Bands Selection for Multispectral Detection Mode of Lunar Mineralogical Spectrometer of China’s Chang’E-5 and Chang’E-6 Missions

Introduction:  In December 2020, China’s Chang’E-5 (CE-5) mission successfully landed in the northeastern part of Oceanus Procellarum on the Moon and achieved its goal of collecting lunar surface samples[1]. Chang’E-6 (CE-6) is China’s second lunar sample return mission following the CE-5 mission, and were launched on 3^rd May 2024 . The CE-6 landing zone has been selected to lie within the lunar farside South Pole–Aitken (SPA) basin in the southern part of the Apollo basin[2]. Both CE-5 and CE-6 missions are equipped with the Lunar Mineralogical Spectrometer (LMS) to conduct surface spectral scanning to obtain the mineralogical infromation of the sampling area. The LMS can acquire hyperspectral data from 480 nm to 3200 nm[3]. Due to in-orbit working time and condition limitations, LMS could not complete whole sampling area scanning using hyperspectral detection mode and has to adopt multispectral detection mode. In this study, we mainly introduced the design of band selection for the LMS multispectral detection mode.

Designed LMS multi-bands for CE-5 mission:  Remote sensing data reveals that the spectra of the CE-5 landing area exhibit longer-wavelengths ~1 μm and 2 μm absorptions, indicating that the CE-5 mare basalts are primarily composed of high-Ca pyroxene (HCP). Spectra of CE-5 landing area also show a much weaker 2 μm absorption relative to their 1 μm absorption, resembling the spectral characteristics of olivine[5]. In addition, CE-5 landing area contains a large amount of ejected materials from surrounding regions, including basaltic materials dominated by low-Ca pyroxene (LCP) from western old IM basaltic regions and materials dominated by plagioclase from the eastern highlands. Products associated with volcanic activity such as volcanic glass and ilmenite are also likely to contribute to the material in CE-5 landing area. Considering the spectral features of these mineral/glass and taking into account wavelength positions that can be used to estimate Fe, Ti and maturity as well as OH^-/H₂O, we finally selected 20 bands combination for CE-5 LMS (Table 1).

Optimization of multi-bands for CE-6 mission: The pyroxene composition of CE-6 landing area has changed significantly, ranging from Mg-rich LCP to Fe, Ca-rich HCP[2]. Spectral interpretation of pyroxene compositions of CE-6 landing area will greatly influence the analysis of the origin of the returned samples (mantle or lower crust). Therefore, for CE-6 mission, we increased the number of the selected bands around ~1 μm and 2 μm absorption to more accurately dipict the variation of absorption center of lunar soils resulting from the changing composition of pyroxene. Besides, to better characterize OH^-/H₂O absorption, we optimized the positions and increased the number of bands for 2200 nm-3200 nm regions (Table 1).

 Evaluation on displaying spectral features:

In general, the designed multi-bands can well display the hyperspectral features (measured by RELAB) of mineral/glass separated from lunar soils. LCP exhibits a shorter-wavelength absorption ~2000 nm, consistent with its hyperspectral data. HCP shows a flattening between ~2000 nm and ~2200 nm, indicating that its absorption center should be located between these two bands. The multispectral data allows for the effective display of olivine’s broader 1 μm absorption, and the hyperspectral features of the three types of lunar volcanic glasses are also well reproduced, particularly the longward 1 μm and shortward 2 μm wide absorptions. Compared to the multi-bands spectra of CE-5, the designed bands combination for CE-6 seems to be more efficient in reflecting the absorptions around ~2 μm and ~2.8 μm because of the optimization and increased number of bands selected for these two spectral regions.

References: [1] Li C. et al. (2022) NSR, 9, 2, nwab188. [2] Zeng X. et al. (2023) Nature Astronomy, 7, 1188-1197. [3] He Z. (2019) J. Appl. Remote Sens, 13(2). [4] Liu D. et al. (2002) Nature Communictions,13, 5965. [5] Staid M. I. et al. (2011) J. Geophys. Res. Planets, 116, E6.

Figure 1. Comparison between hyperspectral and multispectral data of minerals and glasses composing lunar soils. The multispectra were obtained by re-sampling the hyperspectra according to the spectral response function of CE-5 LMS multispectral detection mode.

Figure 2. Comparison between hyperspectral and multispectral data of minerals and glasses composing lunar soils. The multispectra were obtained by re-sampling the hyperspectra according to the spectral response function of CE-6 LMS multispectral detection mode.

Table 1  Designed LMS multi-bands

CE-5 (nm)	CE-6 (nm)	Main Application
485	485	Ilmenite, TiO2
560	560	Ilmenite, TiO2
640	640	Ilmenite, Black bead
750	750	1μm absorption shoulder, maturity, FeO and TiO2
850		olivine
900	900	LCP
	925	LCP
950	950	LCP/HCP, maturity, FeO and TiO2
	970	HCP
1000	1000	HCP
1050	1050	HCP, olivine
1100	1100	olivine, volcanic glass
1250	1250	olivine, plagioclase, volcanic glass
1450	1450	1μm absorption shoulder, olivine
1550	1550	1μm or 2μm absorption shoulder, plagioclase
1800	1800	LCP
	1900	LCP
2000	2000	LCP/HCP
	2100	HCP
2200		HCP
	2250	HCP
	2540	LCP/HCP shoulder,  OH-/H2O shoulder
2600		OH-/H2O shoulder
	2750	OH-/H2O shoulder
2800		OH-/H2O
	2850	OH-/H2O
	2950	OH-/H2O
3000	3000	OH-/H2O
	3100	OH-/H2O
3200		OH-/H2O