According to the WeChat official account "National Center for National Security Science," on March 17, the Chang'e-6 lander's international first ground-based space-specific negative ion analyzer (NILS) successfully achieved the first direct detection of negative ions on the lunar surface, revealing that negative ions produced by solar wind exist on the moon, solving the long-standing mystery of the existence of lunar negative ions.
More than 99% of visible matter in the universe exists in the form of plasma, typically composed of positive ions and electrons. However, negative ions also exist in various astrophysical environments and play important roles. For example, in the outer atmosphere of the sun, negative ions are the dominant factor causing opacity in the visible light band. In the early universe, H⁻ ions can rapidly produce molecular hydrogen through associative desorption reactions, promoting the formation of the first generation of stars. In planetary environments, multiple missions have indirectly detected negative ions in the ionosphere of comets and Mars using electron detectors, proving that negative ions are an essential component of planetary ionospheres. Although it is theoretically expected that negative ions may also exist in the lunar environment, their existence and distribution characteristics have long been a mystery due to the lack of direct observational evidence.
The moon is a representative of airless celestial bodies, with solar wind directly impacting its surface. Recent research results show that when solar wind protons hit the lunar regolith, most are injected into the weathering layer, about 10%–20% are scattered as energetic neutral atoms (ENAs), and about 0.1%–1% are reflected as positive ions. In addition, theoretical and laboratory studies also predict that some protons may capture a second electron during scattering to form negative ions (H⁻). However, due to photo-desorption effects, H⁻ easily loses electrons and dissipates under solar radiation, with a lifetime of only about 0.07 seconds at 1 AU, making it difficult to survive to the altitude of a lunar orbiting spacecraft, which led previous lunar orbiting missions to fail to detect negative ion signals. In-situ lunar surface detection can directly measure negative ions near the source region, which is a key method to overcome this bottleneck. The Chang'e-6 lander is equipped with a negative ion analyzer (NILS) jointly developed by the Swedish Institute of Space Physics and the National Space Science Center of the Chinese Academy of Sciences. This is the world's first dedicated negative ion detector for extraterrestrial space. Within two days of observation, it obtained six valid H⁻ energy spectrum data, achieving the first direct detection of negative ions on the lunar surface by humans.
Figure 1 Schematic diagram of NILS instrument observations by Chang'e-6
Recently, doctoral student Zhong Tianhua, Researcher Xie Lianghai, Researcher Zhang Aibing, and Academician Wang Chi from the National Space Science Center of the Chinese Academy of Sciences, along with domestic and foreign institutions, systematically analyzed the H⁻ energy spectra obtained by NILS and the upstream solar wind parameters observed by the ARTEMIS satellite. The results showed a strong positive correlation (r=0.87) between the integrated flux of H⁻ and the normal flux of the solar wind, and a strong positive correlation (r=0.88) between the average energy of H⁻ and the energy of the solar wind. The H⁻ flux during periods of maximum solar wind flux was approximately three times higher than during periods of minimum flux. These results provide direct observational evidence that H⁻ originates from the interaction between the solar wind and the lunar surface. Additionally, the average energy of H⁻ is concentrated between 250–300 eV, indicating that these negative ions are mainly produced by the scattering process of the solar wind on the lunar surface. Comparing the H⁻ energy spectrum with the empirical energy spectrum of ENAs also revealed that the flux of H⁻ is relatively lower at low energy levels, consistent with the theoretical prediction of velocity-dependent negative ion ejection from solid surfaces, where slower negative ions have a higher probability of tunneling back to the surface, leading to their neutralization by losing an electron.
Figure 2 H⁻ energy spectrum and its correlation with solar wind parameters
To assess the impact of H⁻ on the lunar space environment, this study further used Monte Carlo test particle simulations to reveal its spatial distribution characteristics. On the sunlit side, due to photo-desorption effects, H⁻ is confined within a thin layer close to the lunar surface, with density rapidly decreasing with height, dropping below 10⁵ m⁻³ above 50 kilometers. On the dark side, since this region is in the shadow of the moon without solar illumination, the photo-desorption effect disappears, allowing H⁻ to be picked up by electromagnetic fields and form a long negative ion tail extending several lunar radii. This newly discovered charged particle component can participate in filling the plasma cavity in the lunar wake area. During extreme solar wind density events, H⁻ density can be more than ten times higher than under normal conditions, potentially having significant impacts on the lunar space environment, such as generating certain plasma fluctuations.
Figure 3 Simulated spatial distribution of H⁻ density under different solar wind conditions
In addition to its direct impact on the plasma environment, negative ions may also produce molecular hydrogen (H₂) or hydroxyl (OH) through chemical reactions, providing potential new sources of the lunar exosphere and surface water. Furthermore, considering the porous structure of the lunar soil, H⁻ emitted from one particle may strike adjacent particles and inject electrons, promoting local reduction reactions, possibly contributing to the formation of nanoscale iron. These findings can also be extended to other airless celestial bodies. In environments farther from the Sun, such as the icy satellites of Saturn and Jupiter, where solar radiation is weaker, negative ions may have longer lifetimes and higher concentrations, playing even more significant roles.
This study used the first lunar surface negative ion observation data from the NILS on Chang'e-6, discovering a strong correlation between H⁻ flux and energy with solar wind parameters, providing direct observational evidence that lunar H⁻ originates from the solar wind scattering process. Combined with test particle simulations, it reveals the spatial structural characteristics of a negative ion layer on the sunlit side of the moon and a negative ion tail formed on the dark side. These results significantly enhance our understanding of the lunar plasma environment and provide new perspectives for studying lunar space weathering and the exosphere, while also offering important references for studying the mechanisms and distribution characteristics of negative ions on other airless celestial bodies.
Original article: toutiao.com/article/7618191049660105266/
Statement: This article represents the views of the author(s) alone.