2, which covers the highlands east of Mare Fecunditatis, does not show any particular trend.

The distribution of 1 and 2 µm absorption band centres in cube C4 reveals a systematic shift towards longer wavelengths in the central mare region, consistent with increasing Fe2+ and Ca2+ contents in pyroxenes (Figs.  and ). However, the Maskelyne G crater exhibits band centres shifted towards shorter wavelengths, potentially indicating a different pyroxene composition (Fig. a, green dot). Conversely, band centre estimations in highland regions remain challenging due to weaker absorption features and increased spectral noise (Fig. e). In cube C3, reliable estimates of the 1 µm band centre are not achieved, and no significant variation is observed in the 2 µm band centre. Nevertheless, the band depths at 1 and 2 µm in C4 are clearly correlated (Fig. c), with the deepest bands occurring in fresh crater materials and the central mare, confirming the presence of mafic silicates.

Figures  and  highlight regions within cubes C3 and C4 that exhibit notable mineralogical and spectral diversity. In particular, the ejecta of Messier and Messier A (green box in Fig. ) display deep 2 µm absorption bands and low spectral slopes, characteristic of fresh material. By contrast, the darker region (blue box in Fig. ) shows more moderate absorption features combined with steeper slopes, indicative of a more mature regolith. Average spectra from selected regions in cubes C3 and C4 are presented in Figs.  and . These regions encompass a range of terrain types and spectral characteristics. The spectrum from Maskelyne G crater (green line in Fig. ) is particularly distinct. After continuum removal (Fig. d), it displays markedly deeper 1- and 2 µm absorption bands, consistent with fresher, pyroxene-rich material.

Figure b illustrates the typical trend of band centres for pyroxene-bearing surfaces, with each coloured dot corresponding to the regions of interest selected in Fig. a. The data indicate the presence of both low-Ca and high-Ca clinopyroxenes. Maskelyne G crater, along with two nearby fresh craters (purple and red ROIs), appears richer in low-Ca pyroxene, whereas other regions are dominated by high-Ca pyroxenes. Mare Tranquillitatis is also known to have the highest titanium concentrations on the Moon, associated with ilmenite and Fe-rich basalts, and a correspondingly lower abundance of plagioclase feldspar (Fig. b and d).

Figure d shows spectral slope plotted against 1 µm band depth, following the approach of (who originally used the 0.57–1.50 µm continuum), revealing four distinct spectral populations: 1.

fresh materials with high band depths and low spectral slopes,

Finally, Fig. e demonstrates a correlation between reflectance at 0.55 µm (I/F) and 1 µm band depth: higher reflectance values generally correspond to deeper absorption bands, except in the highlands, further supporting compositional and maturity differences across the studied regions.

RGB colour composite maps offer an intuitive means of visualising spectral heterogeneity. Figure  presents an RGB composite of cubes C2, C3, and C4, where the red channel represents the spectral slope between 0.55 and 2.60 µm (a proxy for surface maturity), the green channel indicates 2 µm band depth, and the blue channel shows 1 µm band depth, indicative of mafic mineral presence. Cube C4 exhibits the greatest spectral variability, with a sharp contrast between highland and mare regions. Even within Mare Tranquillitatis, subtle colour variations are evident. In particular, the central portion of Mare Tranquillitatis and the area surrounding Maskelyne G crater display a cyan hue, indicating a higher abundance of pyroxene-bearing material (Fig. ).

The mineralogical diversity described here is also supported by thermophysical analyses of the same MAJIS observations (, this issue), which reveal systematic terrain-dependent differences in surface temperature and emissivity in the 3–5.56 µm range, consistent with compositional and surface-texture contrasts between mare and highland terrains. In contrast, plagioclase-rich highlands, characterised by weak mafic bands and steep, reddened slopes, display lower emissivity and cooler temperatures. Fresh crater materials, such as those at Maskelyne G, show both enhanced pyroxene signatures and localised thermal anomalies, reflecting reduced maturity and blocky textures. This close agreement between VNIR and IR observations highlights the tight coupling between composition, maturity, and thermophysical behaviour along the MAJIS track (, this issue).

Here we discuss the spectral behaviour and the mineralogical variation highlighted after spectral ratioing described in Sect. . The scatter plot of the spectral slope between 0.75 and 1.50 µm (REFSlope) versus 1 µm band depth (Fig. a) reveals two distinct spectral populations corresponding to the lunar highlands and Mare Tranquillitatis. Highland points occupy the upper-left quadrant, characterised by relatively shallow 1 µm absorption bands and steep spectral slopes in the 0.75–1.50 µm range. These features are indicative of feldspathic compositions dominated by plagioclase with minimal mafic absorption and are consistent with mature, weathered surfaces exhibiting high spectral reddening.

