New Insight Into Lunar Regolith-Forming Processes by the Lunar Rover Yutu-2

The Yutu-2 rover of the Chang'E-4 spacecraft observed many meter-sized shallow pits fully covered with small fragments, distinct from the typical rock-free impact craters with comparable sizes in the landing area. The unique morphology of the pits and the visible and near-infrared spectra of the fragments suggest that the fragments are broken pieces of impact melt-conglutinated regolith breccia projectiles, which were excavated from preexisting craters. The rareness of rock on the landing area surface suggests that the preexisting craters were probably small in size (e.g., <60 m in diameter), not large enough to penetrate the thick regolith (similar to 12 m) and to excavate the beneath rock breccia and/or bedrock. The presence of the impact melts was confirmed by the glass-like spectra of some fragments with unusually high albedo and blue-green tint in the centers of the pits. These observations reveal the gardening and consolidating processes on the Moon. Plain Language Summary There are very rare outcrops of bedrock on the lunar surface because the Moon is covered by the lunar regolith. The detailed formation processes of the lunar regolith are important for understanding the orbiting and landing exploration data, and for planning future sample return missions. The lunar rover Yutu-2 discovered fragment-filled pits and identified in situ impact glass in the fragments. The glass-bearing and regolith-like fragments were probably broken pieces of lunar regolith breccia projectiles, which were excavated from the consolidated bottom and walls of preexisting small craters within the lunar regolith (<12-15 m in thickness). These observations contribute to our understanding of the formation processes of the lunar regolith.