Copernican‐Aged (<200 Ma) Impact Ejecta at the Chang'e‐5 Landing Site: Statistical Evidence From Crater Morphology, Morphometry, and Degradation Models

Chang'e‐5 successfully returned ∼1,731 g of lunar samples on December 17, 2020. We systematically studied the morphology and morphometry of craters surrounding the Chang'e‐5 site based on high‐resolution images. A key ejecta source crater database was produced, incorporating their morphometrical parameters, excavation depth, target rock types, and ages from either crater counting, crater morphology or degradation methods. We found that (a) the regolith at the Chang'e‐5 site is ∼4–6 m thick, (b) ejecta are primarily from five proximal craters, including Xu Guangqi, dominantly with ages <200 Ma; (c) distal materials are mainly from Harpalus, Copernicus, Aristarchus, and Mairan G craters; (d) the drill samples are likely to be mainly from Impact Crater (IC)‐261 and IC‐265 particles at the top and the contribution of Xu Guangqi increases with depth; (e) the ejecta of Xu Guangqi would buried preexisting materials but probably overturned by postimpacts.

at a distance from the impacting site, and also calibrate chronology functions (van der Bogert & Hiesinger, 2020). Therefore, a thorough understanding of the provenance of the CE-5 samples could help the sample analysts locate the key scientific questions and maximize the outcome of CE-5 from a previously unsampled young area (J. . Different researchers have studied the possible distal ejecta sources for the CE-5 region. Xie et al. (2020) proposed that Aristarchus (∼8%), Copernicus (∼2%), Sharp B (∼1%), and Harding (∼0.4%) craters contributed the most distal ejecta to Em4/P58 according to the ballistic sedimentation model. Qian, Xiao, Wang, et al. (2021) directly tracked the ejecta rays surrounding the CE-5 site based on albedos and compositions, and estimated their percentages using a power law model. They concluded that the admixed impact ejecta are mainly from Harpalus (∼6%), Copernicus (∼2%), and Aristarchus (∼1%) craters. By also tracing the secondaries close to the landing site, Qiao et al. (2021) supported the interpretation that distal materials are from Copernicus and Harpalus craters for the SE-NW and NE-SW ejecta. In addition, Fu et al. (2021) studied the nonmare materials by unmixing 12 FeO-Th endmembers in the northern Oceanus Procellarum, concluding that Aristarchus crater contributed highly evolved materials, and thorium is indigenous to basalts. T.  proposed a spatially resolved numerical model to calculate the basin-derived melt abundance from cumulative lateral and vertical mixing. Using this model, Liu, Michael, Zhu, and Wünnemann et al. (2021) suggested that there is ∼40% nonmare materials at the CE-5 site, mainly from the Imbrium basin; and distal materials are from Sharp B, Harding, Copernicus, and Aristarchus craters.
In summary, most previous studies focus on the broad CE-5 region in northern Oceanus Procellarum, and a detailed investigation of the local craters surrounding the CE-5 site has not been reported. However, lunar regolith is mainly from comminution by local impacts and controlled by bedrock composition based on the Apollo/ Luna results (Hörz et al., 1991;McKay et al., 1991): >95% of the regolith is locally derived (<5 km), only <1% comes from sources >10 km distant. Because of the significance of studying impact melts/breccias by modern techniques such as Ar-Ar and U-Pb chronologies and geochemical tracing (Joy et al., 2010;Mercer et al., 2015), it is critical to constrain the local ejecta contributors in the vicinity of the CE-5 site.
Benefitting from the acquisition of high-resolution Lunar Reconnaissance Orbiter Camera (LROC) images (Robinson et al., 2010), it is now possible to analyze the impact craters surrounding the CE-5 site, and estimate the ejecta delivered by them. Here we show that the morphology and morphometry of small craters (<1 km) and crater size-frequency distribution (CSFD) measurements on the continuous ejecta of large craters (>1 km) could constrain their formation ages, which can be compared with the laboratory dating of impact melts/breccias. In addition, even smaller craters (<250 m) can be used to obtain lunar regolith thickness based on the concentric crater method. All results will help the CE-5 sample analysis and data interpretations.

