Structure and Formation Mechanism of Lunar Regolith

Lunar regolith is a thin layer of weakly cohesive detrital materials covering the lunar surface. Studies on returned lunar samples have revealed that the lunar regolith mainly consists of fragments of rocks, minerals, breccia, glasses, and agglutinates with a median grain size of ~40 to 800 μm. The lunar regolith was produced from the space weathering of lunar rocks, including the following processes: meteoritic bombardment, solar wind implantation, solar and galactic cosmic ray irradiation, and gardening. During space weathering, the maturity of the lunar regolith increases as the median particle size decreases, and specific minerals and structures (such as nano Fe⁰) are produced. The chemical and isotopic compositions of the lunar regolith also change via interactions with solar wind and cosmic rays. Volatiles resulting from solar wind, asteroid impacts, and volcanic degassing can be preserved in the lunar regolith and redistributed to the lunar polar regions during micrometeorite bombardment. Cosmic radiation can produce nuclides through spallation and neutron capture reactions, thereby changing the isotopic compositions of the lunar regolith, which could reflect the gardening history of the lunar regolith. Thus, the lunar regolith carries massive information about the space weathering history, impacting processes, and the interior of the Moon. In this review, we discuss the progress that has been made toward understanding the composition, the lateral and vertical structure, and the formation processes of the lunar regolith, in particular, the progress that has been made after the Chang’E mission series.