Geological characteristics of Chang’E-6 landing area in micro-scale unveiled by new observation data

Domain: earth_science
Published: 2025-05-06
Editorial outcome: major_revision

Abstract
Chang’E-6 mission successfully achieved humanity’s first sample return from the farside of the Moon. Here, we report a high-precision terrain dataset with millimeter-to-decimeter spatial resolution around the CE-6 landing site. Based on this dataset, the accurate location of the landing site is determined to be (153.9776°W, 41.6251°S, -5273 m), and a micro-scale geological analysis of the landing area is conducted. Comparative analysis suggests that the surface exposure time of Chang’E-6 landing area is likely between that of Chang’E-4 and Chang’E-5 landing areas. Studies of the ejecta thickness from possible craters in mare and non-mare areas indicate that Chang’E-6 samples may contain materials from diverse provenances. Detailed topographic analysis reveals that the Chang’E-6 landing area is primarily covered by local mare materials, which were excavated from an nearby unnamed crater with a diameter of ~51 m and account for 30%–35% in volume.

Introduction
At 06:24 on June 2, 2024 (Beijing time), the Chang’E-6 (CE-6) lander and ascender composite, part of humanity’s first mission to return samples from the farside of the Moon, successfully landed in the northeastern part of the South Pole-Aitken Basin (SPA), within the mare basalt region between the southern rim crest and peak rim of the Apollo Basin. It successfully returned a total of 1935.3 grams of lunar samples. Laboratory studies of the CE-6 lunar samples can accurately determine the samples’ mineral composition, formation age, etc. Lunar researchers are eager for the laboratory results of CE-6 samples to provide crucial evidence for important scientific issues, such as magma activity on the farside of the Moon and the dichotomy between the near and farsides1. The latest laboratory research results of CE-6 samples indicate that the soil is a mixture of local basalts and non-basaltic ejecta. It contains diverse lithic fragments, including local mare basalt, breccia, agglutinate, glasses and leucocrate2. The majority of CE-6 basalt fragments have a consistent formation age of ~2.8 Ga3,4.

To better constrain how the CE-6 samples’ material composition and mineral content relate to their geological origin and subsequent evolutionary history, it is essential to gain a thorough understanding of critical informations such as the detailed surface landforms and geological characteristics of the landing area. These include surface roughness, depth-to-diameter ratio of fresh craters, regolith thickness, rock abundance, as well as material provenances of CE-6 samples. Previous studies of the geological context of the CE-6 landing area are primarily based on remote sensing data at scales of kilometers to hundreds of kilometers. It is believed that the CE-6 landing area is located within a young lunar mare basalt geological unit with an age estimated to be 2.4 Ga–3.1 Ga5–9, possibly mixed with non-mare ejecta materials from foreign provenances, such as Apollo basin wall and rim, Chaffee S, and White craters9–12, as well as Vavilov crater7,9,13. However, due to the uncertainty of the landing site location prior to the mission and the lack of high-precision detection data, very little is known about the geological context information in the vicinity of the CE-6 landing site at scales of kilometers, sub-kilometers, or even smaller.

The CE-6 mission has obtained high-precision in situ exploration data such as imagery, spectral data, and radar data near the landing site. Research leveraging these data can furnish more accurate information on the geo-positioning of the lander and geomorphology of the landing site. It aids in accurately determining the geographical location of the CE-6 landing site, as well as establishing a correlation between the laboratory analysis outcomes of the lunar samples and the combined remote sensing and in situ detection data of the landing area. Moreover, it enables a micro-scale understanding of the geological context of the CE-6 landing area, with a spatial resolution spanning from millimeters to sub-kilometers. Among the scientific payloads onboard the CE-6 probe, the landing camera (LCAM) and panoramic camera (PCAM) (detailed in the Methods section) captured images with varying coverage, viewing angles, and spatial resolutions during the landing and surface exploration phase, providing essential data for reconstructing a detailed terrain dataset of the landing area.

In this work, the Chang’E-2 (CE-2) global lunar topography data (CE2TMap201514,15) with a spatial resolution of 7 m is selected as the reference base map, and a high-precision terrain dataset of the CE-6 landing area with a unified spatial reference frame and projection coordinate system is produced. On this basis, the exact locations of the CE-6 landing site, the scoop sampling areas, and the drilling sampling point are accurately determined. Micro-scale topographic analysis near the landing site is conducted, to identify and extract craters and rocks, and to analyze geological characteristics such as surface roughness, depth-to-diameter ratio of fresh craters, regolith thickness, and rock abundance. Additionally, a comparative analysis is conducted with the landing areas of Chang’E-4 (CE-4) and Chang’E-5 (CE-5), and the exposure time of the CE-6 landing area is discussed, together with the potential provenances of ejecta on the landing area.

Results
High-precision positioning of the CE-6 landing site
Accurate knowledge of the CE-6 landing site coordinates is essential for placing lunar surface in situ scientific observations and returned samples into their proper geological and geophysical context16,17. In this work, a total of 142 LCAM sequence images and 120 pairs of PCAM stereo images are selected to reconstruct the CE-6 landing area terrain data based on photogrammetric principles with spatial resolution ranging from millimeter to decimeter, including digital orthophoto maps (DOMs) and digital elevation models (DEMs) denoted as CE6_LCAM_DOM, CE6_PCAM_DOM, CE6_LCAM_DEM, and CE6_PCAM_DEM, respectively (Fig. 1, see the Methods section for details).

Based on this terrain dataset, the accurate location of the CE-6 landing site on the reference base map was determined as (153.9776°W, 41.6251°S, −5273 m, see the Methods section for details); the relative positions of the four footplates of the CE-6 lander, scoop sampling areas, and drilling sampling point were marked in relation to the landing site, and their accurate coordinates within the CE-6 landing area high-precision terrain dataset were calculated (Fig. 1), providing foundational background data for the analysis of lunar samples and in situ detection data.

We marked the location of the CE-6 landing site in terrain data and images with different spatial resolutions. The positional relationships between these datasets can be seen in Supplementary Material 1. It can be seen from the LCAM DOM (Fig. 2a) and the PCAM three-dimensional (3D) rendering terrain (Fig. 2b) that the distribution and relative distances of topographic features around the landing site, such as craters (marked as A~C) and rocks (marked as D~E), are consistent across different terrain datasets. In addition, the PCAM image mosaic and raw images taken after landing (Fig. 2c, d) reveal an elevated terrain feature ~24° northwest of the landing site. By comparing with the CE2TMap2015 DOM, the southern rim of the Chaffee crater can be observed in the same direction, over 62 km away from the landing site (Fig. 2e).

Furthermore, NASA’s lunar reconnaissance orbiter (LRO) captured an image of the CE-6 lander using the narrow-angle camera (NAC) while flying over the CE-6 landing area on June 7, 2024 (image ID LROC NAC M1472410644L18, Fig. 2f). The LROC team calculated the CE-6 landing site location as (153.9852°W, 41.6385°S, −5256 m), with a positional accuracy of ±30 m18, which is within the same order of magnitude as other CE-6 landing site positioning results derived from LRO images19. The deviation between this position and our results is 172.2 m in longitude, 406.3 m in latitude, and 17.0 m in elevation. Therefore, the horizontal positional deviation between the two locations is 441.3 m, and the 3D positional deviation is 441.7 m. However, the positional deviation of our results is only about 1 pixel from those derived from CE2TMap201520. The 441.7 m difference between our coordinates and those of the LRO team reflects the positional discrepancy between CE2TMap2015 and LRO terrain data, which is based on the LOLA-plus-GRAIL coordinate framework, on the farside of the Moon21.

By reason of the foregoing, it can be concluded that the location of the CE-6 landing site derived from the high-precision terrain dataset is accurate and reliable.

Micro-scale geological analysis of the landing area
High-precision terrain data and accurate landing site localization can support the micro-scale geological analysis of the CE-6 landing area, and provide evidences for confirming the material characteristics and provenances of the samples. Based on the high-resolution terrain dataset of the CE-6 landing area and the determined landing site, morphological features such as craters and rocks in the landing area were identified and extracted. Geological characteristics, including lunar surface roughness, depth-to-diameter ratio of fresh craters, regolith thickness, and rock abundance of the landing area, were analyzed. The specific results are as follows.

