Space weathering has long been known to alter the chemical and physical properties of the surfaces of airless bodies such as the Moon. The isotopic compositions of moderately volatile elements in lunar regolith samples could serve as sensitive tracers for assessing the intensity and duration of space weathering. In this study, we develop a new quantitative tool to study space weathering and constrain surface exposure ages based on potassium isotopic compositions of lunar soils. We first report the K isotopic compositions of 13 bulk lunar soils and 20 interval soil samples from the Apollo 15 deep drill core (15004-15006). We observe significant K isotope fractionation in these lunar soil samples, ranging from 0.00 %o to + 11.77 %o, compared to the bulk silicate Moon (-0.07 +/- 0.09 %o). Additionally, a strong correlation between soil maturity (Is/FeO) and K isotope fractionation is identified for the first time, consistent with other isotope systems of moderately volatile elements such as S, Cu, Zn, Se, Rb, and Cd. Subsequently, we conduct numerical modeling to better constrain the processes of volatile element depletion and isotope fractionation on the Moon and calculate a new K Isotope Model Exposure Age (KIMEA) through this model. We demonstrate that this KIMEA is most sensitive to samples with an exposure age lower than 1,000 Ma and becomes less effective for older samples. This novel K isotope tool can be utilized to evaluate the surface exposure ages of regolith samples on the Moon and potentially on other airless bodies if calibrated using other methods (e.g., cosmogenic noble gases) or experimental data.

Space weathering alters the surface materials of airless planetary bodies; however, the effects on moderately volatile elements in the lunar regolith are not well constrained. For the first time, we provide depth profiles for stable K and Fe isotopes in a continuous lunar regolith core, Apollo 17 double drive tube 73001/2. The top of the core is enriched in heavy K isotopes (delta 41K = 3.48 +/- 0.05 parts per thousand) with a significant trend toward lighter K isotopes to a depth of 7 cm; while the lower 44 cm has only slight variation with an average delta 41K value of 0.15 +/- 0.05 parts per thousand. Iron, which is more refractory, shows only minor variation; the delta 56Fe value at the top of the core is 0.16 +/- 0.02 parts per thousand while the average bottom 44 cm is 0.11 +/- 0.03 parts per thousand. The isotopic fractionation in the top 7 cm of the core, especially the K isotopes, correlates with soil maturity as measured by ferromagnetic resonance. Kinetic fractionation from volatilization by micrometeoroid impacts is modeled in the double drive tube 73001/2 using Rayleigh fractionation and can explain the observed K and Fe isotopic fractionation. Effects from cosmogenic 41K (from decay of 41Ca) were calculated and found to be negligible in 73001/2. In future sample return missions, researchers can use heavy K isotope signatures as tracers of space weathering effects.

Observations of widespread hydration across the lunar surface could be attributed to water formed via the implantation of solar wind hydrogen ions into minerals at the surface. Solar wind irradiation produces a defectrich outer rim in lunar regolith grains which can trap implanted hydrogen to form and store water. However, the ability of hydrogen and water to be retained in space weathered regolith at the lunar surface is not wellunderstood. Here, we present results of novel and coordinated high-resolution analyses using transmission electron microscopy and atom probe tomography to measure hydrogen and water within space weathered lunar grains. We find that hydrogen and water are present in the solar wind-damaged rims of lunar grains and that these species are stored in higher concentrations in the vesicles that are formed by solar wind irradiation. These vesicles may serve as reservoirs that store water over diurnal and possibly geologic timescales. Solar windderived water trapped in space weathered rims is likely a major contributor to observations of the widespread presence, variability, and behavior of the water across the lunar surface.

Permanently shadowed regions (PSRs) on the Moon are potential reservoirs for water ice, making them hot spots for future lunar exploration. The water ice in PSRs would cause distinctive changes in space weathering there, in particular reduction-oxidation processes that differ from those in illuminated regions. To determine the characteristics of products formed during space weathering in PSRs, the lunar meteorite NWA 10203 with artificially added water was irradiated with a nanosecond laser to simulate a micrometeorite bombardment of lunar soil containing water ice. The TEM results of the water-incorporated sample showed distinct amorphous rims that exhibited irregular thickness, poor stratification, the appearance of bubbles, and a reduced number of npFe0. Additionally, EELS analysis showed the presence of ferric iron at the rim of the nanophase metallic iron particles (npFe0) in the amorphous rim with the involvement of water. The results suggest that water ice is another possible factor contributing to oxidation during micrometeorite bombardment on the lunar surface. In addition, it offers a reference for a new space weathering model that incorporates water in PSRs, which could be widespread on asteroids with volatiles.

