Dielectric properties of the lunar regolith estimated through a combined loss tangent and EM velocity distribution analysis of the Chang'e-4 GPR data set

We study the dielectric properties of the lunar near-surface across a 1.43 km long 2-D Ground Penetrating Radar (GPR) data set, that was acquired by the still-operational Yutu-2 surface rover as part of the Chang'e-4 lunar mission of 2019. The surveyed area is part of the Eastern Floor of the Von Kármán crater, which is located in the northwestern sector of the South Pole–Aitken Basin, on the far side of the Moon. The total attenuation and intrinsic lost tangent are mapped across the entire GPR profile using time-domain amplitude decay and centroid frequency downshift methods, respectively. The cumulative electromagnetic (EM) velocity distribution is obtained by analyzing 136 automatically tracked hyperbolic diffractions spread across the data set, down to a time-depth of ∼200 ns. The resulting vertical trends within three different sections of the GPR profile are then fitted using the Deming regression, in order to estimate the interval EM velocities in the lunar regolith, and subsequently the average electric permittivities, down to a depth of ∼13 m. The novel EM velocity analysis is designed to account for both the GPR antenna height, possible out-of-plane diffractions, and the radii of the scatterers, all of which can significantly affect the reconstructed cumulative EM velocity distribution. We further compare our inversion results with other lunar subsurface models from the available literature, and critically discuss both the advantages and limitations of the applied methodologies, while also taking into account the expectably less-than-ideal survey conditions of the Chang'e-4 mission.