We selected three craters (denoted as C1–C3, Fig. 3a) in the near vicinity of the CE-6 landing site. The landing site is located on the southwestern rim of crater C1, which has a diameter of 50.95 m and a center distance of 26.93 m from the landing site. According to CE6_LCAM_DEM, the maximum depth of crater C1 is 2.79 m, with a depth-to-diameter ratio of ~0.05, indicating that it is a severely degraded crater. About 8 m southwest of the landing site is crater C2, which has a sharp rim and a diameter of about 4.8 m. Crater C2 has a depth of approximately 0.75 m (Fig. 3b, c), with a depth-to-diameter ratio of ~0.16, and numerous small rocks are visible at the crater’s bottom and rim. Concentric structures with a diameter of around 2.5 m can be faintly seen on the crater walls. To the east is crater C3, which has a diameter of 2.3 m and a depth of about 0.44 m, resulting in a depth-to-diameter ratio of ~0.19 (Fig. 3b, d). Concentric structures can also be observed, with a diameter of about 1 m. The depth-to-diameter ratio (close to 0.2) and ejecta (Fig. 3c, d) suggest that craters C2 and C3 are relatively fresh22,23. Based on CE6_LCAM_DEM data, the average slope of the area around the landing site (within the range shown in Fig. 3a) was calculated at a baseline of 1 m. The results show that the average slope of this area is 3.24°, with gentle slopes (below 7°) accounting for ~82% of the total area. This indicates that the region is mostly flat, which is favorable for the safe landing and sampling operations.

The photometry of the lunar regolith is influenced by its roughness, which can be characterized using bidirectional slope and Root Mean Square (RMS) height (see the Methods section for details, the results are in Table 1). Previous studies have shown that at the micro-scale, surface roughness may be related to space weathering, micrometeorite impacts, and regolith gardening experienced by the geological unit currently located24. Based on the CE6_PCAM_DEM data, bidirectional slope25 at a 7 mm scale and RMS height25–28 with a 125 mm window size were calculated for a ~5 m range in the CE-6 landing area. The results indicate that the bidirectional slope in the area can reach up to 73°, with a median of 6° (Fig. 3e). The maximum RMS height is 25 mm, with a median of 3 mm (Fig. 3f). In comparison, at the same scale, the bidirectional slope of the CE-5 landing area is up to 78°, with a median of 7°, and the RMS height has a maximum value of 43 mm and a median of 3 mm. For the CE-4 landing area, the bidirectional slope reaches a maximum of 40°, with a median of 10°, and the RMS height has a maximum value of 18 mm and a median of 4 mm29. Thus, the surface roughness of the CE-6 landing area is closer to that of the CE-5 area and is smaller than that of the CE-4 area.

There is a large number of craters around the CE-6 landing site. Based on the CE6_LCAM_DOM data, 1481 craters were identified (see Supplementary Material 2 and supplementary Fig. 2) within an area of 300 m × 400 m region around the landing site, with diameters up to a maximum of 56.41 m, and an average of 2.28 m30. Based on CE6_LCAM_DEM data, the average depth-to-diameter ratio of fresh craters (i.e., craters with ejecta clearly visible at their rims), with diameters ranging from 2 m to about 30 m, is ~0.214. Based on CE2TMap2015 DEM, the average depth-to-diameter ratio of fresh craters, with diameters ranging from 80 m to about 300 m within an area of 9 km × 8 km around the CE-6 landing site, is ~0.050. At the two scales of 2–30 m and 80–300 m, the average depth-to-diameter ratios of fresh craters at the CE-4 landing area were calculated to be ~0.352 and ~0.064, respectively, while the average depth-to-diameter ratios of fresh craters at the CE-5 landing area were ~0.183 and ~0.059. This indicates that the depth-to-diameter ratio of small fresh craters (2 m to several tens of meters) near the CE-6 landing site is similar to that of the CE-5 landing site, but significantly lower than that of the CE-4 site. But for mid-scale craters larger than 80 m, the average depth-to-diameter ratio for the CE-6 landing area falls between those of the CE-5 and CE-4 landing areas, with minimal differences among the three (see Supplementary Material 2 for details).

Additionally, 26 concentric craters were discovered around the CE-6 landing site, which can be used to analyze the lunar regolith thickness. According to the empirical formula for surface regolith thickness based on crater morphology31,32 (see supplementary material 3 for details), the regolith thickness around the CE-6 landing site was estimated to range from 3.4 to 8.5 m. Radar observations show that the lunar regolith thickness at the CE-5 landing area is ~2.5 m33, while the regolith thickness at the CE-4 landing area is around 12 m34. Using the same concentric craters statistical method, the fine-grained regolith thickness in the CE-4 landing site is estimated to be 2.5–7.5 m35. Beneath this regolith layer, an additional ejecta deposit with a depth of ~13 m is also suggested35. The regolith thickness in the CE-5 landing site is estimated to be 4–6 m36, while the regolith thickness in the CE-6 landing site is estimated to be 1.1 to 7.0 m37. Thus, it is evident that the lunar regolith thickness at the CE-6 landing area (3.4–8.5 m) is similar to that at the CE-5 landing area (4–6 m) but shallower than that at the CE-4 landing area (12–13 m).

Rock abundance is generally related to the exposure time experienced by the geological unit38. Using CE6_LCAM_DOM data, a total of 1142 small rocks were identified and extracted from a 100 m × 100 m area around the landing site (See Supplementary Material 4 and Supplementary Fig. 4a), with a maximum diameter of 0.52 m and an average diameter of 0.11 m. The rock abundance, defined as the ratio of the total area occupied by rocks to the total area of the region, is 0.14%. At the same scale, the rock abundance at the CE-4 and CE-5 landing areas was calculated to be 0.07 and 0.43%, respectively. According to the distribution curve trend of the rock abundance fitting line39 (see Supplementary Material 4 and Supplementary Fig. 4b), the number of larger rocks with diameters exceeding 0.2 m is relatively low within the CE-6 landing area. For comparison, rock abundance data derived from Diviner (with a spatial resolution of 200 m)40 was used to conduct a statistical analysis within a 10 km radius around the CE-6 landing site, resulting in an average rock abundance of 0.31%. The calculated average rock abundance within a 10 km radius around the CE-4 and CE-5 landing sites was found to be 0.20 and 0.50%, respectively. Overall, the rock abundance in the CE-6 landing area is relatively low. It is slightly higher than that in the CE-4 landing area, but lower than that in the CE-5 landing area.

In summary, the geological characteristics of the CE-6 landing area at the micro-scale (from millimeters to sub-kilometers), such as surface roughness, depth-to-diameter ratio of fresh craters, regolith thickness, and rock abundance, all fall between those of the CE-4 and CE-5 landing areas, being closer to those of the CE-5 landing area.

Distribution and potential provenances of ejecta in the landing area
The CE-6 landing area is predominantly covered by dark lunar mare basalts, with a small amount of foreign ejecta. Images captured by CE-6 PCAM revealed a widespread distribution of possible secondary craters near the landing site, characterized by relatively shallow depths and significant debris around their rims (Fig. 4a), indicating that the samples collected by CE-6 are likely to contain ejecta components2,8. To identify the potential provenances of samples from the CE-6 landing area, we searched for craters around the CE-6 landing site that might have contributed ejecta to the landing site using CE2TMap2015 DOM data (Fig. 4b–d). On one hand, based on the USGS geological map41, craters labeled as Copernican or Eratosthenian in geological age were identified around the landing site. On the other hand, for areas near the landing site, the focus was on identifying craters with sharp rims and pronounced bowl-shaped features. These features are indicative of craters that could potentially be significant provenances of ejecta impacting the landing site.

Using the identified craters, the potential thickness of ejecta contributed by these craters is calculated42 (see Supplementary Material 5 for details). Based on the study of CE-6 samples, it has been determined that the age of the mare basalt at the CE-6 landing site is ~2.8 Ga3,4. Following this conclusion, we employed the crater size-frequency distribution (CSFD) dating method to determine the geological ages of all potential provenance craters for the ejecta (see Supplementary Material 6 for details). The results indicate that among the identified craters, crater White (3.26 Ga), Hausen (3.65 Ga), and Finsen (3.5 Ga) are older than the age of CE-6 basalt, making it less likely that they contributed ejecta. In contrast, craters C1 (37.5 Ma), C7 (576 Ma), C5 (1.12 Ga), C4 (1.21 Ga), C8 (1.88 Ga), Chaffee S (2.39 Ga), and C6 (2.48 Ga) are all younger than the age of CE-6 basalt, and are thus potential provenances of ejecta. The geological ages of these craters are similar to those derived from remote sensing data prior to CE-6 landing13. We have also created a geological stratigraphy based on ejecta thickness, where material from younger provenance craters is positioned at the top (Fig. 4e).