Data
The morphology of all craters were studied based on LROC NAC data (1-1.5 m/pixel; Robinson et al., 2010) after registration using ISIS3 (Sides et al., 2017). To measure the morphometry of small craters (<1 km), a shape from shading-based method was used to produce NAC DEMs ; and three NAC DEMs were constructed (M1348581418L, M1173414625L/R). All NAC images used are listed in Table S1 in Supporting Information S2. In addition, SLDEM2015 (Barker et al., 2016) is used to measure the morphometry of large craters, Kaguya TC Morning Map (Haruyama et al., 2008) and LROC Wide Angle Camera (LRO WAC; Robinson et al., 2010) data are used as basemaps.

Regolith Thickness
We updated the regolith thickness maps of the CE-5 landing region (41°-45°N, 49°-69°W) produced by Yue et al. (2019, their Figure 3), benefiting from the accurate landing site coordinates determination since then. A new regolith thickness map (Figure 1c, 42°-44°N, 50°-53°W) was created based on the small fresh crater (<250 m) morphologies. This method relies on the transition of simple, flat-bottomed/central mound, and concentric craters ( Figure S1 in Supporting Information S1) with increasing regolith thickness (Oberbeck & Quaide, 1968). In practice, only concentric craters were used (Yue et al., 2019), and the diameter from rim-to-rim (DA) and of the inner ring (DF) were measured on the NAC images to obtain regolith thickness, using their Equation 1.

Ejecta Thickness and Key Ejecta Source Selection
To select potential craters whose ejecta contribution are nonnegligible, we first applied the power law model of ejecta thickness modified by Huang et al. (2018). The model assumes the relation between r (the distance to the crater center), R (crater radius), and T (ejecta thickness) is: T = 3.95 R 0.399 (r/R) −3 . The excavation efficiency (μ) of ejecta is μ = 2.25 × 10 −5 r 0.87 adopting half of excavation efficiency of Oberbeck (1975) according to Petro and Pieters (2006), and the total ejecta thickness equals the sum of distal ejecta and excavated local material.  (Qian, Xiao, Head, van der Bogert, et al., 2021). The basemap is a LROC WAC image (Robinson et al., 2010). (b) CE-5 landing region. Yellow dots represent key ejecta source craters. The basemap is a Kaguya TC Morning Map image (Haruyama et al., 2008). (c) Regolith thickness of the landing site (Section 3.1). Craters used to calculate regolith thickness are shown in Figure S2 in Supporting Information S1 and Table S5 in Supporting Information S2.
All key ejecta source craters were selected by the criteria described in Text S1 in Supporting Information S1; Tables S2 and S3 in Supporting Information S2, excluding Imbrian-aged ones. In this way, 736 craters were selected (labeled from IC-1 to 736, IC = Impact Crater), and 209 craters with >0.1 cm ejecta were further studied as a database (Table S4 in Supporting Information S2) to constrain their morphology types, target properties, excavation depth, and formation ages. Their topographic profiles were acquired from the NAC DEMs and classified into good/medium/bad qualities; medium/bad ones in most cases are near the resolution limit or disturbed by other craters. Then their morphometric parameters (i.e., diameter, diameter calibrated, depth, depth/diameter, and maximum slope) were measured based on profiles.

Target Rock and Excavation Depth
To constrain the properties of the materials transported to the site, the target type of each critical crater was determined and classified into "mare" or "highland," and further into geological units according to the boundaries of Hiesinger et al. (2011) andQian, Xiao, Head, van der Bogert, et al. (2021). The excavation depth of each crater was calculated by the depth of transient crater (Melosh, 1989). For those craters within Em4/P58, it was determined whether or not each crater excavated the underlying low-Ti mare basalts based on Kaguya MI TiO 2 abundance data (Lemelin et al., 2016).