By contrast, Mare Tranquillitatis points cluster in the lower-right quadrant, displaying deeper 1 µm absorption bands and lower continuum slopes. These spectra reflect higher mafic mineral content, particularly pyroxenes, and spectral properties influenced by the different compositional response of Fe-rich basaltic materials to space weathering processes. In particular, the higher iron abundance in mare basalts enhances optical maturation effects, producing spectral characteristics distinct from those observed in feldspathic highland terrains. REFSlope variability among the mare spectra is reduced relative to the highlands (see also Fig.  in Appendix , right panel), indicating more homogeneous surface properties in terms of maturity.

Within the mare population, two distinct clusters are apparent. The first (orange circle in Fig. a, REFSlope between 0.35 and 0.40) corresponds to spectra closer to the highland boundary, while the second (green circle, REFSlope < 0.37) represents spectra from more central mare regions. This spatial distinction may reflect minor differences in regolith maturity or local mixing with highland material.

A subtle trend within the highland population shows increasing spectral slope with decreasing 1 µm band depth, consistent with high maturity, as previously documented . Between the highland and mare populations, an intermediate group (blue oval in Fig. a) corresponds to a transitional region situated between the highlands and Mare Tranquillitatis near the margins of the Ruin Basin. This scatter plot captures a spectral gradient reflecting the transition from feldspathic highlands to basaltic mare surfaces.

Representative mean spectra for each group are shown in Fig. b. In the highlands, some spectra (black spectrum) exhibit band depths similar to the reference spectrum used for ratioing but with a redder visible slope, typically located in the brightest highland areas. Most highland spectra (red spectrum) display weak 1 µm bands centred slightly shortward of 1 µm. Both highland mean spectra lack the 2 µm absorption feature, as expected for mature highland soils.

In Mare Tranquillitatis, most spectra (green spectrum) show deeper 1 µm bands (centred slightly longward of 1 µm), although the 2 µm band in the ratioed mean spectrum is not prominent. The mean spectrum from the Ruin Basin margin (blue curve) exhibits deeper 1 µm bands than highland spectra, centred around 0.95 µm, with a deeper 2 µm band, indicating a higher content of low-Ca pyroxenes. The magenta curve represents spectra from mare regions with the deepest 1 µm bands; for these, small craters in Mare Tranquillitatis also show the 2 µm band, reflecting enrichment in pyroxenes. Spectra with the lowest REFSlope, corresponding to the least weathered and most mafic-rich surfaces, are associated with the interior of Maskelyne G crater and other small craters in the mare, where fresh basaltic exposures are present.

Although MAJIS acquired only four spectral cubes over the lunar surface, these data provide reflectance spectra extending up to 5 µm. This extended wavelength range opens new possibilities for detecting and analysing spectral features previously inaccessible to imaging spectrometers such as M³. After removing the thermal emission component (see Appendix , Fig. b and c), the resulting reflectance spectra still exhibit a generally red slope, consistent with space-weathered materials.

Several broad and recurring absorption features are evident, most notably near 2.8 µm and around 4.8 µm (see Fig.  in Appendix ). An absorption feature between 2.8 and 3.0 µm has previously been attributed to the presence of hydroxylated or hydrated compounds based on M³ observations . This feature is commonly associated with OH/H2O-bearing phases and is observed across the lunar surface, with generally stronger signatures at higher latitudes and variations related to local time and surface temperature, possibly linked to solar-wind implantation or endogenic processes. In the MAJIS dataset, however, the spectral interval between 2.85 and 2.95 µm is affected by an instrumental artefact corresponding to a spectral gap at the interface between two segments of the Linear Variable Filter (LVF) . This gap was intentionally selected due to the absence of relevant spectral features for the Jovian targets but significantly complicates the reliable interpretation of potential absorptions in this crucial range. Nevertheless, , this issue, discuss this specific feature and its possible interpretation. These instrumental limitations highlight the need for caution when interpreting preliminary MAJIS data.

A broader and more pronounced absorption feature is observed at approximately 4.8 µm (see Fig.  in Appendix ). This spectral region remains largely unexplored in lunar remote sensing, particularly regarding silicate mineralogy. Laboratory measurements of HED meteorites (howardites, eucrites, and diogenites) and olivine reported by and show absorptions in the 4.74–5.10 µm range, associated with pyroxene and olivine components. The presence of a similar absorption in the MAJIS lunar spectra suggests a possible mineralogical origin; however, its spatial characterisation remains challenging due to uncertainties associated with thermal emission removal.

Specifically, lunar surface radiance measurements are affected by a combination of reflected solar radiation and intrinsic thermal emission from the regolith. Correcting for the thermal contribution is inherently degenerate because the emitted component depends on surface emissivity, which is itself related to reflectance through Kirchhoff's law. Consequently, variations in the assumed reflectance modify the emissivity and the modelled thermal emission, allowing different combinations of reflectance, emissivity, and surface temperature to reproduce the same observed signal. Therefore, thermal removal in this wavelength range cannot yield a unique reflectance solution and could account for the systematic signature near 4.8 µm.

