Maria Basalts Chronology of the Chang’E-5 Sampling Site

Abstract

Chang’E-5 is the first lunar sample return mission of China. The spacecraft was landed in the northwest of the Procellarum KREEP Terrane (43.0576°N, 308.0839°E) on 1 December 2020 and returned 1731 g samples from a previously unvisited region. The landing area has been proposed as one of the youngest mare basalt units of the Moon and holds important information of lunar thermal evolution and chronology. However, the absolute model ages estimated in previous studies are quite different, ranging from 2.07 Ga to 1.21 Ga. Such significant difference may be caused by (1) different crater counting areas, (2) different crater diameter ranges, (3) effects of secondary craters, and (4) biases in crater identification. Moreover, the accurate landing site was unknown and the ages were estimated over the Eratosthenian-aged mare unit (Em4) instead. In light of the above unsatisfactory conditions, this study seeks to establish a standard crater size-frequency distribution of the CE-5 landing site. We derived the concentrations of FeO and TiO2 to map out the pure basaltic areas where external ejecta deposits are negligible and thus secondary craters are rare. Based on the geochemistry of basaltic ejecta excavated by fresh craters in the mare unit, the FeO concentration threshold for mapping pure basaltic areas is 17.2 wt.%. The morphologically flat subunits in the pure basaltic areas were selected for crater identification and age dating to exclude the contamination of external ejecta to the best as we could. In the Chang’E-5 sampling site subunit, we detected 313 craters with a diameter greater than 100 m and derived the absolute model age as 1.49−0.084+0.084 Ga. The craters identified in all pure basaltic subunits of Em4 gave the model age of 1.41−0.028+0.027 Ga. As least affected by secondary craters, the crater size-frequency distribution of the sample-collected pure basaltic subunit can provide important constraints for lunar cratering chronology function in combination with isotopic age of returned samples.

Cratering Records in the Chang'e‐5 Mare Unit: Filling the “Age Gap” of the Lunar Crater Chronology and Preparation for Its Recalibration

“…Xu et al. (2021) obtained an almost identical N (1) value with Wu et al. (2018), and these two studies both measured craters on LROC NAC images for smaller count areas.…”

“…Among these derived N(1) values, Qian et al (2018) reported the smallest values, close to Hiesinger et al (2003) from old Lunar Orbiter image data, while and Qian, Xiao, Wang, et al (2021) updated their results later to around 1.3 × 10 −3 km −2 . Xu et al (2021) obtained an almost identical N(1) value with Wu et al (2018), and these two studies both measured craters on LROC NAC images for smaller count areas. Morota et al (2011) got N(1) values around 1.6 × 10 −3 km −2 for the P58 unit.…”

The Geological History of the Chang’e-5 Sample Return Region

“…In this paper, pure basalt and mixture are distinguished according to this threshold. It can be seen from Figure 8 that the FeO concentration at the landing site of CE-5 is high, greater than 17.2 wt.%, but there are non-pure basalt materials less than 17.2 wt.% in the surrounding area, which may be ejecta and rays from sharp B, Harding, Copernicus, and Aristarchus [46].…”

“…Korovet et al [45] analyzed the samples returned from Apollo and Luna and found that the FeO abundance of mare basalt ranged from 16 to 23 wt.%, while that of highland plagioclase ranged from 3 to 6.5 wt.%. Xu et al [46] et al determined the FeO abundance threshold of CE-5 landing unit to distinguish pure mare basalt and non-pure mare basalt, that is, 17.2 wt.%. In this paper, pure basalt and mixture are distinguished according to this threshold.…”

Landing control method of a lightweight four-legged landing and walking robot

“…Luna-2, the first hard lander to reach lunar surface, can only perform a transient probe before colliding with the moon. Probers with landing capability are produced to realize surface exploration on the Moon; they usually adopt three or four foldable legs with invert-tripod mechanism, such as Surveyor-1 [2] and Luna-16 [3], or with cantilever mechanism, such as Apollo project [4] and Chang'e 3, 4, and 5 [5][6][7]. For probers with invert-tripod or cantilever mechanism, their leg often consists of a main branch chain and two multifunctional auxiliary branch chains.…”