Comparison of Dielectric Properties and Structure of Lunar Regolith at Chang'e-3 and Chang'e-4 Landing Sites Revealed by Ground-Penetrating Radar

On 3 January 2019, the Chang'e-4 (CE-4) touched down on the Von Karman crater located inside the South Pole-Aitken Basin, providing for the first time the opportunity for in situ measurements of the lunar regolith at the farside of the Moon. The CE-4 ground penetrating radar reveals that fine-grained regolith, coarse impact ejecta, and fractured bedrocks lie beneath the exploration path of the Yutu-2 rover. The variations of regolith permittivity with depth and the radargrams indicate that the CE-4 site has a fine-grained regolith layer thickness of 11.1 m, which is about 1.3-3 times higher than the in situ measurement results at the Apollo and Chang'e-3 (CE-3) sites except for Apollo 16, possibly due to a faster weathering rate of ejecta deposits compared with coherent basalt substrates. The penetration depth of CE-4 is about 2.85 times (in terms of round-way delay) deeper than CE-3, probably due to the differences in abundances of ilmenite and rocks in the regolith.