Impact Crater Formation Age
The formation age of small craters (<1 km) was computed by crater morphology and degradation method separately. For the first method, we classified each crater into Basilevsky (1976) classes (A/AB/B/BC/C) on the basis of the morphometry measurements and NAC-based morphologies. Using the relationship of each class crater and their residence time on the lunar surface ( Figure 4 in Basilevsky, 1976), the lower/upper limit age of each crater was determined (Table S4 in Supporting Information S2). The crater degradation model is developed quantitatively from the acquisition of lunar high-resolution topography (Fassett & Thomson, 2014;Xie et al., 2017). Here we used nine different crater degradation models (Models 1/2/2'/3/3'/4/4'/5/5') to estimate crater ages if having good profile quality (51 in total), based on the crater degradation models by Fassett and Thomson (2014) and Xie et al. (2017), and a locally calibrated crater diffusion rate for Models 4/4'/5/5'. The local diffusion rate was derived from comparing the CSFD of 10-100 m-diameter craters measured by an automatic crater detector (Wilhelm & Wöhler, 2021) and a Monte Carlo cratering simulation  after degradation. Details of, and comparisons between the different degradation methods are given in Text S2 and S3 in Supporting Information S1, and an example is given in Figure S4 in Supporting Information S1. We find Model 1 (a quantitative model based on Xie et al., 2017), and Model 4 (a simplified model based on the depth/diameter ratio with a locally calibrated diffusion rate) can give the best crater age estimations (Text S3 in Supporting Information S1). Therefore, the following discussions of crater ages are mainly based on the results of Models 1 and 4.

Regolith Thickness
The regolith thickness (Figure 1c) within 42°-44°N, 50°-53°W was obtained through the concentric crater method (Section 2.2). In total, the DA and DF of 1,084 craters were measured ( Figure S2a in Supporting Information S1 and Table S5 in Supporting Information S2). This region has a mean regolith thickness of ∼6.1 m; ∼31% and ∼88% of the area has a thickness between 5-6 and 4-8 m ( Figure S2b in Supporting Information S1), respectively.  Koenig et al. (1977). e "Penetrating" means this crater is within Em4/P58 and can penetrate through Em4/P58 to excavate the underlaying Imbrian-aged mare basalts. f Based on the crater morphology method of Basilevsky (1976) (Section 2.5). g Based on the crater degradation Model 1 and Model 4, respectively (Section 2.5; Text S2 and S3 in Supporting Information S1). h CSFD ages may influenced by involving craters <50 m.

Table 1 Twenty-Nine Most Critical Ejecta Source Craters
The vicinity of CE-5 site has a thickness between 4 and 6 m. The mean regolith thickness is slightly smaller than Yue et al.'s (2019) results (∼7.2 ± 2.6 m). Compared with Yue et al. (2019), the results here have a much higher resolution that may help penetration radar interpretations (Shen et al., 2021).

Impact Crater Age
The small crater (<1 km) ages were estimated by the crater morphology or degradation method (Section 2.5).
Besides, the automatic crater detection coupled with the crater degradation Model 4 (Text S3 in Supporting Information S1) show that crater numbers increase toward young ages and reach the highest number in the 0-10 Ma bin (Figure 3d). Although the automatic method may suffer from misidentifications, the trend is clear and is also seen in the manual-determined database (Figures 3a and 3c). These craters cannot provide considerable ejecta and are therefore excluded from the database.
The age of Harpalus is more controversial, but even critical because the prominent NE-SW rays overlying Em4/ P58 are either from Sharp B or Harpalus or both. Qian, Xiao, Wang, et al. (2021) proposed that Sharp B is not the source but Harpalus instead, because directly dating Sharp B and Harpalus on their ejecta would yield unreliable ages due to self-secondaries and the rough surface. Alternatively, using high-resolution NAC images, we found two impact melt ponds within the west and east wall of Harpalus (Figures S12a-S12c in Supporting Information S1). CSFD measurements on these two flat ponds formed concurrently with the crater should provide more reliable ages than the rough ejecta deposit. On this basis, the age of Harpalus was concluded to be ∼490 Ma (Figures S12d and S12e in Supporting Information S1), younger than the CE-5 basalts and probably transporting distal materials to the site.

