Deconvolution of Chang'e-4 Reflectance Spectra on Rocky versus Soils Targets along the Yutu-2 Traverse across Von Karman crater (lunar days 41-43)

Deconvolution of Chang’e-4 Reflectance Spectra on Rocky versus Soils Targets along the Yutu-2 Traverse across Von Karman crater (lunar days 41-43). P. C. Pinet1, Y.H. Daydou1, C. Li2, D. Liu2, B. Liu2, X. Ren2, 1Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS/CNES, Observatoire MidiPyrénées, Université de Toulouse, France; [email protected]), 2Key Laboratory of Lunar and Deep Space Exploration, NAOC, CAS, Beijing, China; [email protected]). Introduction: Chang’E-4 spacecraft touched down in the 186-km-diameter Von Kármán crater on the floor of the South Pole-Aitken basin on January 3rd, 2019. The results here are based on spectra of reflected light acquired on rock and regolith surfaces, that were recorded by the Yutu-2 rover as it traversed successfully across the Von Kármán crater covering as of today more than 2000m. Observations: In situ reflectance observations of local targets have been made by the VNIS/SWIR spectrometer onboard the rover [e.g.,1]. Among the targets which have been surveyed along the traverse and which include soils and rocks, the focus is put here on a selection of rocky and soil surfaces. We analysed a series of several tens of spectra acquired until lunar day 46 (2022/08/22) but highlight in this presentation only a few of them (see figs. 1 and 3). The location, geomorphological and geologic context of these measurements are indicated here below in two cases (see figs. 2 and 4). Figure 1. VNIS/SWIR spectra acquired on rocky targets along the Yutu-2 traverse Figure 2. Location of rock spectrum LE04104_0239 Figure 3. VNIS/SWIR spectra acquired on soil targets along the Yutu-2 traverse Figure 4. Location of soil spectrumLE04302_0246 Methodology: Advanced spectral deconvolution by means of MGM implementation: The principle of the Modified Gaussian Model is to deconvolve overlapping absorptions of mafic mineral spectra into their fundamental absorption components. Spectra are modeled in the logarithm of reflectance space as a sum of modified Gaussian distributions superimposed on a baseline continuum [e.g., 2, 3]. The procedure is described in [4]. MGM inverse modeling provides with band center, band width and band depth estimates. Band centers are used to determine trend line equations for each individual absorption band [5]. The olivine composition (Molar Forsterite Fo#) is then predicted based on minimizing the deviations in band centers from the established trends for the three absorptions simultaneously, using the integrated rms spectral distance to trend quantity (sdt). Analyses and Results : A systematic search has been implemented with different mineralogic configurations (OL, OL-OPX, OL-OPX-CPX) [3, 5, 6]. The best modeling outputs are produced with the OL-OPX-CPX case and two examples are shown hereafter (figs. 5 and 6). As displayed on the figures, the modeling appears well behaved (with quite low residuals), both in terms of continuum (secondorder polynomial adjusted on the main maxima) and band centers, band widths and depths, which meet a number of spectroscopic constraints associated with pyroxenes and olivine. For both rock and soil spectra acquired in close vicinity (lunar days 41-43), the relative proportion of mafic minerals (Opx, Cpx, Ol) and the olivine composition (Molar Forsterite Fo#) (figs. 6a and b) are determined. Figure 5. MGM deconvolution of rock spectrum LE04104_0239. Measured spectrum (light pink dotted line) and MGM modeled one (thick green solid line) with the Gaussians (solid blue lines, vertically stretched x 3) and polynomial (hatched red line); residuals displayed by the black line along the spectral domain. For clarity, the residuals (observed – modeled quantity) are shifted by +0.1 which means that a perfect fit would be displayed with a 0.1 flat line (here rms : 0.0081) Figure 6. MGM deconvolution of soil spectrum LE04302_0246 (see legend fig. 5) (rms: 0.0082) Acknowledgments : Thanks expressed to Chang’E-4 team for rover operations and data acquisition. Support of French Agency CNES (APR Chang’e-4 VNIS) and CNRS is duly acknowledged. Figure 6a. Fo# determination for rock spectra. Crosses (x) plot the M1-1, M2, M1-2 band centers for each rock spectrum (color from fig.1) determined by MGM deconvolution (Gaussians ‘850’, ‘1050’, ‘1250’). Composition trend lines from [5]. Figure 6b. Fo# determination for soil spectra. Color of crosses refers to the soil spectra shown on Fig.3. We note that the band center associated with M2 site (Gaussian ‘1050’) is shifted longward by about 30nm (already noted in [7]), which calls for the presence of either additional mineral phases or minor amounts of non-stochiometric cations in the crystal structure. Inferences and Prospects: Overall these results obtained on rocks with coarse textures and soils along the Yutu-2 traverse agree with previous studies [7, 8, 9, 10]. They support the view that the rock mineralogy is driven by assemblages of enstatite, augite and olivine (Fo# 60-70) with the soil having the same mineralogy / composition (Fo# 65-80). It is of note that the relative proportion of olivine appears to be higher in rocks than in regolithic soils which appear slightly more cpx/opx-enriched at this location. It will be worth comparing with Chang’e-6 in situ and laboratory spectroscopic measurements. References: [1] Lin H. et al. (2020) Nat. Sci. Rev., 7, 913920. [2] Sunshine J.M. et al. (1990) JGR, 95, B5, 6955-6966. [3] Clenet, H. et al. (2011) Icarus, 213, 404-422. [4] Pinet P.C. et al. (2018) LPSC XLIX, #1899. [5] Pinet P.C. et al. (2022) Icarus, 373, 114765. [6] Pinet P.C. et al. (2016), https://hal.archivesouvertes.fr/hal-03455649/document. [7] Liu C. et al. (2022), Astron. & Astrophys., 658, A67. [8] Li C. et al. (2019), Nature, 569, 378-382. [9] Gou S. et al. (2020), Icarus, 345, 113776. [10] Zeng Q. et al. (2021), Nature, Scientific Reports 11(1):15435, DOI:10.1038/s41598-021-93694-8.