Properties of Lunar Regolith on the Moon's Farside Unveiled by Chang'E-4 Lunar Penetrating Radar

The complex thermal history of the Moon leads to an unequal distribution of volcanic products between the lunar nearside and the farside. So far, no lunar materials have been sampled from the Moon's farside and no detailed properties of lunar regolith on the farside have been detected before. On January 3, 2019, Chang'E-4 (CE-4) touched down onto the Von Karman crater on the Moon's farside. CE-4 Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover is the first surface radar on the Moon's farside. Here we show the subsurface structure and properties of regolith materials at the landing region with LPR data during the first five lunar days. The thickness of lunar regolith is constrained as similar to 12 m, much thicker than that at Chang'E-3 (CE-3) landing site, which is expected since CE-3 landed on lunar maria. The relative permittivity of lunar surface (<30 cm) at CE-4 landing region is identified to be 2.35 +/- 0.20. The loss tangent and TiO2 + FeO content of the regolith materials layer (0-similar to 12 m) are constrained to be (4.4 +/- 0.5)x10-3 and 11.6 +/- 1.1 wt.%, respectively, much lower than those at CE-3 landing site. It indicates that local surface materials possess less attenuation for radiowave, in accordance with the greater penetrating depth of CE-4 LPR than that of CE-3 LPR. Furthermore, the results also prove that the growth rate of lunar weathered regolith successively declines over time and the growth rate of lunar regolith on the Moon's farside may well be higher than that on the nearside due to the more frequent meteorite impacts.