Finally, we directly discerned the provenances of foreign ejecta from the imagery, as depicted in Fig. 4c. Analyzing the distribution of these craters and the orientation of ejecta streaks, the ejecta rays in the northwest-southeast direction from crater Chaffee S are relatively distinct in images (Fig. 4c). Conversely, the provenances of several ejecta streaks in the northeast-southwest direction are more difficult to identify.

Methods
Instruments and datasets
The observation data used in this study mainly include LCAM sequence images and PCAM close-range stereo images. In addition, Chang’E-2 (CE-2) global lunar terrain data (CE2TMap2015)14,15, including the DOM and DEM datasets, is used as the reference base map.

LCAM is one of the scientific payloads mounted on the bottom of the CE-6 lander (see Table 2 for technical specifications), which is used to capture images of the lunar surface during the powered descent phase to document the entire landing process. In this study, 142 LCAM images with a frame frequency of 1 s from the approaching phase of the CE-6 lander (below an orbital altitude of 2500 m) were selected for the reconstruction of the landing area terrain data. These selected images include both oblique views from orbital altitudes above 2000 m and near-vertical views taken as the lander approached the surface. This combination of images provides a wider range of observation angles and greater image overlap, which is beneficial for data processing and improves the accuracy of the 3D coordinates of image tie points51.

PCAM is another scientific payload that contains two optical systems mounted on a rotating platform of the CE-6 lander (see Table 2 for technical specifications). During the lunar surface exploration, by utilizing the rotation of the platform, PCAM captures optical images of the lunar surface within a horizontal range of 0° to 180° north of the landing site and a pitch range of −90° to 0°. These images are used for studying the terrain and landforms of the landing area. In this study, 120 pairs of PCAM stereo images acquired during the lunar surface exploration were selected to develop a detailed terrain dataset of the CE-6 landing area.

CE2TMap2015 is generated using CE-2 stereo images with a spatial resolution of 7 m. The average relative error of CE2TMap2015 in horizontal positioning is 5 m, with a standard deviation of 4 m, while the average relative error in elevation is 2 m, with a standard deviation of 5 m. Compared to the absolute positions of five laser reflectors on the lunar nearside, the horizontal position deviation of CE2TMap2015 ranges from 21 to 97 m, while the elevation deviation ranges from 2 to 19 m15,21.

High-precision terrain data development and CE-6 landing site positioning
The processing workflow and relevant coordinate systems used in this study are detailed in Supplementary Material 7.

In this study, a lunar surface cartesian coordinate system (LSCS) was first established, using the predicted CE-6 landing site location in CE2TMap2015 DOM as the origin. A hybrid matching method52,53 was employed to achieve hybrid matching between LCAM sequence images, and between LCAM images and CE2TMap2015 DOM, resulting in a total of 55,892 image tie points. From these tie points, eight evenly distributed small crater centers were manually selected as lunar surface control points (LGCPs). Among them, LGCP01 was designated as the elevation control point, while the remaining seven served as both horizontal and elevation control points. Their 3D coordinates were extracted from CE2TMap2015. Based on photogrammetric bundle adjustment theory54, the exterior orientation parameters (EOs) of each LCAM image and the coordinates of all tie points were determined in the LSCS. A pixel-level point cloud was then generated using dense image matching based on the adjusted EOs, and two versions of DEMs, referred to as CE6_LCAM_DEM, were interpolated using a triangulated irregular network (TIN), with spatial resolutions of 1 and 10 cm, respectively. Among them, the 10 cm resolution dataset is a global product, covering a 2.4 km (east-west) × 3.0 km (north-south) area around the landing site, while the 1 cm resolution dataset is a local, high-resolution product covering only a 300 m × 400 m area. Finally, using the DEMs and the adjusted EOs, orthoimage reprojection techniques were applied to generate two versions of DOMs, referred to as CE6_LCAM_DOM, with matching spatial resolutions and coverage. The LCAM terrain data were produced in the LSCS.

Similarly, we generated the PCAM DEM and DOM (denoted as CE6_PCAM_DEM and CE6_PCAM_DOM) covering an area up to 18 m north of the landing site, using high-resolution PCAM close-range stereo images, with spatial resolutions of 3 and 1 mm, respectively. The only difference in this process is that no control points were used. Instead, the EOs of PCAM images were derived by combining real-time telemetry data of the lander’s pitch and yaw mechanisms, payload mounting parameters, and other information in the CE-6 mechanical coordinate system (LMCS). Therefore, the PCAM terrain data were produced in the LMCS.

The three types of terrain data, including CE2TMap2015, LCAM terrain data, and PCAM terrain data, were geometrically registered to generate a high-precision terrain dataset for the CE-6 landing area. This dataset features a unified spatial reference framework (consistent with the lunar geographic coordinate system (LGCS), used in CE2TMap2015), a common projection coordinate system, and spatial resolutions ranging from millimeters to meters. Furthermore, based on the CE-6 lander’s 3D model and in-orbit telemetry attitude parameters, the relative positions of the lander’s four footpads, scoop sampling areas, and drilling sampling point were precisely marked, providing accurate coordinates for these elements in the CE-6 landing area terrain dataset.

During the development of the high-precision terrain dataset for the CE-6 landing area, strict geometric registration was achieved between the PCAM and LCAM terrain data by calculating the seven coordinate transformation parameters between the LMCS and LSCS, including three rotation angles, three translation shifts, and one scale factor. Using these parameters, the 3D coordinates of the origin of the PCAM terrain data (i.e., the CE-6 landing site) in the LCAM terrain data could be determined. By transforming these coordinates into the LGCS, the precise position of the CE-6 landing site on the lunar surface was obtained as (153.9776°W, 41.6251°S, −5273 m).

The positional accuracy of the CE-6 landing area high-precision terrain dataset and the landing site depends on the processing accuracy of the LCAM and PCAM terrain data, as well as the registration accuracy between these datasets and the CE2TMap2015.

During the processing of LCAM terrain data, the RMS of the reprojection error of 55,892 LCAM image tie points after photogrammetric adjustment is 0.38 pixels, reflecting the relative positional accuracy of the reconstructed terrain data, with a corresponding spatial position error RMS of 0.66 m. For PCAM terrain data, the RMS of the reprojection error of 162,149 PCAM image tie points is better than 0.04 pixels, corresponding to a spatial position error RMS of 0.06 m. In summary, both the LCAM and PCAM terrain data for the CE-6 landing area exhibit relative positional accuracies better than 1 pixel, with horizontal positional accuracy in the decimeter to centimeter range.

Regarding registration accuracy, during the alignment of PCAM terrain data with LCAM terrain data, ten easily identifiable and evenly distributed small rocks around the landing site were selected as control points, denoted as PGCP01 to PGCP10 (Fig. 5b, d). The RMS of the horizontal and vertical position deviations of the PGCPs are 0.04 mm (1δ) and 0.02 mm (1δ), respectively, with a total RMS position deviation of 0.05 mm (1δ), achieving centimeter-level registration accuracy (see Table 3). For the alignment of LCAM terrain data with CE2TMap2015, eight control points (Fig. 5a, c) were used. Adjustment results show that the RMS of horizontal and vertical position deviations of the LGCPs are 1.346 m (1δ) and 1.817 m (1δ), respectively, with a total RMS position deviation of 2.261 m (1δ), achieving meter-level registration accuracy (see Table 4). Therefore, the positional accuracy of the high-precision terrain dataset for the CE-6 landing area and the landing site relative to CE2TMap2015 reaches meter-level, and its absolute positional accuracy remains consistent with that of CE2TMap201515.

Calculation of bidirectional slope and RMS height
The bidirectional slope is the two-dimensional topographic slope and can be used to measure the anisotropic characteristics of the surface topography25. In a DEM format, we first select a point of interest with coordinates of (x_i, y_j, z_i,j), where i and j represent the indices in the horizontal and vertical directions, respectively. We then select two points in the horizontal direction, denoted by (x_i-1, y_j, z_i-1,j) and (x_i+1, y_j, z_i+1,j), and another two points in the vertical direction, denoted by (x_i, y_j-1, z_i,j-1) and (x_i, y_j+1, z_i,j+1). These four surrounding points form two vectors in the vertical and horizontal directions, and the two vectors further form a plane, whose surface normal can be calculated using vector geometry as:

n^_x,y = (n^_x × n^_y) / |n^_x × n^_y| (1)

Where

n^_x = (x_i+1, y_j, z_i+1,j) - (x_i-1, y_j, z_i-1,j) (2)

n^_y = (x_i, y_j+1, z_i,j+1) - (x_i, y_j-1, z_i,j-1) (3)

Finally, the bidirectional slope is calculated as the angle between n^_x,y and the normal of a flat surface, that is, (0, 0, 1). The effective baseline is the square root of the area of the rhombus formed by the four points, which is ~1.4 times the spatial resolution of the DEM. In our research, the resolution of the CE-6 PCAM DEM data is 5 mm, thus the effective baseline is ~7 mm.