Lunar soils record the history and spectral changes resulting from the space-weathering process. The solar wind and micrometeoroids are the main space-weathering agents leading to darkening (decreasing albedo) and reddening (increasing reflectance with longer wavelength) of visible and near-infrared spectra. Nevertheless, their relative contributions are not well constrained and understood. In this study, we examine the near-infrared spectral variation as a function of lunar latitude and chemical composition using remote spectroscopic analysis of mare basalts and swirl regions. The results indicate that the reflectance of lunar mature soils darkens and the spectral slope flattens (reddening effect saturation) in areas of enhanced solar wind flux. We propose a previously unrecognized stage of space weathering (the post-mature stage), in which solar wind implantation may contribute to the growth and coarsening of metallic iron particles into larger microphase iron. This space-weathering mechanism is dominated by the solar wind and has important implications for understanding the alteration processes of airless bodies across our solar system.

Space weathering is a primary factor in altering the composition and spectral characteristics of surface materials on airless planets. However, current research on space weathering focuses mainly on the Moon and certain types of asteroids. In particular, the impacts of meteoroids and micrometeoroids, radiation from solar wind/solar flares/cosmic rays, and thermal fatigue due to temperature variations are being studied. Space weathering produces various transformation products such as melted glass, amorphous layers, iron particles, vesicles, and solar wind water. These in turn lead to soil maturation, changes in visible and near-infrared reflectance spectra (weakening of characteristic absorption peaks, decreased reflectance, increased near-infrared slope), and alterations in magnetism (related to small iron particles), collectively termed the lunar model of space weathering transformation. Compared to the Moon and asteroids, Mercury has unique spatial environmental characteristics, including more intense meteoroid impacts and solar thermal radiation, as well as a weaker particle radiation environment due to the global distribution of its magnetic field. Therefore, the lunar model of space weathering may not apply to Mercury. Previous studies have extensively explored the effects of micrometeoroid impacts. Hence, this work focuses on the effects of solar-wind particle radiation in global magnetic-field distribution and on the weathering transformation of surface materials on Mercury under prolonged intense solar irradiation. Through the utilization of high-valence state, heavy ion implantation, and vacuum heating simulation experiments, this paper primarily investigates the weathering transformation characteristics of the major mineral components such as anorthite, pyroxene, and olivine on Mercury's surface and compares them to the weathering transformation model of the Moon. The experimental results indicate that ion implantation at room temperature is insufficient to generate np-Fe-0 directly but can facilitate its formation, while prolonged exposure to solar thermal radiation on Mercury's surface can lead directly to the formation of np-Fe-0. Therefore, intense solar thermal radiation is a crucial component of the unique space weathering transformation process on Mercury's surface.

The Moon is a unique natural laboratory for the study of the deep space plasma and energetic particles environment. During more than 3/4 of its orbit around the Earth it is exposed to the solar wind. Being an unmagnetized body and lacking a substantial atmosphere, solar wind and solar energetic particles bombard the Moon's surface, interacting with the lunar regolith and the tenuous lunar exosphere. Energetic particles arriving at the Moon's surface can be absorbed, or scattered, or can remove another particle from the lunar regolith by sputtering or desorption. A similar phenomenon occurs also with the galactic cosmic rays, which have fluxes and energy spectra representative of interplanetary space. During the remaining part of its orbit the Moon crosses the tail of the terrestrial magnetosphere. It then provides the opportunity to study in-situ the terrestrial magnetotail plasma environment as well as atmospheric escape from the Earth's ionosphere, in the form of heavy ions accelerated and streaming downtail. The lunar environment is thus a unique natural laboratory for analysing the interaction of the solar wind, the cosmic rays and the Earth's magnetosphere with the surface, the immediate subsurface, and the surface-bounded exosphere of an unmagnetized planetary body.This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.

