Abstract
The Chinese Chang’E-4 mission for moon exploration has been successfully completed. The Chang’E-4 probe achieved the first-ever soft landing on the floor of Von Kármán crater (177.59°E, 45.46°S) of the South Pole-Aitken (SPA) basin on January 3, 2019. Yutu-2 rover is mounted with several scientific instruments including a lunar penetrating radar (LPR), which is an effective instrument to detect the lunar subsurface structure. During the interpretation of LPR data, subsurface velocity of electromagnetic waves is a vital parameter necessary for stratigraphic division and computing other properties. However, the methods in previous research on Chang’E-3 cannot perform velocity analysis automatically and objectively. In this paper, the 3D velocity spectrum is applied to property analysis of LPR data from Chang’E-4. The result shows that 3D velocity spectrum can automatically search for hyperbolas; the maximum value at velocity axis with a soft threshold function can provide the horizontal position, two-way reflected time and velocity of each hyperbola; the average maximum relative error of velocity is estimated to be 7.99%. Based on the estimated velocities of 30 hyperbolas, the structures of subsurface properties are obtained, including velocity, relative permittivity, density, and content of FeO and TiO2.
Highlights
The Chinese Chang’E-4 mission for moon exploration has been successfully completed
The velocity spectrum analysis method is originally based on common middle point (CMP) ground penetrating radar (GPR) data [19,20], but it can be applied to common offset data processing such as lunar penetrating radar (LPR) data processing
I = 1, . . . nt; j = 1, . . . nx; k = 1, . . . nv where f is the LPR data in t-x domain; Ni denotes the selected horizontal computation region size at ith time; to satisfy the field situation that rock sizes increase vertically, we make Ni variations along longitudinal direction; nt, nx, and nv are the number of sampling points of each trace, number of traces, and number of velocities used in computation; xj represents the horizontal distance between the jth point and the extreme point of the hyperbola; ti,j,k is the two-way time of the jth points of the hyperbola [21,22], which can be obtained using the formula below: ti, j,k
Summary
The Chinese Chang’E-4 mission for moon exploration has been successfully completed. The Chang’E-4 probe achieved the first-ever soft landing on the floor of Von Kármán crater (177.59◦E, 45.46◦S) of the South Pole-Aitken (SPA) basin on January 3, 2019 [1,2,3]. SPA basin is the broadest basin on the Moon. This ancient basin was born from asteroid impacts 4 billion years ago, recording the evolutionary history of the far side of the Moon, and is of great significance for researching the internal materials and structures of the Moon [4,5,6]. Von Kármán crater is one of the primary craters in the SPA basin, with a diameter of 186 km. Recent studies have revealed that the ejecta from adjacent craters have various contributions to the subsurface material of Von Kármán crater, which results in the complex subsurface structure at the Chang’E-4 landing site [1,3,4]
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