RMS height is the standard deviation of elevations within a two-dimensional window, and it is widely used in planetary radar remote sensing25–27. RMS height usually increases with the size of the window, because the number of points grows with the window size. It can be calculated using an equation like:

RMS height = [1/n * Σ_{i=1}^n (z_i - z_mean)^2]^1/2 (4)

For the DEM data, where n is the number of pixels in a defined two-dimensional window, z_i is the elevation of each pixel in the window, and z_mean is the mean value of the elevation in the window. In our research, the window is set to be 125 mm × 125 mm, and the resolution of the CE-6 PCAM DEM data is 5 mm, which means, n is 25 × 25 = 625.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This article presents the geological characteristics of the Chang’E-6 landing site at multiple spatial scales. The manuscript presents the development of a millimeter-to-centimeter terrain dataset derived from in situ images acquired by the landing camera (LCAM) and panoramic camera (PCAM), which allowed to identify the most accurate location of the CE-6 landing site, the scoop sampling areas, and the drilling sampling point to date, as well as some geological characteristics. These characteristics include surface roughness, depth-to-diameter ratio of fresh 2-30 m diameter craters, regolith thickness, and rock abundance. These characteristics are compared with those derived for the Chang’E-4 and Chang’E-5 landing areas. Overall, the analyses suggest that the geological characteristics of the Chang’E-6 landing site fall between those observed at the Chang’E-4 and Chang’E-5 landing sites but are more similar to the Chang’E-5 landing site, likely caused by significant modification of the Chang’E-6 landing area by later ejecta. The manuscript also presents the distribution and potential sources of ejecta in the landing area using PCAM images, USGS geological map, CE2TMap2015 orthomosaic, and an ejecta thickness calculation formula. The results suggest that local material excavated from crater “C1” should constitute the primary material of the Chang’E-6 samples.\n\nOverall, the article provides a relevant analysis of the topography at the Chang’E-6 landing site and enhances our understanding of the sources of materials found on the regolith surface in the landing region. The geolocation of the landing site and the terrain characterization presented in this article will help contextualize the results of sample analyses corresponding the studied area. Similar terrain characterization could be undertaken by upcoming missions and be compared to those derived herein. Notable results include the identification of differences in surface roughness and depth-to-diameter ratio of fresh craters measured at the millimeter to centimeter scale as allowed by landed missions versus at the kilometer scale as allowed by orbital remote sensing missions. At the kilometer scale, ancient highland regions with longer exposure times exhibit greater surface roughness compared to the younger mare regions. At the millimeter to centimeter scale, the authors find minimal surface roughness differences between the Chang’E-4, Chang’E-5 and Chang’E-6 areas, suggesting that the influence of exposure time on surface roughness is not significant at that scale. At the kilometer scale, the depth-to-diameter ratio of craters tends to decrease as exposure time increases. At the several meters to tens of meters scale, the depth-to-diameter ratio of fresh craters is significantly smaller in the relatively younger Chang’E-6 and Chang’E-5 landing areas than in the older CE-4 landing area, suggesting that it is influenced by both exposure time and by the thickness and looseness of the surface regolith. The surface regolith in the Chang’E-6 and Chang’E-5 landing areas is relatively thinner and contains more rocks than the Chang’E-4 landing area, making it more difficult to form deeper small craters.\n\nThe approach presented in this manuscript is mostly sound. The section describing the distribution and potential sources of ejecta in the landing area appears weaker. It would be interesting to relate the abundance of ejected material at the landing site with its expected stratigraphy, which would help put into context the drill sample acquired. The depth-to-diameter values presented should also be better contextualized. The depth/diameter ratios are presented using the mean. A variance metric for the measurements should be included.\n\nThe quality of the presentation needs to be improved. The syntax and wording would benefit from a round of editing to improve the clarity of the text. Some discourse markers are used inappropriately (e.g., \"At the same time\" to introduce new ideas). Some figure items are hard to identify, and the syntax of figure labels should follow a more conventional format (e.g., using full sentences or noun phrases instead of \"(a) is..., (b) is...\").\n\nAdditionally, sections of the text need to be reworked to improve clarity. The introduction needs refinement to clearly outline the main components of the manuscript: the problem being addressed, the expected outcomes, and the relevance of the proposed research within the context of current knowledge. While these aspects become clearer as the text progresses, restructuring the introduction to emphasize these points and establishing a coherent narrative would enhance the understanding of the research and its significance. Including a brief overview of the fundamental questions related to the study site and the Chang'E-6 mission would help situate the research within the broader context of this mission and ongoing scientific studies in the South Pole-Aitken (SPA) basin. The results section contains a lot of information that overlaps with the methodology section. The sections dedicated to results should be streamlined to better highlight the findings. Moreover, the authors suggest that the results provided in this study will benefit the interpretation of laboratory analysis results of the samples. Clarification on this point would enhance the robustness of the article.\n\nIn general, we also find that the manuscript lacks references to recent work conducted at the Chang’E-6 landing site. These papers should be cited, and the results derived herein should be compared to those. Additional papers published following the manuscript’s submission should also be mentioned, including the recent radioisotope dating of basalts in the CE-6 samples.\nCui, Z. et al. A sample of the Moon’s far side retrieved by Chang’e-6 contains 2.83-billion-year-old basalt. Science 0, eadt1093 (2024).\nZhang, Q. W. L. et al. Lunar farside volcanism 2.8 billion years ago from Chang’e-6 basalts. Nature 1–2 (2024) doi:10.1038/s41586-024-08382-0.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "NCOMMS-24-61970\nGeneral Comments\nThis manuscript presents high-precision terrain datasets with mm-to-cm resolution for the world’s first lunar farside sample return mission, Chang’e-6, and conducted detailed geological analyses based on them and found the exposure time of CE-6 is between CE-4 and CE-5. The results of this study are timely, interesting, and convincing that could help the ongoing Chang’e-6 sample analysis especially the produced high-resolution terrain data. I find no major concerns in this work and support to publish it after a revision.\n\nThe authors should improve unclear descriptions of this manuscript, for example 1) what data do they use in this study and what are their IDs, 2) what are bidirectional slope and RMS height and how to produce them including technical details, 3) how do they produce CE-6 LCAM/PCAM DOM and DEM data including technical details, etc. The authors indeed should provide the produced datasets including but not limited to the CE-6 LCAM DOM/DEM and CE-6 PCAM DOM/DEM to a public repository such as Zenodo or Figshare.\n\nThe authors should compare their results with those of other groups, for example 1) the landing site location, 2) the ejecta thickness, 3) the age of potential ejecta source craters especially Chaffee S and White craters, etc. The authors should compare regolith thickness obtained by concentric crater method with those obtained by similar approaches. The authors shouldn’t compare the roughness obtained by this study with Zeng et al. (2023) because of the very different scales as emphasized by themselves.\n\nIn addition, the figures of this manuscript should be improved. The font size of labels in all figures should be increased for a better visibility. The symbol size should also be increased and please avoid using similar colors with the background units. Some of the perspective figures need point the direction of north.\n\nMy detailed comments are embedded below.\n\nDetailed Comments\nLine 2: “New Observation Data” Please specify what are they.\nLine 12: “moon”: “M” should always be capitalized if it refers to the Earth’s Moon.\nLine 13 “resolution” Spatial resolution.\nLine 14 “153.9776°W, 41.6251°S, -5273 m” These numbers should be compared with other works, especially Liu et al. (2024) and Tong et al. (2024).\n• Liu, Z., et al., 2024. High-precision visual localization of the Chang’e-6 lander. National Remote Sensing Bulletin 28, 1648–1655.