Given these complexities related to instrumentation and data interpretation at wavelengths longer than 2.6 µm, a comprehensive analysis of these absorption features requires further dedicated investigation. As this topic lies beyond the scope of the present study, a detailed discussion is deferred to future work.

A comparison between spectral index maps derived from publicly available datasets (e.g., M³) and those produced in this study using MAJIS observations reveals a strong degree of consistency, both in terms of spatial distribution and absolute values (see Fig. ). This agreement is noteworthy given that M³ spectral index maps have a spatial resolution approximately ten times coarser than the M³ Global Mode images (∼ 140 m per pixel), and therefore significantly lower than that of MAJIS (see Table ).

This consistency supports the reliability of the MAJIS products and highlights their potential to complement existing lunar spectral datasets. In particular, the 1 µm band depth map – available only for cube C4, corrected for stray light – shows higher values over mare regions and lower values over the highlands, in agreement with expectations based on lunar mineralogy (see Fig. ). Mafic-rich basalts in the maria typically exhibit deeper absorption features due to their higher pyroxene content, whereas the highlands, dominated by anorthositic compositions, display shallower bands. To further investigate this consistency, a direct comparison of the average spectra of three different terrains was conducted (see Fig. c). While the average spectra of mare soil and Maskelyne exhibit a remarkably good agreement between the two sensors, the highlands spectra display a more noticeable divergence in terms of overall slope and band depth. This discrepancy is highly compatible with the high topographic and macroscopic surface roughness characteristic of the lunar highlands. When observed under MAJIS's high phase angle (∼ 90°), rugged and sloped terrains significantly enhance local shadowing and photometric variations, leading to a more pronounced spectral mismatch compared to the flatter and smoother mare surfaces.

A similar trend is observed in the spatial distribution of the 2 µm band centre. In cube C4, the highest band centre values are concentrated within the Mare Tranquillitatis region. These longer-wavelength band centre positions indicate a predominance of Fe- and Ca-rich clinopyroxenes, consistent with previous findings from M³ data, which also reported a shift towards longer wavelengths in the 2 µm band centre (Fig. a and b).

For cube C3, which lies almost entirely within Mare Fecunditatis, the 1 µm band depth values are more spatially homogeneous, reflecting the geochemical uniformity of this mare region.

When comparing these spectral parameters, several physical, geometric, and instrumental factors must be carefully considered. First, a direct comparison of absolute reflectance levels is intrinsically complex: MAJIS observations are expressed as I/F and were acquired at a high, fixed phase angle of ∼ 90° without photometric correction, whereas M³ data represent photometrically corrected reflectance . This major difference in viewing geometry not only shifts the overall albedo levels but also directly affects the absorption band parameters (such as depth and shape), which are highly sensitive to photometric variations. Second, while thermal emission removal was applied to both datasets to ensure spectral integrity at longer wavelengths, this correction was performed using different thermal models and algorithms. Finally, different spatial resolutions lead to distinct sub-pixel mixing effects within the footprints of each sensor, and both instruments are subject to their minor, inherent calibration limits. Nevertheless, despite these different observational conditions and the early-stage calibration of MAJIS, the outstanding performance of the instrument is confirmed by the highly consistent geological trends and parameter ranges resolved by both sensors.

Overall, both the spatial distribution patterns and the numerical ranges of spectral parameters derived from MAJIS appear broadly consistent with those obtained from M³. This further validates the spectral integrity of MAJIS observations, even at this early stage of data analysis, and demonstrates the instrument's potential for future mineralogical mapping of the Galilean moons of Jupiter.

The MAJIS instrument onboard the JUICE mission represents a major advance in lunar hyperspectral imaging, providing high-resolution reflectance spectra. Although only a limited number of spectral cubes (C2, C3, and C4) were acquired during the lunar flyby, these data provide a valuable opportunity to investigate the spectral, mineralogical, and maturity-related properties of the lunar surface at high spatial resolution. Spectral parameters extracted from MAJIS not only reproduce well-established mineralogical trends identified by previous missions such as Chandrayaan-1/M³ and SELENE/Kaguya, but also extend the analysis into previously unexplored spectral domains.

Analyses of cubes C3 and C4 reveal significant mineralogical diversity within the mare-dominated regions of Mare Tranquillitatis and Mare Fecunditatis. Pronounced 1- and 2 µm absorption bands – particularly in C4 – indicate the dominance of pyroxene-rich basalts, consistent with previous classifications of these maria as mafic and Fe-rich. Spectral trends in C4 show a systematic shift of the 2 µm band centre towards longer wavelengths within Mare Tranquillitatis, indicative of high-Ca clinopyroxenes. However, localised craters such as Maskelyne G exhibit the opposite trend, suggesting a pigeonite-rich composition. This mineralogical variability, observed even within the same mare unit, is especially evident in fresh craters and their ejecta, which expose less weathered and compositionally distinct subsurface material.

