A potential subsurface cavity in the continuous ejecta deposits of the Ziwei crater discovered by the Chang\u2019E-3 mission

Chunyu Ding,Zhiyong Xiao,Yan Su

doi:10.1186/s40623-021-01359-7

Abstract

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.Subsurface cavities of the Moon are the ideal natural shelter for future lunar explorations since they provide ideal insulation from cosmic radiation, meteoroid impacts, temperature fluctuations, etc. In this paper, we first report that a potential subsurface cavity with a height of ~3.1 m is revealed by the lunar penetrating radar onboard Chang’E-3 Yutu Rover

Highlights

In 2013, the Chinese Chang’E-3 mission (CE-3) carried out high-resolution ground penetrating radar profiling on the Moon and it was the first in situ application of ground penetrating radar on an extraterrestrial body
Using the high-frequency lunar penetrating radar (LPR) data returned along the route of the Yutu rover, subsurface structures were revealed in unprecedented detail (Fa et al 2015; Feng et al 2017; Xiao et al 2015; Zhang et al 2015; Zhang et al 2019)
The different subsurface structures are another possible reason for the different signal magnitudes observed, i.e., the lava tube studied by Miyamoto et al (2005) and regolith interior structures identified by CE-3

Summary

Introduction

In 2013, the Chinese Chang’E-3 mission (CE-3) carried out high-resolution ground penetrating radar profiling on the Moon and it was the first in situ application of ground penetrating radar on an extraterrestrial body. The landing site and the traverse route of the Yutu rover were both located on the continuous ejecta deposits of the ~ 450-m-diameter Ziwei crater (Fig. 1a and b). Using the high-frequency lunar penetrating radar (LPR) data returned along the route of the Yutu rover, subsurface structures were revealed in unprecedented detail (Fa et al 2015; Feng et al 2017; Xiao et al 2015; Zhang et al 2015; Zhang et al 2019). From the perspective of echo strengths, there is a general consensus that the top ~ 50 ns of the radargram was returned from the continuous ejecta deposits of Ziwei crater This interpretation is consistent with the geological context of the landing site (Qiao et al 2016)

Methods

Results

Discussion

Conclusion