\n• Tong, X., et al., 2025. Illumination Robust Landing Point Visual Localization for Lunar Lander With High-Resolution Map Generation. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 18, 1577–1591.\nLine 18: “accounting for about 40%” In mass or in volume?\nLine 19: “its geological characteristics (e.g., surface roughness) are more closely aligned with those of the CE-5 landing area.” What “its” stands for? I don’t understand the meaning of this sentence. Please rewrite it.\nLine 22: “At 06:24 on June 2, 2024” Please specify the time zone.\nLine 22: “CE-6 lander” It is the lander and ascender composite.\nLine 24-25: “within the mare basalt region on the southern rim of the Apollo Basin.” It is not on the southern rim but between the southern rim crest and peak rim. Please correct it.\nLine 29-30: “such as magma activity on the farside of the moon and the dichotomy between the near and far sides” A reference is needed. I suggest this one: Yang, W., et al., 2024. Scientists eager for Chang’e-6 lunar farside samples to bring new discoveries. The Innovation 5, 100660.\nLine 32: Suggest changing “material sources” to “material provenances”\nLine 34-37: “At a macro scale of kilometers to hundreds of kilometers, it is believed that the CE-6 landing area is located within a young lunar mare basalt geological unit, possibly mixed with non mare ejecta materials from exotic sources 1-6.” References might be improved here. After “mare basalt geological unit” you could cite: Zeng et al. (2023, NA), Qian et al. (2024, EPSL), and Gou et al. (2024, EPSL). After “mixed with non mare ejecta materials from exotic sources”, you could cite: Qian et al. (2024, ApJL), Yue et al. (2024, The Innovation), and Xu et al. (2024, GRL).\nLine 48: “multi-source image data” If only LCAM and PCAM, it should be “bi-source” rather than “multi-source”.\nLine 53: “analysis of the landing area” Suggest changing to “sampling area”.\nLine 70-71: “142 LCAM sequence images and 120 pairs of PCAM stereo images” Please list them in the supplementary materials.\nLine 73: “spatial resolutions of 1 cm and 10 cm” What do you mean? 1 cm spatial resolution for CE6_LCAM_DOM, 10 cm for CE6_LCAM_DEM? If so, please rewrite this sentence.\nLine 74-77: “(noted as CE6_LCAM_DOM and CE6_LCAM_DEM) were produced, as well as the PCAM orthoimage (noted as CE6_PCAM_DOM) with a spatial resolution of 1 mm and digital elevation model (noted as CE6_PCAM_DEM)” Is it possible to constrain the vertical resolution of CE6_LCAM_DEM and CE6_PCAM_DEM.\nLine 86: “153.9776°W, 41.6251°S, -5273 m” These numbers should be compared with other works, especially Liu et al. (2024) and Tong et al. (2024).\nLine 109-110: “The horizontal position deviation from the positioning result of this paper is 441.3 m, and the three-dimensional position deviation is 441.7 m” How do you get these two numbers.\nFigure 2: Please indicate the north direction in Fig. b and Fig. c. Please add scale bar in Fig. d.\nLine 147-148: “at a scale of 1-meter” What do you mean? Please specify.\nLine 149-151: The comparison of slope between this work and Zeng et al. (2023) is not appropriate as emphasized by yourself because of the very different working scales.\nLine 153: “by bidirectional slope and RMS height” I don’t understand what they are and how do you calculate them. Please explain here, in the method, or in the supplementary materials.\nLine 152-166: It is hard to catch the numbers of bidirectional slope and RMS height for Chang’e-4, 5, 6 landing sites. Could you summarize them in a table?\nLine 171-172: Green dots and red cross are hard to see in this figure.\nLine 179: “1,481 craters were identified” Please mark them here or in the supplementary materials.\nLine 181-183: “Based on CE6_LCAM_DEM data, the average depth-to-diameter ratio of fresh craters ranging from 2 m to approximately 30 m in diameter, where ejecta is clearly observable around the crater rims, was calculated to be ~0.214.” The grammar is poor, please rewrite it.\nLine 183-185: “Within a several-kilometer range, the average depth-to-diameter ratio of fresh craters ranging from 80 m to ~300 m in diameter, calculated from LRO and CE2 data, was ~ 0.050.” The grammar is poor, please rewrite it.\nLine 194, 197", "reviewer_214": "see Supporting Material for details", "reviewer_201": "the CE-5 landing area is approximately 2.5 meters", "reviewer_basin": "Chang’E-4 Landing Site Region. Journal of Geophysical Research: Planets 123, 1684–1700.\n• Qian, Y., et al., 2021. Copernican-Aged (<200 Ma) Impact Ejecta at the Chang’e-5 Landing Site: Statistical Evidence From Crater Morphology, Morphometry, and Degradation Models. Geophysical Research Letters 48, e2021GL095341.\nLine 203-206: “It is evident that the lunar regolith thickness at the CE-6 landing area falls between that of the CE-4 and CE-5 landing areas, suggesting that the geological age of the CE-6 landing area should also lie between those of the CE-4 and CE-5 areas.” First of all, you need to compare regolith thickness obtained by similar methods (see above). Secondly, even if the regolith thickness of the CE-6 landing site is between those of CE-4 and CE-5 landing sites, you cannot conclude that its geological age is between those of CE-4 and CE-5 because the regolith thickness doesn’t grow linearly with age. Because the geological age of CE-6 is already known (Cui et al., 2024, Science; Zhang et al., 2024, Nature), you could discuss the regolith thickness based on that age.\nLine 215-218: “For comparison, rock abundance data derived from Diviner (with a spatial resolution of 200 m) 23 was used to conduct a statistical analysis within a 10 km radius around the CE-6 landing site, resulting an average rock abundance of 0.31%.” Please explain why the rock abundance yielded by the Diviner data is different from this study.\nLine 230-231: “a widespread distribution of secondary craters near the landing site (seen in Fig. 4a)” How do you know they are secondary craters. Please explain.\nLine 231-232: “indicating that the samples collected by CE-6 are likely to contain ejecta components.” You could refer Li et al. (2024, NSR). It clearly shows there are ejecta components in the returned CE-6 samples.\nLine 242-243: “(a) is identifiable ejecta in images from CE-6 PCAM, indicated by white arrows” How do you know they are secondary craters. Please explain.\nLine 244: Change “related” to “transported”.\nFigure 4: I can barely see the cross representing the CE-6 landing site.\nLine 258-260: “the craters White and C8, both located to the south of the landing site on the rim of crater Apollo, contribute to the ejecta thickness at the landing site of 0.2303 m and 0.0871 m, respectively.” Please check the work of Qian et al. (2024, ApJL) and Jia et al. (2024, Icarus). Whether or not the formation of White crater postdates the CE-6 basalt eruption is still in debate. Most likely, its formation predates the CE-6 basalt eruption.\nLine 265-268: Please mark the northwest-southeast and northeast-southwest ejecta streaks in Fig. 4c. I cannot see them clearly from this figure.\nLine 271: “the geological characteristics of the CE-6 landing area” Please specify what are they.\nLine 274-275: “This is consistent with the latest CSFD geological age of the CE-6 landing area being 2.79- 2.87 Ga 26” At the moment, the mare basalt age at the CE-6 landing area has been constrained by samples using isotopic approaches (Cui et al., 2024, Science; Zhang et al., 2024, Nature). Therefore, please use these ages.\n• Cui, Z., et al., 2024. A sample of the Moon’s far side retrieved by Chang’e-6 contains 2.83-billion-year-old basalt. Science 386, 1395–1399. https://doi.org/10.1126/science.adt1093\n• Zhang, Q.W.L., et al., 2024. Lunar farside volcanism 2.8 billion years ago from Chang’e-6 basalts. Nature.\nLine 275-276: “the geological age of the CE-4 landing area being approximately 3.6- 3.7 Ga 27” Not only Ling et al. (2019) studied the geological age of the CE-4 landing area. Please summarize the ages of other people’s work as well.\nLine 277-280: “However, our analysis results also indicate that, at the micro-scale, geological characteristics such as surface roughness and the depth to-diameter ratio of fresh craters exhibit inconsistencies with those observed at the macro-scale.” Any further explanations?\nLine 281-283: “Previous studies have suggested that, at the kilometer scale, ancient highland regions with longer exposure times exhibit significantly greater surface roughness compared to the younger mare regions.” At lease a reference is needed.\nLine 297-300: “A likely explanation for this phenomenon is that the fresh small craters selected in this study are relatively young and have experienced similar degradation times, but the depth of these craters is also influenced by the thickness and looseness of the surface regolith in these areas.” Please rewrite this sentence. I don’t understand what you mean. Do you mean the shallow depth of these craters is controlled by the thickness and looseness of the surface regolith in these areas.\nLine 307-308: “0.7-0.9 Ga younger than that of CE-4 landing area” Please remember that CE-4 landed on the ejecta of Finsen crater with an age much younger than CE-4 basalts. The CE-4 landing site is also significantly modified by later ejecta. This phenomenon should be included in the discussions.