On 3 January 2019, China's Chang'E-4 (CE-4) successfully landed on the eastern floor of Von Kármán crater within the South Pole-Aitken Basin, becoming the first spacecraft in history to land on the Moon's farside. Here, we report the observations made by the Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover during the first two lunar days. We found a signal penetration at the CE-4 landing site that is much greater than that at the CE-3 site. The CE-4 LPR images provide clear information about the structure of the subsurface, which is primarily made of lowloss, highly porous, granular materials with embedded boulders of different sizes; the images also indicate that the top of the mare basal layer should be deeper than 40 m. These results represent the first high-resolution image of a lunar ejecta sequence ever produced and the first direct measurement of its thickness and internal architecture.

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The Chang'E-4 landing site is depleted with boulders seen from both orbit and surface.However, the Yutu-2 rover came across thousands of concreted fragments in and around an abnormally fresh crater that has more elevated northwestern rims. The origin of the fragments is crucial to resolve the provenances of surface materials detected by the rover. The lunar penetrating radar performed two in-and-out scans for the blocky ejecta, revealing that the subsurface materials have indistinguishable radar permittivity with the surrounding regolith. Forward modeling of electromagnetic wave propagation shows that the fragments were not an original component in the subsurface. This crater is among the several fresh craters photoed by the rover, and they are located in an eastern extension ray of the Zhinyu crater. The small craters are likely secondaries of Zhinyu, and the fragments contain a mixture of shattered projectiles and most likely compacted regolith clumps formed during the secondary impacts.Plain Language Summary The Chang'E-4 landing site lacks boulders seen from both orbit and surface, but several abnormally fresh craters were encountered by the Yutu-2 rover. The blocky rims and interiors are in sharp contrast with the heavily degraded crater population in the landing area. These craters are less than 3 m in diameter, and the fragments are less than 10 cm long and appear earthy in color. The origin of the fragments needs an explanation because it is the basis for the interpretation of data returned by both the lunar penetrating radar and reflectance spectrometer onboard the rover. At the ninth lunar day, the rover was carefully driven into the blocky ejecta deposits of such a crater, and ground-penetrating radar detections were conducted. Radar measurements reveal that the physical properties beneath the blocky ejecta deposits are essentially the same with that of the surrounding regolith, and the observed fragments were not originated from the subsurface. The high spatial density, more pronounced northwestern rims, and colocation with an impact ray suggest that the small craters are most likely secondaries formed by the Zhinyu crater. The fragments are mainly formed by impact compaction of the preimpact target regolith, and the shattered secondaries-forming projectiles are a minor component.

Chang'e-3 (CE-3) has been the first spacecraft to soft land on the moon since the Soviet Union's Luna 24 in 1976. The spacecraft arrived at Mare Imbrium on December 14, 2013, and the same day, Yutu lunar rover separated from lander to start its exploration of the surface and the subsurface around the landing site. The rover was equipped, among other instruments, with two lunar penetrating radar systems having a working frequency of 60 and 500 MHz. The radars acquired data for about two weeks while the rover was slowly moving along a path of about 114 m. At navigation point N0209, the rover got stacked into the lunar soil and after that only data at a fixed position could be collected. The low-frequency radar data have been analyzed by different authors and published in two different papers, which reported totally controversial interpretations of the radar cross sections. This paper is devoted to resolve such controversy by carefully analyzing and comparing the data collected on the moon by Yutu rover and on earth by a prototype of LPR mounted onboard a model of the CE-3 lunar rover. Such analysis demonstrates that the deep radar features previously ascribed to the lunar shallow stratigraphy are not real reflectors, rather they are signal artifacts probably generated by the system and its electromagnetic interaction with the metallic rover.

Radargrams obtained by the Yutu‐2 rover reveal that the post‐mare deposits at the Chang'E‐4 landing site are ∼45 m thick, consistent with estimations based on orbital observations. Besides obvious ejecta from Finsen, polarized interpretations exist on whether or not the Alder crater is another contributor to the post‐mare deposits, although predicted thicknesses of ejecta from all potential source craters are not adequately large compared to observations. We recognize that the rover has been exploring along a shared crater wall of two secondaries from Finsen. Mechanics of secondary impacts suggests that the discontinuous layers of coarse and fine materials observed in the radargram are older than Finsen. Cross‐cutting relationships and crater density comparisons show that Alder is older than the mare basalts, and the Orientale basin was the major source for the post‐mare materials. Lower‐crust materials excavated by Orientale constituted a substantial portion of the surface regolith detected by the rover.

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Accurate relative permittivity is essential to the further analysis of lunar regolith. The traditional hyperbola fitting method for the relative permittivity estimation using the lunar penetrating radar generally ignored the effect of the position and geometry of antennas. This paper proposed a new approach considering the antenna mounting height and spacing in more detail. The proposed method is verified by numerical simulations of the regolith models. Hence the relative permittivity of the lunar regolith is calculated using the latest high-frequency radar image obtained by the Yutu-2 rover within the first 24 lunar days. The simulation results show that the relative permittivity is underestimated when derived by the traditional method, especially at the shallow depth. The proposed method has improved the accuracy of the estimated lunar regolith relative permittivity at a depth of 0–3 m, 3–6 m, and 6–10 m by 35%, 14%, and 9%, respectively. The thickness of the lunar regolith at the Chang’E 4 landing site is reappraised to be 11.1 m, which improved by ~8% compared with previous studies.

The lunar penetrating radar (LPR) onboard the Chinese Chang'e-3 (CE-3) mission obtained high-resolution profile data for the continuous ejecta deposits of the Ziwei crater. Geological background suggests that the continuous ejecta deposits contain few large boulders, and the ejecta deposits were largely originated from the pre-impact regolith. Using the top~50 ns of radar data, we estimate the bulk density and porosity for the ejecta deposits based on hyperbolic echo patterns in the radargram that are caused by subsurface boulders. The physical properties are close to those of typical lunar regolith. Numerous subparallel and discontinuous short layers are visible in the radargram of the continuous ejecta deposits. The dielectric coefficients of the layering structures are estimated, and their permittivity is slightly larger than that of typical lunar regolith and less than that of basaltic rocks. Cratering physics together with the geological context of this area suggest that the layering structures are most likely ground gravels and/or melt-welded breccias that were sheared due to the horizontal momentum of the impact ejecta. This interpretation is indicative of the origin of the enigmatic layering structures in regolith core samples returned by the Apollo and Luna missions. The results also highlight the importance of ejecta emplacement in shaping the structure of lunar regolith. Plain Language Summary Large discrepancies existed in previous geological interpretations using the Chang'e-3 high-frequency LPR data. A comprehensive review of previous studies that used the Chang'e-3 radar data noticed that most previous studies agree that the top~50 ns of the radargram is restricted within the continuous ejecta deposits of the Ziwei crater. Analyses for the stratigraphy of the landing site suggested that the continuous ejecta deposits were largely composed by pre-impact regolith. Using the high-frequency LPR data, we reconstructed the depth profiles of physical parameters (i.e., relative permittivity, bulk density, and porosity) for the continuous ejecta deposits of the Ziwei crater, which are similar with those of typical lunar regolith. Many subparallel and discontinuous layers are observed in the radargram, which were not deciphered before. We carried out numerical simulations to study the nature of the layering features. Results suggest that these structures have a permittivity slightly larger than that of typical lunar regolith. Geological context of the landing site suggested that the layering structures are likely shear bands within the continuous ejecta deposits, and they may be composed by ground rock fragments and/or melt-welded breccia, which were laterally deformed due to the horizontal momentum of the ejecta deposits.