Lunar soil is an important material to study the surface processes on airless bodies, and the geological evolution of the Moon. Since the Apollo era, considerable progresses in lunar soil research have been achieved, including the origin, composition, and properties of the soil. The results from early research have already been summarized and reviewed. With the lunar soil samples returned by the Chang'E-5 mission, all research concerning the Moon has drawn unprecedented attention in China. Therefore, it is timely to summarize the latest advances related to lunar soil research. The lunar soils represent the interior materials that were modified by external processes after formation. This paper reviews the latest achievements of lunar soil from two aspects: the space weathering and the lunar evolution. The space weathering of lunar soil includes the interaction between the soil on the Moon's surface and the materials and energy (such as meteorites, solar wind, and cosmic rays) outside of the Moon, which has also recorded the evolution of the solar system. The effects of space weathering on the surface of the Moon are discussed with respect to the water, volatiles (carbon, nitrogen, oxygen, fluorine, sulfur, chlorine, and noble gas), nanophase minerals (npFe and npSiO(x)), and micrometeorites and meteorite fragments in lunar soil. The contents of water in lunar soil are much higher than those of basalt. However, the water in lunar soil is much more depleted in deuterium than that of basalt. This striking contrast suggests that hydroxyl in the lunar soil mainly results from the implantation of hydrogen ions in the solar wind. On the other hand, volatiles in lunar soil could have originated from various sources, including lunar interior, solar wind, cosmic rays, and even Earth wind. The contributions of the sources for each volatile are different, evidenced by the isotopic composition of the respective volatile. The npFe grains in lunar soil can alter the surface optical spectra of the Moon. It has been demonstrated that the npFe grains are the product of vapor deposit generated by charged particle puttering and/or micrometeorite impact followed by re-condensation of metallic iron. Micrometeorite impacts have not only caused crushing and mixing of lunar soil particles, but also resulted in partial melting and volatilization of lunar soil, leading to the escape and possible redeposition of volatiles on the lunar surface and thus increasing the maturity of lunar soil. Therefore, the npFe concentration is often used as a parameter to represent the maturity of the lunar regolith. On the other hand, the lunar soil can be used to study the evolution of the Moon. This paper mainly reviews the bulk compositions of lunar soil, and the volcanic glasses and lithic clasts as well. The last 60-year study of lunar soil has revealed the main contributors of the soil on the Moon. The chemical and isotopic compositions of bulk soil can be used to reveal the genesis and trace the material sources of lunar soil. The volcanic glasses in lunar soil are products generated by magmatism on the Moon and have recorded the magmatic processes and the mantle compositions. The glass beads with various compositions (and thus colors) in lunar soil are intensively studied to explore the crystallization differentiation and degassing processes of early lunar magma. The lithic clasts have unique scientific significance in revealing the origin and evolution of the parent rocks of lunar soil, in which, anorthosite clasts can be utilized to explore the formation of the lunar crust, whereas basalt clasts play an irreplaceable role in studying the diversity of lunar magma and the evolution of the lunar mantle. Finally, this paper summarizes the related research on the utilization of lunar soil as resources, including metal, and water as well as energy (such as helium, uranium, and thorium), and lunar soil molding. The early study on lunar soil can provide important references for studying the soil samples returned by Chang'E-5 mission. Therefore, this review could facilitate the ongoing research of lunar soil and future lunar exploration of China.

Impact gardening is a mixture of excavation by impacts and burial under continuous proximal ejecta. An existing analytical model describes the rate at which impacts excavate material on the Moon (Gault et al., 1974; Costello et al., 2018, ; Costello et al., 2020, ). We expand the model to include a treatment of burial under proximal ejecta. Using the models for excavation and burial, we explore the effects of impacts in the evolution of the lunar surface over the last few billion years. We find that excavation of material by gardening outpaces burial in all reasonable ejecta coverage test scenarios. Thus, gardening does not act as a shield for ice in permanent shadow. However, gardening fails to eradicate the surface expression of compositional contrasts, such as those associated with pyroclastic deposits and compositional rays, which are not vulnerable to removal by thermal or ionization processes. Explorers seeking ice at the lunar poles should not expect regions of permanent shadow to have pure ice within the top 1-10 m because that ice will have been disrupted by gardening.

Molecular dynamics simulations are carried out to investigate dielectric breakdown of lunar regolith induced by space weather events and its potential effects on water ice formation on lunar surface. We find that dielectric breakdown can trigger the water formation process by breaking the chemical bonds of regolith grains and exposing the oxygen atoms to react with the hydrogen implanted by solar wind. In the permanently shadowed region, the water molecules formed become attached to regolith grains in the molecular structure of ice after the event. Thus, dielectric breakdown can also enable the preservation of water molecules by changing the hydrophobicity of regolith grains.