Dielectric properties and stratigraphy of regolith in the lunar South Pole-Aitken basin: Observations from the Lunar Penetrating Radar

Aims. We examine data obtained by the Lunar Penetrating Radar (LPR) onboard the Chang'E-4 (CE-4) mission to study the dielectric properties and stratigraphy of lunar regolith on the far side of the Moon. Methods. The data collected from January 2019 to September 2020 were processed to generate a 540 m radargram. The travel velocity of the radar signal and the permittivity of the regolith were deduced from hyperbolas in the radargram. As CE-4 LPR detected distinct planar reflectors, we evaluated the dielectric loss from the maximum penetration depth based on the radar equation. The derived dielectric properties are compared with the measurements of Apollo samples and Chang'E-2 microwave radiometer observations. Results. The results suggest that regolith at the landing site has a permittivity of 2.64-3.85 and a loss tangent of 0.0032-0.0044, indicating that the local regolith is composed of a fine-grained, low-loss material that is much more homogeneous than that found at the Chang'E-3 landing site. The total thickness of weathered material is similar to 40 m, with several regolith layers and a buried crater identified in the reconstructed subsurface structure. Conclusions. These layers clearly record a series of impact events from the adjacent regions. We suggest that the top layer is primarily made up of the ejecta from a large crater 140 km away. In contrast, the material source of other thinner layers comes from nearby smaller craters.