Sub-surface stratification and dielectric permittivity distribution at the Chang'E-4 landing site revealed by the lunar penetrating radar

Context. In 2019, China's Chang'E-4 (CE-4) probe landed on the far side of the Moon: a first in lunar exploration. The Lunar Penetrating Radar (LPR) mounted on the Yutu-2 rover allows the mapping of the near-surface structure and the dielectric permittivity of the landing area. The dielectric properties of the lunar soil affect the propagation of the LPR signals, which can be used to infer the depth of sub-surface boundaries and derive the composition of the component materials. Aims. Our objectives are to estimate the fine-resolution spatial distribution of relative permittivity and to improve the interpretation of the geological processes combined with the radargram of the CE-4 landing area. Methods. We used a modified method that combines the F-K migration and the minimum entropy of the ground penetrating radar (GPR) signals to estimate the velocity and permittivity values; this has the advantage of obtaining the appropriate velocity and permittivity, even with the incomplete or unnoticeable hyperbolic curves in the radar image Results. The sub-surface stratification of the CE-4 landing area is seen in the first 31 lunar days of the LPR data. A fine-resolution dielectric permittivity profile ranging from similar to 2.3 to similar to 6.3 is obtained with our method, and the actual depths of the observed prominent sub-surface interfaces are determined, giving a maximum average depth of similar to 38 m. The thickness of the regolith layer is in the range of similar to 5.7-15.6 m, with an average of 11.8 m. The permittivity of the near-surface regolith (< 30 cm) is similar to 2.78 +/- 0.01, the bulk density is 1.57 +/- 0.01 g cm(-3), which is close to the results of similar to 1.61 g cm(-3) at the Apollo 15 landing area. The permittivity map is consistent with the radargram; the regolith and the paleo-regolith layer have relatively low permittivity and low echo strengths, while the rock debris has high permittivity and shows strong echos in the radargram. Two buried craters of different diameters beneath the navigation sites 4-11 and 16-31 are revealed in the radar profile. The permittivity distribution map can show detailed variations of material properties both inside and outside craters.