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.

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.

Many studies exist on magmatic volatiles (H, C, N, F, S, Cl) in and on the Moon, within the last several years, that have cast into question the post-Apollo view of lunar formation, the distribution and sources of volatiles in the Earth-Moon system, and the thermal and magmatic evolution of the Moon. However, these recent observations are not the first data on lunar volatiles. When Apollo samples were first returned, substantial efforts were made to understand volatile elements, and a wealth of data regarding volatile elements exists in this older literature. In this review paper, we approach volatiles in and on the Moon using new and old data derived from lunar samples and remote sensing. From combining these data sets, we identified many points of convergence, although numerous questions remain unanswered. The abundances of volatiles in the bulk silicate Moon (BSM), lunar mantle, and urKREEP [last similar to 1% of the lunar magma ocean (LMO)] were estimated and placed within the context of the LMO model. The lunar mantle is likely heterogeneous with respect to volatiles, and the relative abundances of F, Cl, and H2O in the lunar mantle (H2O > F >> Cl) do not directly reflect those of BSM or urKREEP (Cl > H2O F). In fact, the abundances of volatiles in the cumulate lunar mantle were likely controlled by partitioning of volatiles between LMO liquid and nominally anhydrous minerals instead of residual liquid trapped in the cumulate pile. An internally consistent model for lunar volatiles in BSM should reproduce the absolute and relative abundances of volatiles in urKREEP, the anorthositic primary crust, and the lunar mantle within the context of processes that occurred during the thermal and magmatic evolution of the Moon. Using this mass-balance constraint, we conducted LMO crystallization calculations with a specific focus on the distributions and abundances of F, Cl, and H2O to determine whether or not estimates of F, Cl, and H2O in urKREEP are consistent with those of the lunar mantle, estimated independently from the analysis of volatiles in mare volcanic materials. Our estimate of volatiles in the bulk lunar mantle are 0.54-4.5 ppm F, 0.15-5.3 ppm H2O, 0.26-2.9 ppm Cl, 0.014-0.57 ppm C, and 78.9 ppm S. Our estimates of H2O are depleted compared to independent estimates of H2O in the lunar mantle, which are largely biased toward the wettest samples. Although the lunar mantle is depleted in volatiles relative to Earth, unlike the Earth, the mantle is not the primary host for volatiles. The primary host of the Moon's incompatible lithophile volatiles (F, Cl, H2O) is urKREEP, which we estimate to have 660 ppm F, 300-1250 ppm H2O, and 1100-1350 ppm Cl. This urKREEP composition implies a BSM with 7.1 ppm F, 3-13 ppm H2O, and 11-14 ppm Cl. An upper bound on the abundances of F, Cl, and H2O in urKREEP and the BSM, based on F abundances in Cl carbonaceous chondrites, are reported to be 5500 ppm F, 0.26-1.09 wt% H2O, and 0.98-1.2 wt% Cl and 60 ppm F, 27-114 ppm H2O, and 100-123 ppm Cl, respectively. The role of volatiles in many lunar geologic processes was also determined and discussed. Specifically, analyses of volatiles from lunar glass beads as well as the phase assemblages present in coatings on those beads were used to infer that H-2 is likely the primary vapor component responsible for propelling the fire-fountain eruptions that produced the pyroclastic glass beads (as opposed to CO). The textural occurrences of some volatile-bearing minerals are used to identify hydrothermal alteration, which is manifested by sulfide veining and sulfide-replacement textures in silicates. Metasomatic alteration in lunar systems differs substantially from terrestrial alteration due to differences in oxygen fugacity between the two bodies that result in H2O as the primary solvent for alteration fluids on Earth and H-2 as the primary solvent for alteration fluids on the Moon (and other reduced planetary bodies). Additionally, volatile abundances in volatile-bearing materials are combined with isotopic data to determine possible secondary processes that have affected the primary magmatic volatile signatures of lunar rocks including degassing, assimilation, and terrestrial contamination; however, these processes prove difficult to untangle within individual data sets. Data from remote sensing and lunar soils are combined to understand the distribution, origin, and abundances of volatiles on the lunar surface, which can be explained largely by solar wind implantation and spallogenic processes, although some of the volatiles in the soils may also be either indigenous to the Moon or terrestrial contamination. We have also provided a complete inventory of volatile-bearing mineral phases indigenous to lunar samples and discuss some of the unconfirmed volatile-bearing minerals that have been reported. Finally, a compilation of unanswered questions and future avenues of research on the topic of lunar volatiles are presented, along with a critical analysis of approaches for answering these questions.

[1] Both steady and episodic sources have been proposed as sources of hydrogen observed by Lunar Prospector in association with the regions of permanent shadow at the poles of the Moon. Either source could supply significant quantities of water to the poles. However, space weathering processes affect the retention of water in the cold traps. We investigate those effects by simulating the evolution of a column of regolith in the region of permanent shadow over time. We determine the hydrogen concentration as a function of depth using a Monte Carlo model of discrete impacts and of delivery from the solar wind. We treat the delivery, sublimation, sputtering, and very small scale impacts as continual processes. Comparing the amount of water delivered to the poles to the amount remaining after space weathering, we find a retention efficiency of 5.6%. The retention efficiency of the polar cold traps is adequate for preserving volatile deposits over long periods of time. The average hydrogen concentration in the regolith column is 4100 ppm in the top meter after 1 Gyr. This is a saturation level in the regolith. Increasing the amount of time deepens the enriched layer but does not lead to increased concentrations. In 1 Gyr, about 1.6 m of the regolith is gardened. Therefore the top meter, which is probed by the neutron spectroscopy technique, has reached steady state in the simulations. The 4100 ppm saturation level is about half of the amount of hydrogen inferred from the Lunar Prospector neutron data.