“Weathering” processes describe the breaking down or dissolving of rocks and minerals on the surface of the Earth. The Moon has essentially no atmosphere, hydrosphere, or magnetic field; thus, the lunar surface has been exposed to the space environment for a long time and is constantly affected by the solar wind, cosmic rays, and impacts. The “weathering” processes on the Moon are termed "space weathering", including mainly four processes (Figure 1): 1) Comminution: breaking of rocks, minerals, and glasses into smaller particles; 2) Agglutination: welding of rock, mineral, and glass fragments together by micrometeorite-produced, impact-generated melt (quenched to glass); 3) Solar-Wind Spallation and Particle Implantation: erosion and vaporization caused by sputtering from impacting high-energy particles and reduction of ferrous iron to metallic iron by hydrogen from solar wind; 4) Impact-Melt Vaporization and Deposition: vaporization of volatile components in the micrometeorite-impact-generated melt, dissociation of iron from iron-bearing phases, and deposition of metallic iron particles in the glassy rim of mineral grain.
Metallic iron particles have large influences on the albedo and color of lunar soils and the effects vary with the particle size. Smaller metallic iron particles could redden reflectance spectra, while larger metallic iron particles do not. The “reddening” effect refers to that a positive slope is introduced in the reflectance spectra owing to stronger absorption at shorter wavelength.
Although the spectral effects of metallic iron have been extensively studied based on Apollo samples and data from satellites orbiting the Moon, Chang’e-5 mission provides an opportunity to in situ study space weathering on the Moon and compare with undisturbed lunar soils as seen by previous orbiters and soils returned to Earth (Figure 2). Chang’e-5 landed in the northern Oceanus Procellarum (Figure 3), ~10 km north of Mons Heng, southeast of a small crater (named after Xu Guangqi) with a diameter of ~400 m, where it is covered by the most iron-rich (22.5 wt.%) and the youngest (~2.0 Ga) lava flows, compared with previous Apollo and Luna landing sites.
We calculated the abundance of metallic iron particles by spectroscopic method and found that the production rate of large-sized metallic iron in Chang’e-5 soils is faster than that observed in Apollo and Luna soils. The production rate of metallic iron depends on composition of host materials, especially ferrous iron, the source of metallic iron particles. Thus, we suggest that Chang'e-5 soils provide evidence for the unique space weathering process: rapid aggregation of nanosize iron particles and more production of larger metallic iron clusters. Relatively abundant metallic iron particles, especially more large-sized iron clusters, inferred from Chang’e-5 soils, indicate that a layer of mature soils has been developed on this young mare unit.
The space weathering degrees (soil maturity) of the lunar surface can be used to determine the extent of the crater ejecta. Some very fresh craters (formed in just millions of years) feature very bright rays. For example, the ejecta rays of one famous crater, Tycho (~85 kilometers), can be easily seen by our naked eyes on Earth (Figure 3). However, small craters (diameter of hundreds of meters) even if formed in millions of years, their weak ray features would be degraded and indistinguishable by albedos.
Nevertheless, ejecta rays of small craters and their surrounding region can be distinguished by metallic iron abundance, because the longer the surface is weathered, the more metallic iron would be produced.
We mapped the metallic iron abundance distribution around the Xu Guangqi crater to confirm its ejecta region, and found that Chang’e-5 exactly landed on the ejecta ray (~200 m-long and ~50 m-wide) from Xu Guangqi crater. This indicates that lunar soils returned by Chang’e-5 mission are primarily from this small crater. The formation age of Xu Guangqi crater, i.e., the exposure age of Chang’e-5 soils, are estimated to be ~240-300 Ma, based on literature data of Apollo soil maturity and CRE age. Among Apollo, Luna, and Chang’e-5 soils, such an exposure age belongs to the “elderly group”.
Chang’e 3-5 missions are invaluable because these in situ explorations on the lunar surface bridged the gap between orbital and laboratory data. However, the lunar surface’s physical states would be altered by the landing and sample return activities. The in situ explorations by current and upcoming lunar missions, which do not disturb the primary state of lunar soils, are expected to bring more insights on the formation and evolution of lunar soils.