Ejecta Composition
The potential ejecta compositions transported to the CE-5 site are constrained by the excavated material amounts and the target rock properties. According to the investigations of key craters (  (Qian, Xiao, Head, van der Bogert, et al., 2021) and excavating the underlying Imbrian-aged low-Ti basalts. This type of crater includes Mairan G, Pei Xiu, Rümker H, IC-10, and IC-12 ( Figure S20 in Supporting Information S1), able to excavate 495, 186, 339, 135, and 180 m deep materials, respectively. However, the total amount of ejecta derived by the local-penetrating craters is minor (∼0.1% of all ejecta), compared with distal large craters, agreeing with the results from spatially resolved numerical model (Liu, Michael, Zhu, & Wünnemann, et al., 2021).
All the other craters (190 in number, Table S4 in Supporting Information S2) within Em4/P58, but not penetrating through the overlaying unit, still impact and transport the same type of local material as seen at the CE-5 site. If we only consider distal/different materials (ejecta thickness = distal crater ejecta + excavated local material, Section 2.3), the total distal/different ejecta thickness is ∼8 cm, and if totally mixed with the top ∼74 cm regolith by gardening (Costello et al., 2020;Qian, Xiao, Wang, et al., 2021), which leads us to conclude that the distal/different materials are likely to be mainly from Harpalus (∼6%), Copernicus (∼2%), Aristarchus (∼1%), and Mairan G (∼1%) craters.

Ejecta Age
We have compared crater ages determined by different methods (Text S3 in Supporting Information S1) and find that they generally follow the same trend: the older the degradation age, the older the morphology age, and vice versa. The relation between the results of Model 1 and other models are shown in Figure S8 in Supporting Infor- mation S1. All models have a good correlation with each other, in terms of r values (≥0.8). There may be some systematic difference from model assumptions that can be tested by samples.

