NATURAL ANALOGUES OF PLANET MERCURY’S SURFACE: MINERALOGICAL AND SPECTROSCOPIC ASPECTS

Understanding the nature and evolution of Mercury’s surface materials is crucial to constrain the plan-et’s formation processes. Data from NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEo-chemistry, and Ranging) mission indicate that the surface is dominated by Na-rich plagioclase, FeO-poor enstatite and forsterite, and Mg–Ca–Fe sulphides, suggesting that Mercury formed in the inner part of the protoplanetary disk from highly reduced precursor materials. However, the interpretation of Mercury’s spectral properties remains complex due to the effects of space weathering (SpWe) and extreme temperature variations affecting the surface. Since it is not possible to have direct information on the surface, from meteorites from Mercury nor from returned samples, the study of analogous ma-terials becomes crucial to prepare for the interpretation of the upcoming data form the ongoing ESA-JAXA BepiColombo mission. Therefore, I conducted a comprehensive minero-petrological and spectroscopic investigation on highly reduced meteorites and terrestrial analogues. The investigated meteorites include five aubrites (Rantila, Tiglit, Peña Blanca Spring, Norton County, NWA 14185) and four enstatite chondrites, achondrites and melt rocks (Itqiy, NWA 4945, NWA 13266, NWA 13210). SEM (Scanning Electron Microscope) and EPMA (Electron Probe Micro Analyzer) analyses revealed that the samples are mainly composed of nearly FeO-free enstatite, with variable amounts of forsterite, diopside, and Na-rich plagioclase. Sulphides and metals, particularly abundant in enstatite chondrites and achondrites, include troilite, daubréelite, oldhamite, alabandite, keilite, and kamacite with variable Si contents. The mineralogical and composi-tional features indicate that these meteorites come from multiple parent bodies. The visible–near infra-red (VNIR) reflectance spectra show diagnostic features that reflect both the very low FeO content (<0.4 wt%) in silicates and the minero-chemical variations among the samples. Aubrites exhibit weak mafic-related absorption bands at 0.9 µm, attributed to Fe²⁺ electronic transitions in low-Ca pyroxenes and olivine, with an additional minor absorption feature around 0.5 µm in olivine-rich portions, due to spin-forbidden Fe²⁺ transitions. Enstatite chondrites and achondrites show nearly flat, featureless spectra with only weak or no absorptions, and in some cases a slight red slope likely related to the higher abundance of metal and sulphides. Minor features at 1.4 µm and 1.9 µm are occasionally ob-served, indicating incipient terrestrial weathering. Rantila aubrite presents slightly higher FeO con-tents in silicates, which are observable in VNIR spectra: the potential detection limit of mafic minerals from crystal field absorptions in VNIR reflectance spectra could be placed between the composition of Rantila and that of the other reduced meteorites. For Rantila aubrite Laser-Induced Time-Resolved Lu-minescence (LITRL) analysis was conducted, revealing the presence of different activators in different mineral phases, giving us additional information on the mineralogy and chemistry of the sample. To simulate SpWe processes on Mercury’s surface, particularly the effects of solar wind irradiation, ion irradiation experiments with He⁺ (20 keV) and C⁺ (30 keV) were performed on aubrite powders and rock fragments. The observed spectral effects are darkening and reddening in VNIR spectra, accompanied by a reduction in spectral contrast and shifts of Christiansen Features (CFs) and Reststrahlen Bands (RBs) positions toward longer wavelengths in MIR spectra. C⁺ irradiation produces stronger darkening ef-fects than He⁺, and the presence of carbon in the sample prior to irradiation appears to cause more pronounced spectral darkening. Different responses to ion irradiation are observed between powders and rock fragments. Therefore, the extent of the spectral changes induced by ion irradiation depends on both the nature of the ion species and the physical state of the target (powder vs rock), revealing that surface mineralogy and texture play a key role in spectral alteration under solar wind exposure. The study of boninites provided several insights relevant to Mercury’s surface exploration. First of all, their mineralogy and bulk composition generally match Mercury’s surface. The primary compositional difference is the higher FeO content in the terrestrial samples (~8 wt% compared to estimated <2 wt% on Mercury). Major mineralogical discrepancies include the presence of hydrated alteration minerals, which are absent on Mercury, and the absence of sulphides typical of highly reduced environments such as Mercury’s surface. Reflectance spectra show a red slope in the ultra-violet (UV) region and a subtle red slope in the visible (VIS) region. The main absorption feature is in the VNIR range, at ~1 μm, associated with mafic minerals. CFs are located at lower wavelengths for the samples with higher pla-gioclase content. Increasing grain size leads to spectral flattening and reduced reflectance and absorp-tion band intensities. Heating to Mercury-like temperatures (up to 450°C) induces a reddening of the VIS slope, disappearance of the band linked to aqueous alteration, a decrease in the MIR spectral con-trast, and shifts of RBs positions. The emissivity spectra show three changes occurring for all the sam-ples as the temperature increases from 150°C to 450°C: CFs positions shift, emissivity minima shift, and the spectral contrast between CFs and RBs increases. An opposite trend is observed for the emis-sivity minima shifting between mafic-richer and felsic-richer samples. Still, the minima positions re-main distinct, suggesting that mafic-richer and felsic-richer terranes should be easily identifiable with the MERTIS instrument. By integrating mineralogical, chemical, and spectroscopic data from natural analogues of Mercury’s surface, we can better constrain how composition, reduction state, and space weathering can influence spectral properties, highlighting the value of a multi-methodological approach. Moreover, this study provides a detailed analogues spectral library that could help in the interpretation of Mercury’s surface spectra from BepiColombo SIMBIO-SYS (Spectrometer and Imagers for MPO BepiColombo Integrated Ob-servatory SYStem) and MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instruments.