\nLine 316-319: “From the high-resolution terrain dataset of the CE-6 landing area, it is evident that the CE-6 lander landed right on the rim of crater C1, which has a diameter of 50 m. According to the impact ejecta model, the ejecta thickness from crater C1 at this location is about 0.4861 m.” I noticed CE-6 landing site is located within the rim of crater C1. Can the ejecta thickness model mentioned in the Supporting Material 4 (Pike et al., 1974) calculate the ejecta thickness for a location within the crater rim crest?\nLine 330-331: “the ejecta thickness sourced from Chaffee S are predicted to be ~5cm in the CE-6 landing site.” This number is much smaller than those given by Qian et al. (2024, ApJL), Yue et al. (2024, The Innovation), and Xu et al. (2024, GRL). Please compare with them.\nLine 335-337: “it is likely that the Mg-rich ejecta from Chaffee S could be included in the lunar samples collected by CE-6.” “Mg-rich ejecta” occurs like a sudden. At least a reference is needed.\nLine 339-361: All information in this paragraph has been discussed previously. Please summarize them concisely.\nLine 359: Be careful of “White crater” for its formation time.\nLine 367: You might show where LCAM and PCAM installed in Fig. 1b or a separated figure.\nLine 421-425: “Based on the principle of photogrammetric adjustment 33, the LCAM terrain data covering an area of 2.4 km (east-west direction) × 3.0 km (north-south direction) around the landing site was produced (noted as CE6_LCAM_DOM and CE6_LCAM_DEM), with spatial resolutions of 1 cm and 10 cm.” I don’t understand how you produce CE6_LCAM_DOM and CE6_LCAM_DEM. Please give more details.\nLine 426-430: “the high resolution PCAM orthoimage (noted as CE6_PCAM_DOM) with a spatial resolution of 1 mm and digital elevation model (noted as CE6_PCAM_DEM) with a spatial resolution of 3 mm within an 18-meter range north of the landing site were created using close-range stereo images of the lunar surface captured by PCAM.” I don’t understand how you produce CE6_PCAM_DOM and CE6_PCAM_DEM. Please give more details.\nLine 441: “the seven coordinate transformation parameters” What are they?\nLine 594: There are lots of data archived in this website. You need to list what data you used and provide their IDs.\nFigure S2: Why there are much more rocks in the blue box than outside the blue box?", "reviewer_author_response": "We appreciate the reviewers for their detailed evaluation and recognition of our work.\n\nAccording to the reviewer's comments, we supplemented the analysis of the potential provenances of ejecta in the landing area. We conducted age dating of the craters that could be the potential provenances of ejecta, and created a geological stratigraphy based on these dating results (see Fig. 4e). These analyses could provide contextual information for the study of the drilled samples.\n\nIn addition, following the reviewer's suggestion, we have also supplemented more detailed statistical information on the depth-to-diameter ratio of craters, including not only the mean value but also variance metric (see Supplementary Table 1).\n\nAccording to the reviewer's comments, the revised manuscript has been edited for the syntax and wording, with inappropriately markers removed to ensure more accurate expression. We have also revised the titles and captions of the figures and tables, following a more conventional format.\n\nThe introduction section of the revised manuscript has been refined to clearly outline the main components of the paper. Redundant information between the Results and Methods sections has been removed from the revised manuscript.\n\nIn addition, the relationship between the results provided in this study and the interpretation of laboratory analysis results of the samples were supplemented in the “Discussion and conclusions” section as follows.\n“The results of this study can provide evidence for revealing the characteristics of the topography, landforms, lunar regolith and possible foreign ejecta of the CE-6 landing area at scales from millimeters to sub-kilometers. The local mare basalt is from the ejecta of C1, and foreign ejecta with Mg-rich materials might be originated from Chaffee S. Combined with recent remote sensing analyses of the CE-6 landing area at kilometers to sub-kilometers, as well as research on the CE-6 lunar samples, these findings can offer new constraints for understanding the material characteristics and provenances of the lunar samples, which would benefit the interpretation of laboratory analysis results of the samples.”\n\nBased on the reviewers' comments, we have cited the recent works on the CE-6 landing site, and compared their results with our findings. For details, please refer to the revised manuscript and the reference list.\n\nWe would like to thank Reviewer #2 for the co-reviewed of this manuscript, and we acknowledge and appreciate the Early Career Researchers co-review mechanism provided by NC.\n\nWe appreciate the reviewer's positive evaluation and the recognition of our work.\n\nAccepted.\n1) Based on the reviewer's comments, we have organized the relevant data used in this study, such as CE-6 LCAM/PCAM DOMs and DEMs data. The corresponding data descriptions and IDs can be found in a “Data introduction” document uploaded with the revised manuscript. Additionally, we have made these data available in a public online repository, accessible at: https://moon.bao.ac.cn/Moon/CE6SourceData.jsp\n2) We have supplemented the technical details of Bidirectional slope and RMS height, including their definitions, specific calculation methods, and references. For detailed information, please refer to the \"Methods\" section of the revised manuscript.\nReference:\nCai, Y., & Fa, W. (2020). Meter-scale topographic roughness of the Moon: The effect of small impact craters. Journal of Geophysical Research: Planets, 125(8), e2020JE006429. https://doi.org/10.1029/2020je006429\nShepard, M. K., Campbell, B. A., Bulmer, M. H., Farr, T. G., Gaddis, L. R., & Plaut, J. J. (2001). The roughness of natural terrain: A planetary and remote sensing perspective. Journal of Geophysical Research Planets, 106(E12), 32777–32795. https://doi.org/10.1029/2000je001429\nGuo D, Fa W, Wu B, et al., (2021). Millimeter- to Decimeter-Scale Surface Roughness of the Moon at the Chang'e-4 Exploration Region. Geophysical Research Letters, 48.DOI:10.1029/2021GL094931.\n3) The technical details of the production of CE-6 LCAM/PCAM DOMs and DEMs data have been supplemented in the section of “Method- High-precision terrain data development and CE-6 Landing Site positioning”.\n\nAccepted. We have compared our results with those of other groups", "reviewer_including": "n1) In terms of the landing site location, we have compared our results with those of Liu et al. (2024), Tong et al. (2025), and the LRO team. The main conclusion is that the CE-6 landing site location derived from LRO images is within the same order of magnitude, such as the results from the LROC team and Tong et al. (2024), while the coordinates derived from the CE2TMap2015 also show only about 1-pixel positional deviation, as seen in our results and those from Liu et al. (2024). The discrepancy between these two sets of coordinates reflects the positional deviation between the CE2TMap2015 and LRO terrain data on the farside of the Moon. For further details, please refer to the “Results: High-precision Positioning of the CE-6 Landing Site” section in the revised manuscript.\n2) In terms of the ejecta thickness, we compared our results with those of Qian et al. (2024), Yue et al. (2024), Gao et al. (2024), Xu et al. (2024), and Gou et al. (2024), and others. We adopted the classic Pike (1974) model, whereas other studies are primarily based on the models by Sharpton (2014) or Xie et al. (2020). A comparison of the results shows that our study closely aligns with the Sharpton (2014) model when calculating ejecta thickness for craters smaller than 17 km in diameter, but the differences become more significant for larger craters. Similarly, our results are consistent with the Xie et al. (2020) model for craters smaller than 19 km, while the discrepancies increase for larger craters. For example, for the crater Chaffee S with a diameter of 19.93 km, our calculation gives an ejecta thickness value of 5 cm, while other studies report 15-16 cm. We believe this discrepancy is due to the use of different models.\n\nPike (1974) model:\ne = 0.033R (r/R )−3.0\nt t\nR = 1.07√R /0.86 R >7.5\nt c c\nR =0.84R R <7.5\nt c c\nSharpton (2014) model:\ne = 0.014R 1.01(r/R )−3.0 R <8.5\nc c c\ne = 3.95R 0.399(r/R )−3.0 R >8.5\nc c c\nXie (2020) model:\ne = 0.033R (r/R )−3.0\nt t\nR =0.84×9.50.15R 0.85 R >9.5\nt c c\nR =0.84R R <9.5\nt c c\nWhere, e denotes the ejecta thickness, r denotes the distance from the crater center, Rc refers to the crater radius (in km), and Rt represents the crater transient radius (in km).\n\nReference:\nSharpton V. Outcrops on lunar crater rims: Implications for rim construction mechanisms, ejecta volumes and excavation depths. Journal of Geophysical Research: Planets. 