Nature of the lunar far-side samples returned by the Chang’E-6 mission

Introduction: The returned lunar samples have provided critical information on the formation and evolutionary history of the Moon, contributing to the development of hypotheses such as the Moon’s giant impact into early Earth origin, the Lunar Magma Ocean and the Late Heavy Bombardment [1]. However, all these samples from the 10 Apollo, Luna and CE-5 missions were collected from the lunar nearside [2,3], leaving the far side unexplored from a sample perspective. A significant dichotomy exists between the nearside and far side of the Moon [4,5]. Nearside samples alone, without adequate sampling from the entire lunar surface, especially from the far side, cannot fully capture the geologic diversity of the entire Moon [6]. On 25 June 2024, the Chang’E-6 (CE-6) mission collected 1935.3 g of lunar samples from the South Pole-Aitken (SPA) basin (41.625°S, 153.978°W) representing the first time in history that lunar samples have been retrieved from the Moon’s far side. We focus on the preliminary investigation into the basic physical properties, rock types, petrography, mineralogy and geochemistry of the scooped samples obtained by the CE-6 mission, so as to provide foundational data for future in-depth scientific research to be carried out on these newly returned far-side samples.

Samples and methods: The lunar soils analysed in this study were from the scooped samples of CE-6 mission allocated by the China National Space Administration (CNSA). The bulk density of the soil samples was measured in its natural state and the true density was determined by Quantachrome ULTRAPYC 1200e analyser. The quantification of the mineral phases was conducted using the X-ray diffraction analyser (XRD). The mineral compositions was analysed by electron probe microanalyser (EPMA). The bulk chemical composition of CE-6 lunar soil and basaltic fragments were determined by X-ray fluorescence spectrometry (XRF) [7] and inductively coupled plasma optical emission spectroscopy (ICP–OES), respectively. The trace elements abundance of lunar soils were further determined by laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q–ICP–MS).

Results and discussion: The CE-6 soil has a significantly lower bulk density (0.983 g/cm³) and true density (3.035 g/cm³) than the Chang’E-5 (CE-5) samples. This suggests the apparent influence of light components, such as feldspar and glass, and may also indicate a higher porosity for CE-6 lunar soil.

In the particle-size mass distribution, 95% of particles in the CE-6 soil ranges between 5.12 (Φ7.61) and 336.81μm (Φ1.57), with a mean value (= (Φ16+Φ50+Φ84)/3) of 38.98 μm (Φ4.68), a mode value of 27.97 μm (Φ5.16) and a median value (Φ50) of 35.03μm(Φ4.83) (Figure 1). The grain size of CE-6 soils exhibits a bimodal distribution, indicating a mixture of different compositions.

Petrologically, the lithic fragments from scooped soils comprise basalt (∼30%–40%), breccia (∼30%–40%), agglutinate (∼20%–30%), and minor leucocratic clasts (∼10%) (Figure 2). Basalt dominates as the primary lithology, displaying poikilitic and subophitic textures, with subordinate porphyritic and vitrophyric varieties. Breccias include both regolith breccias and impact melt breccias, while leucocratic fragments are dominated by anorthosite and norite.

Mineralogically, the CE-6 soil consists of 32.6% plagioclase (anorthite and bytownite), 19.7% augite, 10% pigeonite and 3.6% orthopyroxene, and with low content of olivine (0.5%) but high content of amorphous glass (29.4%) (Figure 3).

Geochemically, the bulk composition of CE-6 soil is rich in Al₂O₃ (14%) and CaO (12%) but low in FeO (17%), and trace elements of CE-6 soil such as K (∼630 ppm), U (0.26 ppm), Th (0.92 ppm) and rare-earth elements are significantly lower than those of the lunar soils within the Procellarum KREEP Terrane (Figure 4). The local basalts are characterized by low-Ti (TiO₂, 5.08%), low-Al (Al₂O₃ 9.85%) and low-K (∼830 ppm), features suggesting that the CE-6 soil is a mixture of local basalts and non-basaltic ejecta (Figure 5).

The returned CE-6 sample contains diverse lithic fragments, including local mare basalt, breccia, agglutinate, glasses and leucocrate. These local mare basalts document the volcanic history of the lunar far side, while the non-basaltic fragments may offer critical insights into the lunar highland crust, SPA impact melts and potentially the deep lunar mantle, making these samples highly significant for scientific research.

References: [1] Neal et al., 2023, The mineralogical Society of American. [2] McCubbin et al., 20019, Space Science Reviews, 215, 48. [3] Li et al., 2022, National Science Review, 9, nwab188. [4] NRC, 2007, National Academies Press. [5] Jolliff et al., 2000, Journal of Geophysical Research, 105, 4197-216. [6] Wadderburg G.J., 1972, Journal of Aeronautics Astronautics And Aviation, 10, 16-21. [7] Xue et al., 2020, Journal of Analytical Atomic Spectrometry, 35, 2826-2833. [8] Lucy et al., 2006, Reviews in Mineralogy and Geochemistry, 60, 83-219.

Figure 1: The modal mass-grain size distribution of CE-6 lunar soils. The red-colored area represents CE-6 lunar soils, while the orange-dotted reference line derived from CE-5 lunar soil [2].

 

Figure 2: Typical stereomicrographs of lithic fragments collected from the CE-6 scooped sample. a, typical basalt; b, regolith breccia; c, agglutinate, d, leucocratic; e and f, glass fragments.

Figure 3: Triangular plot of major mineral abundances. CE-6 lunar soils are significantly enriched in pyroxene and low in olivine compared with CE-5.

Figure 4: Elemental variations of CaO, FeO and Th. Database and triangles from Ref. [8]

Figure 5: TiO₂, Al₂O₃ and K classification scheme for mare basalts. The basaltic fragment from CE-6 belongs to the low-Ti/low-Al/low-K species.