Origins of Lunar Metallic Iron have been significantly clarified in a groundbreaking study published in Nature Astronomy. Profs. Shen Laiquan, Bai Haiyang, and their colleagues from Prof.
Wang Weihua’s group at the Institute of Physics of the Chinese Academy of Sciences have made substantial progress in understanding this phenomenon. Their research sheds light on how irradiation and impacts contribute to the formation of metallic iron nanoparticles (npFe0) on the moon.
The study, based on detailed observations of glass beads returned by the Chang’e-5 mission, reveals that the origins of lunar metallic iron are influenced by two distinct processes: solar wind irradiation and micrometeorite impacts. The research demonstrates that the formation of both small and large npFe0 with unique optical effects is controlled independently by these two factors.
Origins of lunar metallic iron are critical to understanding the changing optical spectrum of the lunar surface over time. The size of npFe0 plays a pivotal role in this process: smaller npFe0 tend to redden the reflectance spectra, while larger npFe0 cause a darkening effect. These variations significantly complicate remote sensing studies and have long puzzled astronomers. Before this study, the origins of different-sized npFe0 were not well understood.
The research clarifies that npFe0, products of space weathering, arise from micrometeorite impacts and solar wind irradiation. However, the specific contributions of these agents to the formation of different-sized npFe0 had remained unclear, obscuring our understanding of lunar surface color variations and asteroid environments.
“We discovered that the glass beads from the Chang’e-5 lunar soil preserve iron particles of varying sizes, from approximately 1 nanometer to 1 micrometer,” said Prof. Bai. This discovery is crucial for unraveling the origins of lunar metallic iron.
The study highlights the difficulty in distinguishing npFe0 of different origins in single samples. The team utilized the rotation features of impact glass beads to separate npFe0 formed before and after the solidification of the glass beads. This method allowed them to identify large npFe0, tens of nanometers in size, which concentrate at the extremities of the beads—a phenomenon consistent with hypervelocity impact scenarios.
Conversely, the study identified numerous small npFe0, several nanometers in size, densely distributed on the surfaces of the glass beads. These small npFe0 display characteristics similar to those caused by irradiation-induced vesicle damage. The study notes that as the depth of the glass beads increases, both small npFe0 and vesicles decrease in size and abundance, corresponding to the reduction of solar wind ions with depth.
Additionally, when lunar grains are smaller than twice the penetration depth of solar wind ions, small npFe0 can completely fill the grains. These findings underscore that solar wind irradiation is the primary driver of small npFe0, further elucidating the origins of lunar metallic iron.
This research demonstrates that solar wind irradiation and micrometeorite impacts both play crucial but distinct roles in the formation of npFe0. The independent growth patterns of small and large npFe0 observed in this study align well with remote sensing measurements, offering valuable insights into the optical properties of airless bodies exposed to various space environments.
Understanding the origins of lunar metallic iron is essential for predicting these properties and advancing our knowledge of extraterrestrial surfaces.
