Origin and VNIR Featureless Spectra on the Moon from Combined M3 and Diviner Observations

ORIGIN OF VNIR FEATURELESS SPECTRA ON THE MOON FROM COMBINED M³ AND DIVINER OBSERVATIONS. M. Martinot1, J. Flahaut1 C. Wöhler2 and K.L. Donaldson Hanna³. 1Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France ([email protected]), 2TU Dortmund University, D-44227 Dortmund, Germany, ³Department of Physics, University of Central Florida, 4111 Libra Drive, Orlando, FL 32816, USA. Introduction: The visible to near-infrared wavelength range (VNIR) has been widely used to constrain the mineralogy on planetary surfaces, as rockforming minerals (i.e., olivine, pyroxene, plagioclase) have diagnostic signatures in this wavelength range (e.g., Ohtake et al., 2009; Donaldson Hanna et al., 2014; Lemelin et al., 2015). However, featureless (FL) spectra are those that do not present absorption bands near the characteristic 1.0 and 2.0 µm positions. FL spectra have been reported at the surface of the Moon, Mercury and small bodies (Figure 1). On the Moon, featureless spectra are often associated to anorthositic terrains (e.g., Yamamoto et al., 2015). Several formation mechanisms linking plagioclase and FL spectra have been put forward: 1/ Adams, Hörz and Gibbons (1979) proposed that shock pressures may erase the absorption band of plagioclase; 2/ the composition of plagioclase, and more specifically its Fe content, influences the presence and depth of the plagioclase absorption band (e.g., Adams and Goullaud, 1978); 3/ Lucey (2002) proposed that FL spectra may be caused by anorthosite affected by space weathering. In this abstract, we investigate a number of locations presenting both pure anorthosite signatures (PAN) and FL spectra, including Alphonsus, BuysBallot, Carnot and Humboldt craters. PAN exposures and regions exhibiting FL spectra were first mapped with data from the Moon Mineralogy Mapper (M³) VNIR instrument, then Diviner Lunar Radiometer data (Diviner) were extracted from the same regions to further investigate the mineralogy from the thermal infrared (TIR) wavelength range. Figure 1: PAN and FL VNIR continuum-removed spectra. Modified after Martinot et al. (2020). Data Processing: M³ was a VNIR hyperspectral imager onboard Chandrayaan-1 orbiting the Moon between 2008 and 2009, and acquiring data from the lunar surface between 0.43 and 3 µm (Pieters et al., 2009). Level 1B M³ data from the Planetary Data System (PDS) were downloaded and thermally corrected using the method of Wöhler et al. (2017). FL spectra were searched for using a series of filtered band parameters, calculated on the M³ hyperspectral cubes. Band parameters include the IBD1250 (a band parameter developed by Cheek et al., 2014 and designed to highlight Fe-bearing crystalline plagioclase); the IBD2000 (a band parameter developed by the M³ team (Mustard et al., 2011) and designed to highlight minerals with an absorption band centered at 2 µm such as pyroxene and spinel); minimisation of the band depths at 0.95, 1.05, 1.25, 2 µm; and minimisation of the residual calculated from a 2-line continuum fit over each individual spectrum. IBD3, a spectral parameter developed by Wöhler et al. (2017) assessing hydration of the surface was also calculated on the regions of interest to examine if there are notable differences between PAN exposures and regions exhibiting FL spectra. Diviner is a TIR imaging radiometer onboard the Lunar Reconnaissance Orbiter, orbiting the Moon since 2009. It acquires data between 0.3 and 400 µm (Paige et al., 2010). The three thermal channels in the 8 µm region were used to produce Christiansen feature (CF) maps of the regions of interest. The CF position is an emission maximum in the 8 µm region which position can be used to constrain the degree of polymerization of minerals and the bulk silicate composition (e.g., Greenhagen, 2009). Level 1B data from the PDS and a data reduction presented in Martinot et al. (2025) were used to construct mosaics of the selected areas. Data obtained from an altitude of 80 km were used to map the CF position and normalised to lunar equatorial noon (called corrected CF position). The CF positions of the plagioclase endmembers, albite and anorthite, are different (Donaldson Hanna et al., 2012; 2014). Therefore, the CF maps provide crucial compositional data over regions exhibiting FL spectra. Results: Humboldt crater is presented here as an example study area that exhibits both PAN and FL spectra (Donaldson Hanna et al., 2011; Martinot et al., 2018; 2020). Figure 2 shows a colour composite highlighting the mineralogical diversity of the Humboldt crater, where regions exhibiting FL spectra are exposed in red, PAN exposures in green, and mafic minerals in blue. Regions of interest were hand drawn onto the image to select PAN exposures and regions exhibiting FL spectra, and statistics were calculated on the corrected CF position maps and the IBD3 maps (Table 1). Table 1: Corrected CF position (in µm) and the IBD3 for regions exhibiting FL spectra and PAN exposures in Humboldt crater. FL Min Max Mean Stdev CF 7.87 8.11 7.97 0.09 IBD3 0.40 0.9 0.57 0.08 PAN Min Max Mean Stdev CF 7.89 8.26 7.98 0.1 IBD3 0.21 0.70 0.49 0.08 Figure 2: Humboldt crater colour composite (R = regions exhibiting FL spectra; G = IBD1250, highlighting PAN exposures; B = IBD1000, developed by the M³ team, (Mustard et al., 2011) to highlight minerals with a strong band at 1 µm, such as mafic minerals). Discussion and Conclusions: All investigated areas in Humboldt crater are located in the central peak and the peak alignment to the north of the central peak, which means these areas all underwent high shock pressures as expected from the impact process (Melosh, 1989). This gives an opportunity to test the shock hypothesis for the formation of FL spectra in Humboldt crater. The mean CF positions associated with both PAN exposures as well as regions exhibiting FL spectra are consistent with immature Apollo regolith samples returned to Earth and measured in simulated lunar environment (SLE). Indeed, immature highland sample 67701 has a CF position measured under SLE at 7.95 µm, and the mature highland soil sample 66031 has a CF position measured under SLE at 8.04 µm (Donaldson Hanna et al., 2017). A difference in CF position less than 0.02 µm is not considered significant (Greenhagen et al., 2010), so the materials associated with the PAN and FL spectra in Humboldt crater have similar compositions. This means that the shock pressures associated with the impact event that created the Humboldt crater might have erased the VNIR signature associated with areas that have FL spectra. The IBD3 band parameter, also called OHIBD, defined by Wöhler et al. (2017), is associated with the absorption band around 3 µm, which was shown by Hibbitts et al. (2011) to carry information about the speciation of hydration (i.e., adsorbed hydroxyl or water, internal hydroxyl, liquid water and water ice). It seems that FL areas have a higher IBD3 than PAN areas, potentially meaning that FL areas capture more hydration than PAN areas. 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