Pattern Analyses of Lunar Penetrating Radar Images at Chang'E-3, 4, and 5 Landing Sites: A New Insight Into the Evolution of Lunar Regolith

An "atlas" of the Lunar penetrating radar (LPR) patterns is organized and a comprehensive interpretation of the lunar subsurface layers in the areas of the Chang'E (CE) mission is proposed. There are three layers inferred by the LPR high-frequency profiles of the CE-3 and CE-4. The first two layers (i.e., the fine regolith and the ejecta layer) at CE-3 and CE-4 sites have the same strata type but differ in internal structure and thickness. The third layer at the CE-3 site is inferred as the paleoregolith, while at the CE-4 site, it is inferred as the alternating layer with fine materials, ejecta, and fragments. Due to the limited penetration depth of the CE-5 LPR, the data disclose only the fine regolith, with support from the empirical ejecta distribution model. The thicknesses, physical properties, and equivalent growth rates of lunar regolith at three landing sites are estimated and discussed in detail. The lunar regolith thicknesses inferred by the CE-3 and CE-5 LPR data are both approximately 1.0 m. In contrast, the thickness of the lunar regolith at the CE-4 site is 10.0 m, which is greater than that at the other two landing sites. However, the equivalent growth rate of the regolith at the CE-3 and CE-5 landing sites is four times greater than that at the CE-4 landing site. This illustrates that with the increase in lunar regolith thickness, the influence of small impact gardening events on the lunar regolith reconstruction gradually weakened, and the self-buffering effect avoids the fast growth of lunar regolith.