Stratigraphy of the CE-5 Site and Ages of IC-259, 261, 265, 266, and Xu Guangqi
A stratigraphic column was constructed of the CE-5 site (Figure 3e) in order to better understand the sample provenance and subsurface structures. The CE-5 site is located on a basaltic regolith developing from solidified lava flow protolith . The impact ejecta at the CE-5 site are expected to be mainly from Xu Guangqi, IC-265, IC-266, IC-259, and IC-261, contributing ∼75% of all the ejecta (Table 1).  and  are two class C craters with subdued morphology and no exposed boulders (Figure 2c), indicating relatively older ages than the other three. Therefore, their ejecta are expected more at the bottom of the column (Figure 3e), overlying the Eratosthenian-aged intermediate-Ti bedrock (Qian, Xiao, Head, van der Bogert, et al., 2021).
Above the IC-265 and IC-266 ejecta are the Xu Guangqi (IC-396) ejecta. In terms of expected ejecta volume, Xu Guangqi is the most significant crater among all studied craters because it is the largest (419 m-diameter) crater adjacent to the site (424 m-distance). Qiao et al. (2021) studied this crater and proposed that it is older than Copernicus crater because Copernicus secondaries appear to be superposed on Xu Guangqi ejecta blanket. However, the small secondary craters (a few tens of meters) described by Qiao et al. (2021d) are unlikely to be from Copernicus (∼796 Ma; Iqbal et al., 2020), because craters in this size are unlikely to be preserved for >200 Ma on the lunar surface (Basilevsky, 1976;Fassett & Thomson, 2014). In addition, abundant boulders occur on the rim, wall, and floor of Xu Guangqi, especially on the southeast wall ( Figure 2b and Figure S19e in Supporting Information S1). The survival time of meter-sized boulders on the lunar surface is about 150-300 Ma (Basilevsky et al., 2013), suggesting that Xu Guangqi is younger than 150-300 Ma. Furthermore, according to crater degradation Model 1 (Section 2.4), the age of Xu Guangqi is interpreted to be ∼60 Ma, at the lower end of the crater morphology age . Therefore, Xu Guangqi may have formed ∼60-75 Ma ago; its ejecta overlying the IC-265 and IC-266 ejecta (Figure 3e). The age of Xu Guangqi derived from Model 1 and Model 4 is further compared in Figure S15 in Supporting Information S1. It is also possible that Xu Guangqi has an age of ∼200 Ma based on Model 4, but that would not change the main conclusions.
Xu Guangqi penetrated through a depth of ∼35 m, not deep enough to excavate the underlying low-Ti basalts, but certainly deliver deep basalts, possibly affecting their isotopic systems and generating impact melts. All these materials, if sampled, can be used to date the age of Xu Guangqi. The retrieved samples can test and improve crater degradation models on a flat mare surface without complex geology such as Apollo sites (Cone, North and South Ray craters, Figure S16 in Supporting Information S1). In addition, Robinson (2021) used a newly obtained LROC NAC DTM to study samples that might have been collect by CE-5, concluding that the crater we designate IC-265 formed on the ejecta deposit of the larger crater Xu Guangqi, and that both of these craters sampled the Em4/P58 unit. These findings are consistent with our results.
At the top of the geological column (Figure 3e) are the ejecta from IC-259 and IC-261. They are small (11 and 9 m diameter) craters next to the CE-5 site (16 and 14 m distance). IC-259 (6-15 Ma) and IC-261 (5-12 Ma) are among the last-formed craters whose ejecta are abundant at the CE-5 site (∼3% and ∼2% of all ejecta).
The regolith thickness at the site is around 4-6 m (Section 3.1), while the top ∼74 cm are mixed up at least one time (Qian, Xiao, Wang, et al., 2021). Thus, the surface samples collected are primarily from the ejecta of IC-259, IC-261, and minor contributions from other nearby craters younger than Xu Guangqi. With increasing depth, the abundance of particles from IC-396 increases; after ∼74 cm, the regolith may be largely from Xu Guangqi. Particles from craters older than Xu Guangqi should be buried by its ejecta, including those from IC-265 and IC-266. However, the excavation of local craters postdating IC-396, may penetrate through the Xu Guangqi ejecta blanket and transport the buried particles to the site. Similarly, ejecta from four key distal craters (Harpalus, Copernicus, Aristarchus, Mairan G, Section 4.1) are also buried by the Xu Guangqi ejecta; subsequently, small craters may dig out a small portion of them. If distal materials are discovered, they are most likely from these four craters.
CE-5 is the first mission to return mare samples younger than 2.8 Ga (Tartèse et al., 2019), which is dated to be ∼2.0 Ga by isotopic measurements (Che et al., 2021). However, CE-5 in northern Oceanus Procellarum only represents a single point for the late-stage volcanism. There are still a multitude of questions remaining concerning the nature of late volcanism on the Moon, such as: (a) What are the heat sources responsible for the generation of the young volcanism, (b) Are the mantle sources the same for the young and the old mare basalts, (c) Do Oceanus Procellarum and Mare Imbrium have the same magma source, (d) What is the age of the youngest. More missions are required in order to improve our understanding of late lunar volcanism, especially from different sites around the Moon (Cohen et al., 2021;Draper et al., 2021).

Data Availability Statement
The LROC NAC and Kaguya TC Morning Map image IDs and download links are listed in Table S1 in Supporting Information S2.