119(1), 154-168 https://doi.org/10.1002/2013JE004523 (2014)\nXie, M., Liu, T., & Xu, A. (2020). Ballistic sedimentation of impact crater ejecta: Implications for the provenance of lunar samples and the resurfacing effect of ejecta on the lunar surface. Journal of Geophysical Research: Planets, 125(5), e2019JE006113. https://doi.org/10.1029/2019JE006113\n3) In terms of the age of potential ejecta provenance craters, we dated all the potential craters and compared the results with Xu et al. (2024). We calculated the age of the Chaffee S crater to be 2.39 Ga and the age of the White crater to be 3.26 Ga, while Xu et al. (2024) gave the ages to be 2.79 Ga and 3.48 Ga, respectively. The ages are relatively consistent between the two studies.\n4) In terms of the regolith thickness obtained by concentric crater method, we compared our results with those from Li et al. (2024), Huang et al. (2018), and Qian et al. (2021), and other studies. Our study estimates the regolith thickness at the CE-6 landing area range from 3.4 m to 8.5 m, while Li et al. (2024) obtained a range of 1.1 to 7.0 m. The two results are relatively close. Qian et al. (2021) estimated the regolith thickness at the CE-5 landing site to be 4-6 m, and Huang et al. (2018) estimated the regolith thickness at the CE-4 landing site to be 2.5-7.5 m. Additionally, Huang et al. (2018) suggested that the CE-4 regolith contains a layer of LCP material ejecta, bringing the total depth to 13 m. Based on these results, we conclude that the regolith thickness at the CE-6 landing site is comparable to that at the CE-5 landing site and shallower than at the CE-4 site.\n5) Following the reviewer's comments, we have removed the comparison of the roughness obtained in this study with the results of Zeng et al. (2023).\n\nAccepted. Following the reviewer's comments, we have revised the figures and tables in the manuscript. We increased the font size to make it more readable and enlarged some of the symbols. Additionally, we used colors that contrast with the background for better differentiation. We also added a north arrow to the perspective figures.\n\nRevised. “moon” has been changed to “Moon”.\n\nRevised. “resolution” has been changed to “spatial resolution”.\n\nAccepted. Comparisons have been supplemented in the section of “Results: High-precision Positioning of the CE-6 Landing Site”.\nThe main conclusion is that the CE-6 landing site location derived from LRO images is within the same order of magnitude, such as the results from the LROC team and Tong et al. (2024), while the coordinates derived from the CE2TMap2015 also show only about 1-pixel positional deviation, as seen in our results and those from Liu et al. (2024). The discrepancy between these two sets of coordinates reflects the positional deviation between the CE2TMap2015 and LRO terrain data on the farside of the Moon.\n\nThis is calculated based on the two coordinate sets of '153.9776°W, 41.6251°S, -5273 m' and '153.9852°W, 41.6385°S, -5256 m'. The positional deviation between these two coordinates is 172.2 m in longitude, 406.3 m in latitude, and 17.0 m in elevation. Therefore, the horizontal positional deviation is 441.3 m, and the three-dimensional positional deviation is 441.7 m.\n\nIn the revised manuscript, Fig 2 has been split into six panels to comply with the journal's formatting requirements, denoted as Fig 2a to 2f. The north direction has been added in Fig. 2b, 2c and 2e, and a scale bar has been added in Fig. 2f according to the reviewer's comments.\n\nThe sentence has been rewritten as “Based on CE6_LCAM_DEM data, the average slope of the area around the landing site (within the range shown in Fig. 3a) was calculated at a baseline of 1-meter” in the revised manuscript.\n\nThe inappropriate comparison in this section has been removed according to the reviewer's comments.\n\nThe calculation methods for bidirectional slope and RMS height have been added according to the reviewer's comments. For details, please refer to the \"Methods\" section in the revised manuscript.\n\nThe bidirectional slope and RMS height data for the CE-4, CE-5, and CE-6 landing areas have been summarized in a table as follows.\nMaximum Median Maximum RMS Median RMS\nbidirectional bidirectional height (mm) height (mm)\nslope(°) slope(°)\nCE-6 73 6 25 3\nCE-4 40 10 18 4\nCE-5 78 7 43 3\n\nThese symbols have been revised according to the reviewers' comments. Green dots have been changed to red dots for better visibility.\n\nA distribution map of these craters (Supplementary Fig.1) has been added to the supplementary materials 1 following the reviewers' comments. Detailed information on the geographical location, size, and other data of these craters can be found in the “Data introduction” document.\n\nThe sentence has been rewritten as “Based on CE6_LCAM_DEM data, the average depth-to-diameter ratio of fresh craters (i.e., craters with ejecta clearly visible at their rims), with diameter ranging from 2 meters to about 30 meters, is approximately 0.214” in the revised manuscript.\n\nThe sentence has been rewritten as “Based on CE2TMap2015 DEM, the average depth-to-diameter ratio of fresh craters, with diameters ranging from 80 meters to about 300 meters within an area of 9 km × 8 km around the CE-6 landing site, is ~0.050.\n\nThe revised manuscript has added locations to the supplementary materials. Line 194, 197, and 214 in the original manuscript correspond to supplementary materials 1, 2, and 3, respectively.\n\nWe agree with the reviewer’s opinion that it is difficult to determine whether the penetrating radar has detected the bottom of lunar regolith. The regolith layer constrained by the radar method is not the same as that constrained by the concentric crater method. Here we just reference the layered interpretation results based on radar echo interface methods from existing works.\nFollowing the reviewer's comments, we have supplemented the regolith thickness statistics for the CE-4 and CE-5 landing areas, as calculated by Huang et al. (2018) and Qian et al. (2021) using the same concentric craters method. Among them, Huang et al. (2018) calculated the regolith thickness at the CE-4 landing area to be 2.5–7.5 m. However, Huang considered that this thickness represents the surface fine-grained regolith thickness at the CE-4 landing site, with an additional layer of ejecta mixed with LCP materials beneath it, approximately 13 meters deep. Combining this with the interpretation of the CE-4 radar data, which suggests a surface regolith thickness of 12 meters, we believe that a regolith thickness of around 12 to 13 meters for CE-4 landing area is reliable. Qian et al. (2021) calculated the regolith thickness at the CE-5 landing area to be 4–6 m. More recently, Li et al. (2024) used the same method to calculate the regolith thickness at the CE-6 landing area, and they obtained a range of 1.1 to 7.0 meters. Following the reviewer's suggestion, we re-examined the concentric crater data and found that two craters near the landing site, with regolith thickness estimates of 0.38 and 0.40 m, should not be included in the regolith thickness statistics. These values might be the thickness of the ejecta blanket of crater C1. Additionally, we removed some craters with less distinct concentric structures. Finally, we narrowed the statistical range of concentric craters to a range similar to that in Li et al. (2024). Based on this updated dataset, our revised regolith thickness results at the CE-6 landing area range from 3.4- 8.5 m. The updated results are close to those of Li et al. (2024). Overall, we believe that the thickness of the lunar regolith at the CE-6 landing area should be between 3.4- 8.5 m, which is comparable to that of CE-5 landing area and shallower than that of CE-4 landing area.\n\nWe agree with the reviewer’s opinion. We have now included a comparison of the regolith thickness obtained using the same method and found that the regolith thickness at the CE-4, CE-5, and CE-6 landing sites is comparable when using the concentric crater method. The original manuscript’s speculation about the relationship between regolith thickness and geological age was not accurate. We have removed this conclusion in the revised version.\n\nWe believe this discrepancy may result from differences in statistical scale. The rock abundance of 0.14% in our study was conducted within a 100 m×100 m range using cm-level resolution data, while the 0.31% result was obtained from Diviner data with a 200m resolution over a 10km×10km range. However, a comparison between the results based on Diviner data and our findings shows that the relative trends in rock abundance at the CE-4, CE-5, and CE-6 landing sites are consistent across different scales.\n\nWe based our judgment on the small crater morphology observed in the CE-6 PCAM images. These small craters are relatively shallow and have significant debris around their rims. Based on these morphological features, we hypothesize that these craters may be secondary craters. Of course, this is only a hypothesis, and to avoid ambiguity, we have revised this section in the manuscript to refer to them as \"possible secondary craters.\"\n\nReferences have been supplemented in the revised manuscript as Ref. 2.