The complex thermal history of the Moon leads to an unequal distribution of volcanic products between the lunar nearside and the farside. So far, no lunar materials have been sampled from the Moon's farside and no detailed properties of lunar regolith on the farside have been detected before. On January 3, 2019, Chang'E-4 (CE-4) touched down onto the Von Kármán crater on the Moon's farside. CE-4 Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover is the first surface radar on the Moon's farside. Here we show the subsurface structure and properties of regolith materials at the landing region with LPR data during the first five lunar days. The thickness of lunar regolith is constrained as ∼12 m, much thicker than that at Chang'E-3 (CE-3) landing site, which is expected since CE-3 landed on lunar maria. The relative permittivity of lunar surface (<30 cm) at CE-4 landing region is identified to be 2.35  0.20. The loss tangent and TiO₂ + FeO content of the regolith materials layer (0–∼12 m) are constrained to be and  wt.%, respectively, much lower than those at CE-3 landing site. It indicates that local surface materials possess less attenuation for radiowave, in accordance with the greater penetrating depth of CE-4 LPR than that of CE-3 LPR. Furthermore, the results also prove that the growth rate of lunar weathered regolith successively declines over time and the growth rate of lunar regolith on the Moon's farside may well be higher than that on the nearside due to the more frequent meteorite impacts.

中文翻译：

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嫦娥四号探月雷达揭示月球背面月球风化层的特性

月球复杂的热历史导致月球近侧和远侧火山产物的分布不均。到目前为止，还没有从月球背面采样过月球材料，之前也没有探测到月球背面月球风化层的详细特性。2019 年 1 月 3 日，嫦娥四号（CE-4）降落在月球背面的冯卡门陨石坑。Yutu-2 漫游车上的 CE-4 探月雷达 (LPR) 是月球背面的第一个表面雷达。在这里，我们用 LPR 数据显示了月球前五个日着陆区风化层材料的地下结构和特性。月球风化层的厚度被限制为~12 m，比嫦娥三号（CE-3）着陆点的厚度要厚得多，这是自CE-3登陆月球海角以来的预期。  0.20。风化层材料层（0-~12 m）的损耗角正切和 TiO ₂ + FeO 含量分别被限制为和 wt.%，远低于 CE-3 着陆点的。这表明局部表面材料对无线电波的衰减较小，这与 CE-4 LPR 的穿透深度大于 CE-3 LPR 的穿透深度一致。此外，研究结果还证明，月球风化风化层的增长率随着时间的推移而逐渐下降，由于更频繁的陨石撞击，月球背面月球风化层的增长率很可能高于月球背面。