Understanding the shallow subsurface structure of the lunar mare is crucial for unraveling the history of basaltic volcanism and the Moon's thermal evolution. The thickness and area of basalt layers constrain lava eruption volumes. Previous research using remote sensing data (impact crater morphology, gravity data, reflectance spectra) has contributed to models of lunar evolution, but ground-penetrating radar (GPR) provides higher resolution data. The Apollo Lunar Sounder Experiment and the Lunar Radar Sounder on Kaguya provided initial subsurface data, but the LPR on Chang'e-3 and Chang'e-4 rovers offer significantly higher range resolution (1–2 m in mare basalt for the 60 MHz channel), enabling detailed shallow subsurface surveys. Chang'e-4's landing in the South Pole-Aitken (SPA) basin, the largest impact structure on the Moon, provides an ideal location to address key geological questions, particularly concerning the basin's formation and volcanic history. The asymmetric distribution of volcanic materials between the lunar nearside and farside is thought to relate to differences in crustal thickness, radioactive element abundance, or the geological consequences of the SPA-forming impact. This study presents LPR results from the first 9 months of the Chang'e-4 mission, using data from the 60 MHz channel (CH-1) and simulations to interpret the subsurface stratigraphy and infer the local geological history of the landing site.

Numerous studies have investigated the lunar subsurface using various methods. Remote sensing techniques, such as analyzing impact crater morphology, high-resolution gravity data, and reflectance spectra of crater ejecta deposits, have provided valuable insights into lunar evolution and the distribution of basaltic materials. Spaceborne radar experiments, like the Apollo Lunar Sounder Experiment and the Kaguya Lunar Radar Sounder, have detected deep subsurface reflectors, interpreted as interfaces between mare and bedrock or distinct basaltic rock layers. However, these experiments had limitations in range resolution. The Chang'e-3 and Chang'e-4 missions incorporated a Lunar Penetrating Radar (LPR) with significantly improved resolution, providing an unprecedented opportunity to investigate the shallow subsurface structure in greater detail. Previous analyses of Chang'e-3 LPR data have revealed subsurface layering in the nearside maria. Studies on the Chang'e-4 landing site region have used remote sensing data to infer the geological characteristics of Von Kármán crater, but direct subsurface data were lacking. This paper builds upon these prior investigations by providing the first detailed analysis of high-resolution LPR data from the Chang'e-4 landing site.

The study utilizes data from the 60 MHz channel (CH-1) of the Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover during its first nine months of exploration. The data were processed to remove noise, apply filtering, and perform amplitude compensation. Subsurface reflectors were identified by analyzing both the radargrams and aggregated data traces from waypoints along the rover's path. The actual depth of subsurface reflectors was calculated using the two-way travel time of the radar signal and the permittivity of lunar basalt. To test interpretations, the researchers conducted LPR simulations using a two-dimensional finite-difference time-domain method (gprMax). Various subsurface models with different loss tangent and permittivity values were used to generate simulated radargrams, which were then compared to the observed data. Trend surface analysis was employed to assess the large-scale systematic variations in subsurface layer depths. The thickness of ejecta deposits from nearby impact craters was estimated using empirical formulas relating ejecta thickness to crater radius and distance from the crater. The study also incorporated existing geological maps and spectral data to refine the interpretation of the LPR data.

The LPR data revealed a complex subsurface stratigraphy at the Chang'e-4 landing site. Several prominent, relatively continuous horizontal reflectors (A-E) were identified at depths ranging from approximately 52 m to 226 m. The reflectors are interpreted as interfaces between distinct layers, primarily consisting of mare basalts and ejecta from nearby impact events. At least four layers of distinct lava flows were identified, suggesting multiple lava-infilling events within the Von Kármán crater. The thicknesses of these layers range from 12 m to approximately 100 m. The analysis indicates that the subsurface layers are relatively uniform between the major reflectors, with some scattered features interpreted as small-scale ejecta deposits or thin regolith layers. The simulations using different loss tangent values demonstrate the sensitivity of penetration depth and the clarity of subsurface reflectors to this parameter. The best-fit loss tangent for the mare basalts is estimated to be between 0.0040 and 0.0061. Trend surface analysis of reflector depths suggests possible lava flow sources to the west of the lander and a potential ejecta source to the east. The deepest strata (E) is interpreted as large-scale ejecta deposits, likely from the Imbrium impact event or a combination of Imbrium and Orientale impacts. The estimated thickness of the combined basalt layers is consistent with previous estimates from spectral analysis of crater ejecta, but suggests a more complex and punctuated history of lava flows. The study also identified evidence for at least three distinct layers in ejecta from the nearby Zhinyu crater, further supporting the interpretation of multiple layered subsurface structures.

The findings address the research question by providing direct subsurface evidence of multiple lava-infilling events within the Von Kármán crater. This supports the hypothesis of a complex and punctuated volcanic history in the South Pole-Aitken basin, contrasting with previous assumptions of a relatively uniform mare infill. The results highlight the power of high-resolution ground-penetrating radar in revealing the detailed stratigraphy of the lunar surface. The identification of multiple basalt layers provides constraints on the timing and volume of volcanic activity, which can be integrated with other geological and remote sensing data to improve our understanding of lunar thermal evolution and the processes that shaped the lunar surface. The estimated loss tangent provides valuable information about the physical properties of mare basalts on the lunar farside, which can be compared to nearside data and used in future geophysical modeling of the Moon. The results underscore the heterogeneity of the lunar subsurface and the importance of considering both volcanic and impact processes in interpreting the geological record.

This study presents the first detailed subsurface stratigraphic structure of the lunar farside based on ground-penetrating radar data from the Chang'e-4 mission. The findings reveal a complex layered structure comprising multiple lava flows and ejecta deposits from different impact events. At least four major lava-infilling events were identified, suggesting a more complex and punctuated volcanic history than previously thought. The study also provides new estimates of the loss tangent for mare basalts on the lunar farside. Future work could involve integrating these findings with other datasets (geochemical, thermal modeling) to further refine our understanding of lunar evolution and the processes that shaped the South Pole-Aitken basin. Higher resolution GPR surveys could also reveal additional details of the subsurface structure.

The study is limited by the spatial extent of the Yutu-2 rover's traverse. The interpretations are based on a relatively short path, and extrapolating these findings to the entire Von Kármán crater requires caution. The resolution of the 60 MHz channel might limit the identification of very thin layers or small-scale features. The interpretation of the subsurface structure relies on assumptions about the physical properties of lunar materials, which might have some uncertainties.