\n\nWe based our judgment on the small crater morphology observed in the CE-6 PCAM images. These small craters are relatively shallow and have significant debris around their rims. Based on these morphological features, we hypothesize that these craters may be secondary craters. Of course, this is only a hypothesis, and to avoid ambiguity, we have changed \"ejecta in images from CE-6 PCAM\" to \"possible ejecta in images from CE-6 PCAM\" in the revised manuscript.\n\nRevised. “related” has been changed to “transported”.\n\nThe symbols have been bolded for clearer visibility.\n\nWe agree with the reviewer’s opinion. Qian et al. (2024) and Jia et al. (2024) both mentioned that ejecta from crater White might be present at the CE-6 landing site. However, the returned samples indicate a geological age of 2.8 Ga at the CE-6 landing area, while the most recent study by Xu et al. (2024) dated White crater to be approximately 3.48 Ga. We also conducted dating of the White crater, and the result is 3.26 Ga. Therefore, the ejecta from White crater should have formed prior to the eruption of CE-6 mare basalts. Based on this, we have excluded the possibility of the White crater being the provenance of the ejecta at CE-6 landing area in the revised manuscript. This section has been rewritten accordingly in the revised manuscript.\n\nAccepted. The northwest-southeast and northeast-southwest ejecta streaks have been marked with dashed lines in Fig. 4c.\n\nRevised. The geological characteristics here specifically include surface roughness, depth-to-diameter ratio of fresh craters, regolith thickness, and rock abundance.\n\nThese ages have been used and references have been supplemented in the revised manuscript as Ref. 3/4.\n\nAccepted. The geological age of the CE-4 landing area has been well studied by many researchers, and relevant references, including Huang et al. (2018), have been cited accordingly in the revised manuscript.\n\nWe have provided further explanation in the following paragraph.\n“Previous studies have suggested that, at the kilometer scale, ancient highland regions with longer exposure times exhibit significantly greater surface roughness compared to the younger mare regions 44-46. By contrast, at the meter scale or smaller, the difference in surface roughness between highlands and mare regions becomes less pronounced 25. Our statistical results further show that, at the millimeter to centimeter micro-scale, the surface roughness of CE-6 landing area is quite similar to that of CE-5 landing area but lower than that of CE-4 landing area. However, the differences among the three areas are minimal. This indicates that even within mare regions of different exposure ages, the variation in surface roughness is also quite small, suggesting that the influence of exposure time on surface roughness is not significant”\n\nReferences have been supplemented in the revised manuscript as Ref. 44-46.\n\nThe reviewer’s understanding is completely correct. The shallow depth of crater is not only related to the degradation time, but also to the initial thickness and looseness of the surface regolith. In the revised manuscript, this sentence has been rewritten as “A likely explanation for this phenomenon is that the shallow depth of these craters is controlled by the thickness and looseness of the surface regolith in these areas.”\n\nPrevious studies have suggested that the geological age of the mare basalts at the CE-4 landing area is approximately 3.6-3.7 Ga (Huang, et al, 2018; Ling et al., 2019), while the geological age of crater Finsen is 3.5 Ga (Gou et al., 2021). Our dating result for crater Finsen is also consistent with this result. Therefore, although the CE-4 landing area is modified by later ejecta from Finsen crater, the similar geological ages of Finsen crater and the mare basalts at the CE-4 landing site suggest that the modified surface still retains an ancient exposure age. According to the latest dating results of CE-6 sample (2.8Ga), this sentence has been updated in the revised manuscript as “0.8-0.9 Ga younger than that of the CE-4 landing area”.\n\nWe carefully checked the previously identified rim crests (black dots in Fig. 3a) and removed certain dots that should not be considered as part of the C1 crater rim crests. For example, the western rim crests might have been affected by squeezing of the adjacent crater, while parts of the southern rim crest are likely to be influenced by small craters. Based on the new rim crests, we refitted the rim of crater C1. The newly fitted diameter of C1 crater is 50.95 m, and its center is now 26.93 m from the CE-6 landing site. From the updated Fig. 3a, it can be seen that the CE-6 landing site is located outside the rim crest, which is identified in the terrain data. Therefore, the newly fitted C1 crater rim is more reliable. Based on these updated data, we recalculated the ejecta thickness of C1 crater at the landing site using the formula from different ejecta thickness models, obtaining a value of 0.31- 0.35 m.\n\nWe have used a different ejecta thickness model compared to other studies. We adopted the classical Pike (1974) model, whereas other studies used different models such as Sharpton (2014) or Xie et al. (2020). For example, our calculated ejecta thickness from crater Chaffee S is approximately 5.4 cm, whereas Qian et al. (2024, ApJL) reported 15.1 cm, Yue et al. (2024, The Innovation) obtained 16.2 cm, Xu et al. (2024, GRL) calculated 15.6 cm, Gao et al. (JGRP) estimated around 16.4 cm, and Gou et al. (2024, EPSL) determined approximately 16.6 cm. This discrepancy is primarily due to the use of different models. In the Sharpton (2014) model, a new formula is applied for calculating ejecta thickness when the crater diameter exceeds 17 km. Since the diameter of crater Chaffee S is 19.93 km—above this threshold—the resulting ejecta thickness is increased.\nWe believe that both the Pike (1974) model and models like Sharpton (2014), Xie (2020) are all reasonably valid. Using these kinds of models, the calculated ejecta thickness from the nearby crater C1 at the landing site is 0.354m, 0.312 m, and 0.354m, respectively, showing close agreement. The ejecta abundance in the 0–1-meter depth range by BSM simulation is 30- 35%. In addition, using the Pike (1974) model, the calculated ejecta thickness from Chaffee S crater is approximately 5 cm, while using Sharpton (2014) model and Xie (2020), the result is approximately 16 cm. Based on the current research on CE-6 samples, it is difficult to determine which results is more accurate. Therefore, we provided the ejecta thickness range of craters that may have affected the landing site based on these results in the revised manuscript for future reference in sample research.\n\nAccepted. The relevant references including Qian et al. (2024) and Wang et al. (2024) have been supplemented in the revised manuscript.\n\nThis section has been removed in the revised manuscript to avoid redundancy.\n\nAccepted. Thank you for the reviewer’s reminder. We have conducted dating of the White crater, and have excluded the possibility of White crater being the provenance of the ejecta at CE-6 landing area in the revised manuscript.\n\nThe PCAM is located at the top of the CE-6 lander, while the LCAM is positioned at the bottom. We have added the locations of these payloads in the perspective view of CE-6 lander in Fig. 1.\n\nWe have supplemented the detailed methodology and processing procedures for CE6_LCAM_DOM and CE6_LCAM_DEM in the “Method: High-precision terrain data development and CE-6 Landing Site positioning\" section of the revised manuscript.\n\nWe have supplemented the detailed methodology and processing procedures for CE6_PCAM_DOM and CE6_PCAM_DEM in the “Method: High-precision terrain data development and CE-6 Landing Site positioning\" section of the revised manuscript.\n\nThe seven coordinate transformation parameters include three translation parameters, three rotation parameters, and one scale parameter. This has been supplemented in the revised manuscript.\n\nAccepted. The data has been organized and their IDs have been provided, as detailed in the “Data introduction” document.\n\nThis is due to the characteristics of our data. We used LCAM_DOM in this work for rock identification. The data near the landing site has a higher effective resolution, allowing us to identify more and smaller rocks (with diameters of a few centimeters, marked by the blue box in Supplementary Fig.3). However, as the distance from the landing site increases beyond 100 m, the effective resolution of the data gradually decreases, and only larger rocks (with diameters of several centimeters) can be identified.\n\n1）Line 1, “Micro-sacle”, find a more representative term to summarize \"millimeter to hundreds of meters\"", "reviewer_2015": "nhttps://astrogeology.usgs.gov/search/map/kaguya_lunar_multiband_imager_mi_derived_feo_weight_percent_50n50s_512ppd_59mpp (also possible to see on Quickmap).\nYou could also cite Li et al. (2024) who present the nature of the returned samples: \"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...\"\nhttps://pmc.ncbi.nlm.nih.gov/articles/PMC11495410/"}, "author_response": ""}]

Decision letter