Cube C3, covering Mare Fecunditatis, exhibits more spatially homogeneous 2 µm band centre values, consistent with a relatively uniform basaltic composition. Nevertheless, localised features such as the Messier and Messier A craters display deeper absorption bands and spectral characteristics typical of fresher material.

Analysis of spectral slope and band depth across the region demonstrates MAJIS's capability to capture variations in soil maturity. The RGB colour composite map (Fig. ), combining slope and band depth parameters, clearly distinguishes mature highland terrains from less weathered mare regions. Fresh ejecta, such as those associated with Messier and Maskelyne G, exhibit low spectral slopes and strong absorption bands, characteristic of immature soils. However, the spectral effects of lunar space weathering are generally expected to progressively saturate on timescales of the order of ∼ 1 Gyr. Consequently, the maturity variations discussed here should primarily be interpreted in relative terms, reflecting differences in regolith evolution, exposure conditions, and resurfacing processes rather than direct absolute age differences beyond this timescale .

Scatter plots (e.g., Fig. a) illustrate the complex relationship between spectral slope and 1 µm band depth. Highland terrains show a strong anticorrelation between these parameters, indicating the dominant influence of space weathering. Nevertheless, because weathering-related spectral reddening and attenuation of mafic absorption bands tend to approach saturation over geological timescales, the observed correlations are unlikely to represent a simple linear progression with surface age . In contrast, the maria display two distinct clusters characterised by nearly constant spectral slope values (0.035 and 0.030) and increasing 1 µm band depth, reflecting variations in pyroxene abundance and/or maturity. Spectra exhibiting anomalous or negative band depth values (Fig. a, magenta oval) are spatially associated with highland terrains, particularly in shadowed areas, supporting the interpretation that unresolved topography and illumination geometry significantly influence apparent spectral characteristics.

Whereas previous lunar hyperspectral observations provided limited coverage and/or usability in the 2.6–5.4 µm range, MAJIS enables the investigation of potential absorption features within this spectral domain. The 2.8–3.0 µm feature is discussed in , this issue, and may be related to OH/H2O-bearing phases. A preliminary thermal correction reveals a broad feature near 4.8 µm that could be attributed to pyroxene and/or olivine, although thermal removal remains uncertain owing to degeneracy between thermal emission and reflected solar components. Despite challenges related to signal saturation and increased noise beyond 4 µm, this wavelength range holds considerable promise for future mineralogical investigations.

Comparison of MAJIS-derived spectral maps with those from M³ demonstrates excellent agreement in both spatial distribution and spectral parameter ranges. MAJIS reproduces the expected deeper band depths in mare regions and shallower features in feldspathic highlands. Similarly, 2 µm band centres derived from MAJIS follow trends consistent with Ca- and Fe-rich clinopyroxene distributions mapped by M³. This validation is particularly robust for cube C4, which was corrected for stray light and currently represents the most reliable MAJIS lunar dataset. The strong agreement with legacy datasets confirms the quality of MAJIS calibration, providing reliable spectral parameters and establishing a solid foundation for future observations of the Jovian moons.

Analysis of spectral parameters derived from MAJIS data highlights the instrument's strong potential for lunar mineralogical studies. Despite the limited spatial coverage, this dataset represents an important milestone, an in-flight assessment of MAJIS performance and the first remotely acquired hyperspectral lunar observations since Chandrayaan-1/M³ , Cassini/VIMS , and the more recent Imaging Infrared Spectrometer (IIRS) onboard Chandrayaan-2 e.g..

Although preliminary, MAJIS lunar observations demonstrate the instrument's suitability for high-resolution mineralogical mapping of planetary surfaces.

The principal results of this study may be summarised as follows:

Validation of spectral parameters: MAJIS successfully reproduces well-established spectral trends identified by previous missions (M³, Kaguya, Clementine), including deeper 1 and 2 µm absorption bands in mafic-rich mare regions and weaker features in feldspathic highlands.

Overall, the MAJIS lunar dataset constitutes a critical test of hyperspectral imaging performance and demonstrates the instrument's capability to resolve both compositional and maturity-related variations at regional scales. Its successful application to the Mare Tranquillitatis–Mare Fecunditatis region not only validates instrument performance but also provides an essential baseline ahead of MAJIS observations of the Jovian system.

Beyond delivering new spectral observations, this study highlights the broader scientific value of MAJIS for planetary surface characterisation and underscores the importance of cross-mission spectral analyses in advancing our understanding of lunar lithological diversity, magmatic evolution, and surface modification processes.