In the radargram obtained by the high-frequency lunar penetrating radar onboard the Chang’E-3 mission, we notice a potential subsurface cavity that has a smaller permittivity compared to the surrounding materials. The two-way travel time between the top and bottom boundaries of the potential cavity is ~ 21 ns, and the entire zone is located within the continuous ejecta deposits of the Ziwei crater, which generally have similar physical properties to typical lunar regolith. We carried out numerical simulations for electromagnetic wave propagation to investigate the nature of this low-permittivity zone. Assuming different shapes for this zone, a comprehensive comparison between our model results and the observed radargram suggests that the roof of this zone is convex and slightly inclined to the south. Modeling subsurface materials with different relative permittivities suggests that the low-permittivity zone is most likely formed due to a subsurface cavity. The maximum vertical dimension of this potential cavity is ~ 3.1 m. While the continuous ejecta deposits of Ziwei crater are largely composed of pre-impact regolith, competent mare basalts were also excavated, which is evident by the abundant meter-scale boulders on the wall and rim of Ziwei crater. We infer that the subsurface cavity is supported by excavated large boulders, which were stacked during the energetic emplacement of the continuous ejecta deposits. However, the exact geometry of this cavity (e.g., the width) cannot be constrained using the single two-dimensional radar profile. This discovery indicates that large voids formed during the emplacement of impact ejecta should be abundant on the Moon, which contributes to the high bulk porosity of the lunar shallow crust, as discovered by the GRAIL mission. Our results further suggest that ground penetrating radar is capable of detecting and deciphering subsurface cavities such as lava tubes, which can be applied in future lunar and deep space explorations.

在the娥三号飞行器上的高频探月雷达获得的雷达图中，我们注意到潜在的地下空腔与周围材料相比具有较小的介电常数。潜在腔的顶部和底部边界之间的双向传播时间为〜21 ns，整个区域位于紫微陨石坑的连续喷射沉积物中，这些沉积物通常具有与典型月球重石相似的物理特性。我们进行了电磁波传播的数值模拟，以研究该低介电常数区域的性质。假定该区域的形状不同，我们的模型结果与观测到的雷达图的全面比较表明，该区域的屋顶是凸形的，并且向南略微倾斜。对具有不同相对介电常数的地下材料进行建模表明，低介电常数区很可能是由于地下空腔而形成的。该潜在空腔的最大垂直尺寸约为3.1 m。紫微火山口的连续射出沉积物主要由撞击前的re石组成，同时还挖掘出了合格的母马玄武岩，这在紫微火山口的壁和边缘上有大量的米级巨石就是明显的。我们推断地下空腔是由开挖的大石块支撑的，这些大石块在连续射出沉积物的高能进驻过程中堆叠在一起。但是，使用单个二维雷达轮廓无法约束该腔的确切几何形状（例如宽度）。这一发现表明，在撞击弹射器放置期间形成的大空洞应在月球上丰富，这有助于GRAIL任务发现月球浅层地壳的高孔隙度。我们的结果进一步表明，探地雷达能够探测和破译地下洞，例如熔岩管，可用于未来的月球和深空探测。