A Novel Radar Processing Tool for Estimating the Permittivity Profile of the Shallow Lunar Ejecta: A Case Study at the Von Kármán Crater

<p>On 3<sup>rd</sup> of January 2019, the Lunar probe Chang’E-4 landed at Von Kármán (VK) crater at South-Pole Aitken (SPA) crater. The transient cavity of SPA has been estimated at 840-1400 km, which implies that the SPA basin excavated through the Lunar’s crust and into the mantle. Due to that, the geology of the area has attracted a lot of interest, since mantle materials can provide useful insights on the mineralogical composition of the upper mantle and the formation of the Moon.</p><p>Lunar Penetrating Radar (LPR) has been applied for both satellite and in situ measurement configurations resulting to fruitful insights regarding the dielectric structure of the Moon. The Yutu-2 rover from the Chang’E-4 mission is equipped with a low-frequency (60 MHz) and two high-frequency (500 MHz) antennas. Previous research [1] using the high-frequency data from the Yutu-2 rover, concluded that a homogenous ~12 m weathered layered overlays the ejecta from the near-by Finsen crater. This model is based on typical hyperbola-fitting and the lack of layers on the measured radagram for the first ~150 ns [1].   </p><p>Typical hyperbola-fitting is not suitable for complex media with varying permittivity with depth. To mitigate that, we propose a novel interpretation tool that fits multiple hyperbolas simultaneously by estimating the optimum one-dimensional permittivity profile. The suggested scheme is successfully validated via a series of numerical experiments and subsequently applied to the data acquired by the Yutu-2 rover during the first two Lunar days of the mission. Four distinct layers were identified in the first ~12 m that were previously non-visible due to their smooth dielectric boundaries. This differs from previous results [1] where the first ~12 m are assumed homogeneous, part of the weathered fine-grained regolith of the Finsen crater. Based on these results, we suggest a new stratigraphic model in which the ejecta of VK L' (~ 5.5 m) were deposited on top of the Finsen ejecta. Space weathering degraded the first ~1.5 m of the ejecta decreasing its density and electric permittivity. The ejecta from VK L were subsequently deposited on top of the weathered layer creating a top layer with ~6 m width. The long weathering process, from early Eratosthenian till now, gave rise to a ~3 m of loose Lunar soil with low electric permittivity. The suggested model is consisted with the LROC NAC images [2], the expected Lunar weathering rates [3] and the mineralogical content of the area [2].</p><p><strong>References </strong></p><p>[1] Zhang, L., Li, J., Zeng, Z., Xu, Y., Liu, C., & Chen, S, (2020), Stratigraphy of the Von Kármán crater based on Chang’E-4 lunar penetrating radar data. Geophysical Research Letters, 47.</p><p>[2] Huang, J., Xiao, Z., Flahaut, J., Martinot, M., Head, J., Xiao, X., & et al. (2018), Geological characteristics of Von Kármán crater, northwestern South Pole-Aitken basin: Chang’E-4 landing site region, Journal of Geophysical Research: Planets, 123, 1684-1700.</p><p>[3] Gou, S., Yue, Z., Di, K., Cai, Z., Liu, Z., & Niu, S. (2021), Absolute model age of Lunar Finsen crater and geologic implications, Icarus, 354, 114046.</p>