Subsurface Dielectric Permittivity and Structure Along Chang’E-4 Rover's 42-Lunar-Day Traverse Using Diffraction Focusing Methods

Abstract

China's Chang'E-4 probe successfully soft-landed on the lunar far side at Von Kármán crater in January 2019. Onboard, the Lunar Penetrating Radar (LPR) detected subsurface structures and properties such as dielectric permittivity, aiding in our understanding of regolith composition and origin. This study introduces an automatic method for estimating dielectric permittivity using radar diffraction focusing analysis. While developed from traditional seismic analysis, the method is tailored for LPR data by incorporating random noise removal, entropy-based focusing, and lunar-specific parameter optimization to address the challenges of noise interference, complex diffraction overlapping and applicability to LPR data. Applied to Chang'E-4's first 42 lunar-day data, this method revealed subsurface structure and permittivity distributions. Combined with instantaneous amplitudes, centroid frequencies, and geological features, we present a preferred geological interpretation. Our findings suggest that, after experiencing the latest basaltic magma intrusion and subsequent weathering, the region underwent multiple episodes of high ilmenite content ejecta deposition at a depth of 20–33 m. This was followed by several impacts, leaving craters visible today. Subsequently, the area was overlain by at least two ejecta layers with low ilmenite content at a depth of 13–25 m. Later, two meteoroids struck the paleo-surface; the larger one created the most prominent irregular crater on the current surface. Thereafter, ejecta from nearby craters such as Finsen and Von Kármán L covered the area, weathering into the current lunar regolith. Our inversion results demonstrate high reliability, align with previous studies and geological context, and can offer methodological and empirical insights for future planetary missions.