The mineral composition and space weathering of the lunar soil at the Chang’e-5 landing site were revealed

China’s first lunar sample return mission, Chang’e 5 (CE-5), landed on the young KREEP body in the northern part of the lunar storm ocean and successfully returned to 1.73 kg of lunar soil. Predecessors have used orbital remote sensing data to conduct a large number of studies on the topography and material composition of the CE-5 landing zone, and a series of recent sample analyses have advanced the understanding of lunar chronology, late lunar volcanic activity and magmatic evolution. Different from large-scale remote sensing observation and fine sample analysis, in situ spectral detection can not only provide local background information of the sampling area, but also facilitate the study of the characteristics of the unturbed and perturbed state of the lunar soil. The Lunar Mineral Spectrometer (LMS) carried by the CE-5 lander obtains the visible-infrared reflective spectra of the lunar soil, providing important data support for the study of the composition of lunar soil material and space weathering.

Researchers at the Key Laboratory of Solar Activity and Space Weather of the National Space Science Center of the Chinese Academy of Sciences (hereinafter referred to as the “Space Center”) analyzed the material composition and maturity of the lunar soil at the landing site using the chang’e-5 LMS in situ detection data, and the spectral parameters and demixification studies showed that the iron-magnesium minerals in the lunar soil were mainly monoclinic pyroxene, which was consistent with the chemical analysis and orbital spectral analysis of the CE-5 sample in the laboratory. There were no significant changes in the optical maturity and sub-microscopic iron (SMFe) of the lunar soil before and after rocket purging and shoveling sampling, and this study provides spectral evidence for the rapid ploughing and sufficient mixing of the lunar soil surface.

Figure 1 (A) The measured field of view of the LMS spectral data is shown in the red box. The panoramic camera shows images of (B) lunar soil and (C) stones, respectively. (D) LMS reflection spectral curve.

The red box of Figure 1A shows the observational field of view of the LMS spectroscopic data, where D11 is a stone block and D14-D16 is the spectrum of the shoveled lunar soil. The enlarged images of lunar soil and stones are shown in Figures 1B and C, which show the reflection spectral curves after pre-treatment such as radiation calibration and thermal correction.

By the spectral parameter plot of the absorption characteristics of 1 μm and 2 μm (Figure 2A), it was found that as the calcium content increased, the pyroxene moved in the direction of long waves at the absorption positions of 1 μm and 2 μm, and the lunar soil and stones at the CE-5 landing site were high calcium pyroxene, which was further confirmed by the 1 μm absorption center and the 2 μm vs. 1 μm absorption area ratio projection (Figure 2B). In this study, the mineral composition and abundance of the in situ spectrum were obtained by inversion of the Hapke model and sparse dismixing algorithm, and the inversion results were consistent with the analysis results of the CE-5 sample and the inversion results of orbital remote sensing (Figure 2C).

Figure 2 LMS in situ spectral component analysis

Space weathering is a common phenomenon of spectral reddening and darkening on atmosphereless objects caused by micrometeorite impacts and solar wind injections. Firstly, the maturity of the LUNAR soil of the CE-5 landing site was qualitatively analyzed by using the spectral slopes R950/R750 and R1600/R700 and R700, respectively (Figures 3A, B), and it was found that the lunar soils of the CE-3 and CE-5 landing zones were relatively less mature than the CE-4 landing site lunar soils, which may be related to the landing of CE-3 and CE-5 on the young impact crater sputtering blanket (<100 Ma). In this study, the optical maturity (OMAT, Figure 3C) and submicroco iron (FIG. 3D) content were further calculated, and no significant changes in the maturation of the lunar soil before and after the rocket purging and before and after the shoveling were found, indicating that the lunar soil in the CE-5 landing area experienced an equivalent exposure history within the shoveling depth (<3 cm). This result coincides with the results of radionuclide analysis of Apollo’s drilled samples and the simulation results of the lunar soil ploughing model. This study provides spectral evidence that the lunar surface has undergone rapid tillage and sufficient mixing.

Figure 3 LMS in situ spectral maturity analysis

The above research results were published in the international authoritative academic journal Earth and Planetary Science Letters, the first author of the paper is Dr. Wu Xing, special research assistant of the Space Center, and the corresponding author is Researcher Liu Yang. The research was supported by the National Natural Science Foundation of China (11941001, 42072337), the Strategic Pilot B Project of the Chinese Academy of Sciences (XDB41000000), the Civil Aerospace Technology Pre-research Project (D020201, D020203), the Key Deployment Project of the Chinese Academy of Sciences (ZDBS-SSW-JSC007) and the “Climbing Plan” of the Space Center.

Thesis information: Xing Wu, Yang Liu*, Yazhou Yang, Dijun Guo, Jun Du, Shuai Li, Honglei Lin, Xiaohui Fu, Zhiyong Xiao, Yuchen Xu, Rui Xu, Zhiping He, Feng Zhang, Yangting Lin, Yongliao Zou. (2022). Mineralogy and regolith maturity at the ChangE-5 landing site inferred from the Lunar Mineralogical Spectrometer. Earth and Planetary Science Letters, 594, 117747.

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