A Novel Radar Processing Tool for Estimating the Permittivity Profile of the Shallow Lunar Ejecta: A Case Study at the Von K&#225;rm&#225;n Crater
Authors: Giannakis Iraklis , Zhou Feng , Warren Craig , Giannopoulos Antonios 
Published: March 2021

&lt;p&gt;On 3&lt;sup&gt;rd&lt;/sup&gt; of January 2019, the Lunar probe Chang&amp;#8217;E-4 landed at Von K&amp;#225;rm&amp;#225;n (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&amp;#8217;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.&lt;/p&gt;&lt;p&gt;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&amp;#8217;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].&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;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].&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1] Zhang, L., Li, J., Zeng, Z., Xu, Y., Liu, C., &amp; Chen, S, (2020), Stratigraphy of the Von K&amp;#225;rm&amp;#225;n crater based on Chang&amp;#8217;E-4 lunar penetrating radar data. Geophysical Research Letters, 47.&lt;/p&gt;&lt;p&gt;[2] Huang, J., Xiao, Z., Flahaut, J., Martinot, M., Head, J., Xiao, X., &amp; et al. (2018), Geological characteristics of Von K&amp;#225;rm&amp;#225;n crater, northwestern South Pole-Aitken basin: Chang&amp;#8217;E-4 landing site region, Journal of Geophysical Research: Planets, 123, 1684-1700.&lt;/p&gt;&lt;p&gt;[3] Gou, S., Yue, Z., Di, K., Cai, Z., Liu, Z., &amp; Niu, S. (2021), Absolute model age of Lunar Finsen crater and geologic implications, Icarus, 354, 114046.&lt;/p&gt;