Bulk Density of the Lunar Regolith at the Chang'E‐3 Landing Site as Estimated From Lunar Penetrating Radar

摘要

Bulk density of the lunar regolith is a key factor affecting its geophysical and geotechnical properties. In this study, a new method for estimating the bulk density of the lunar regolith is developed based on the geometric characteristic (i.e., hyperbolic shape) of radar echoes in ground penetrating radar (GPR) image. As an application, bulk density of the lunar regolith at China's Chang'E‐3 (CE‐3) landing site is estimated using the Lunar Penetrating Radar data. In total, 57 hyperbolas are identified in the Lunar Penetrating Radar image and their eccentricities are used to estimate relative permittivity of the regolith. Then, bulk density of the lunar regolith is estimated using an empirical relation through its dependence on relative permittivity. The results show that bulk density of the regolith at the CE‐3 landing site increases with depth from 0.85?g/cm at the surface to a steady‐state value of 2.25?g/cm at 5?m, with an average gradient much smaller than that based on the Apollo regolith samples. The bulk density corresponds to a regolith porosity of 74.5% at the surface and 32.3% at 5?m depth over the CE‐3 landing region. Given that the landing site is only 50?m from the east rim of a 500?m diameter crater, named as Zi Wei, the steady‐state bulk density indicates a 29% volume fraction of subsurface rocks within the continuous ejecta of this crater. Plain Language Summary In ground penetrating radar observations, the distance between a radar antenna and a discrete subsurface object can produce a hyperbolic curve that is very common in ground penetrating radar images. The shape of a hyperbolic curve depends on antenna height, depth of the object, and dielectric permittivity of subsurface material. Based on these relationships, relative permittivity (i.e., the ratio of the dielectric permittivity of a material to that of vacuum) of the lunar regolith to a depth of 5?m at China's Chang'E‐3 (CE‐3) landing site is first estimated using the Lunar Penetrating Radar observation. Then, bulk density and porosity of regolith and volume fraction of subsurface rocks are estimated through their dependences on relative permittivity, grain density of lunar regolith, and density of lunar rocks. The results show that bulk density increases with depth from 0.85?g/cm at the surface to 2.25?g/cm at 5?m, indicating a regolith porosity of 74.5% at the surface and 32.3% at 5?m depth. The results also indicate a volume fraction of 29% for subsurface rocks at the CE‐3 landing site. All these results can improve our understanding of the subsurface property of the lunar regolith, which was not revealed by previous missions because of the limited penetration depth (e.g., Apollo core tube experiment and Diviner radiometer estimation).