The Moon has a global dichotomy, with its near and far sides having different geomorphology, topography, chemical composition, crustal thickness, and evidence of volcanism.
To better understand this dichotomy, Professor XU Yigang's team from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences investigated lunar soil samples from the far side South Pole-Aitken (SPA) Basin of the Moon returned by the Chang'e-6 mission.
Their work was published in Science on Nov. 15.
"The samples returned by Chang'e-6 provide a best opportunity to investigate the lunar global dichotomy," said Professor XU.
Volcanic eruptions flooded parts of the surface with lava, producing rocks known as mare basalts, which are more common on the near side, where they cover ~30% of the surface compared to 2% of the far side. It is obvious that to investigate the lunar global dichotomy, samples from both the near and far sides are needed.
Figure 1 Landing site of the Chang'e-6 mission on the Moon's far side (Image by GIGCAS)
The Chang'e-6 lunar soils contain two types of mare basalts: low-Ti and very low-Ti (VLT). The predominant low-Ti basalt represents the local basalt unit around the landing site, whereas the VLT basalt possibly came from the unit to the east of the landing site (Figure 1B).
The high-precision Pb-Pb dating of Zr-bearing minerals and Rb-Sr dating of plagioclase and late-stage mesostasis of the low-Ti basalt yield consistent isochron ages of 2.83 Ga (Figure 2), indicating that "young magmatism also exits on the lunar far side," according to the study.
Figure 2 The two types of basalts in Chang'e-6 soils and isochrons of the Chang'e-6 low-Ti basalt. (Image by GIGCAS)
Compared to the near side samples returned by the Apollo and Chang'e-5 missions, the Chang'e-6 low-Ti basalt has a low μ value and 87Sr/86Sr and a very high εNd value (Figure 3), suggesting a very depleted mantle source.
Figure 3 Initial Pb and Sr-Nd isotopes of lunar basalts. (Image by GIGCAS)
Crustal thickness has been suggested as a key factor in accounting for asymmetry in the abundance of volcanism between the lunar near side and far side. However, this model has been questioned since the SPA basin on the far side, which has an anomalously thin crust, appears deep and significantly underfilled by volcanism.
Based on the investigation of Chang'e-6 low-Ti basalt, XU's team suggested that the composition of the mantle source is another important factor controlling the generation of lunar volcanic activity.
"Although the SPA basin has a thin crust, the depleted and refractory mantle source beneath the SPA basin hinders partial melting to a large degree," said XU.
This work also provides an additional calibration point at 2.83 Ga for the lunar crater chronology and implies a constant impact flux after 2.83 Ga. This newly calibrated chronology model improves the age estimation tool based on crater statistics for both the Moon and other terrestrial bodies, and also has additional implications for the evolution of lunar impactors, potentially related with early planet migration in the early Solar System.
The work was financially supported by the Chinese Academy of Sciences (grant ZDBS-SSW-JSC007-11) and the lunar research program of GIGCAS (grant 2022SZJJZD-03).
Figure 4 Incorporating the Chang'e-6 landing site into a lunar crater chronology model. (Image by GIGCAS)
