Space Weathering Research Articles

Analyzing the Mineralogy and Space Weathering Characteristics of the Finest Fraction in Apollo Core Sample 73002

AbstractApollo 17 core sample 73001/2 was recently made available to researchers for analysis using state‐of‐the‐art techniques in the framework of a modern understanding of lunar surface processes. In this work, we employ transmission electron microscopic analysis to observe the mineralogy, microstructural, and chemical characteristics of space weathering and solar energetic particle (SEP) track distribution in soil grains in the <20 μm size fraction in core sample 73002. The modal mineralogy and stratigraphic space weathered grain abundance suggests that a geologically recent mixing event affected the top 3 cm of 73002. Surface exposure age distributions derived from SEP tracks demonstrate that individual regolith grains rarely reside on the surface for longer than ∼4 million years. The abundance of surface exposed monomineralic fragments with respect to depth correlates well with bulk measurements of space weathered soils using other techniques, such as ferromagnetic resonance. Exposure age distributions suggest the presence of two unique in situ reworking zones spanning the top 8 cm of the core and median exposure ages decrease with increasing depth for both reworking zones, albeit at different rates. These rates were compared to reworking models and suggest a relationship between median exposure age and reworking rate with respect to depth. Applications of modern transmission electron microscopy to core sample 73001/2 have proven useful in understanding lunar regolith evolution both within the context of the Apollo 17 field site and more broadly via in situ reworking.

Understanding the Effects of Micrometeoroid Bombardment on Graphite-rich Mercury Analogs through Laboratory Experiments and Electron Microscopy Analysis

Abstract Space weathering alters the spectral, microstructural, and chemical characteristics of the surface of Mercury, and an effective way to understand it is in the laboratory. To simulate energetic micrometeoroid impacts on the surface of Mercury, we irradiated pellets containing forsteritic olivine and graphite with a pulsed laser. Our analysis with scanning electron microscopy showed two morphological textures: one that is melt-rich and one consisting of fluffy, C-rich globules. Performing transmission electron microscopy on these textures, we identified the presence of new microstructural and chemical features resulting from simulated space weathering, including C-rich inclusions in the melt and vesiculated textures in the C-rich globules. Separate analysis of Si wafers, which were suspended near the target to collect the vapor and melt ejected during irradiation, show a variety of ejected material, including forsterite grains, melt splash, and C-rich particles. These observations suggest the possibility that space weathering products on the surface of Mercury may be different than on other airless bodies, which will be further explored by the BepiColombo mission.

Atomic‐Level Structural Responses of Chang'e‐5 Ilmenite to Space Weathering

AbstractSpace weathering records provide insights to better understand the formation and evolution of the lunar regolith. Ilmenite has contrasting responses to different space weathering processes. However, the atomic‐scale structural modification of ilmenite induced using different space weathering processes remains poorly understood. Here, we investigate the effects of spacing weathering on lunar ilmenite grains returned from Chang'e‐5 (CE‐5) mission using a combination of transmission electron microscopy and thermodynamic modeling approaches. Experimental results show that melt shock induces the formation of twining structures and vein‐like Si‐Ca‐rich nanostructures in the outermost and sub‐outermost layers of ilmenite, respectively. In contrast, solar wind causes the formation of multilayered nanostructures surrounding the ilmenite grains. These structures are characterized by an outermost amorphous Si‐rich vapor deposited layer, a middle layer rich in titanium (Ti) oxides and zero‐valent iron (Fe0) nanoparticles, and an innermost layer hosting crystallographic orientation defect. The Ti oxides were identified as poorly crystallized anatase. Thermodynamic calculations indicate that the disruptive sputtering of solar wind and the reduction of hydrogen under lunar surface pressure conditions can promote ilmenite transformation into Fe0 and Ti oxides; nevertheless, the pressure increase associated with melt shock can lead to a rise in the decomposition temperature of ilmenite. In other words, solar wind irradiation plays a more significant role in promoting nanoparticle (such as anatase and Fe0) formation as compared to melt shock. Thus, unlike the chemical alteration of ilmenite induced by the solar wind irradiation, melt shock mainly causes physical changes in ilmenite grains.

Iron isotopes of Chang'e-5 soil and mineral components: Implications for post-eruption processes on lunar surface

Due to rapid magma cooling and extensive space weathering, significant disequilibrium crystallization and secondary modification widely occur in lunar mare basalt after its eruption on the lunar surface. In this study, we conducted bulk and in-situ Fe isotope analyses to investigate the post-eruption processes on Chang'e-5 (CE-5) samples. The CE-5 soil shows a minor elevation of δ56Fe value (∼0.05 ‰) relative to the CE-5 basalt clasts. Correlations between Ni and Cu contents with δ56Fe values suggest that the minor increase in the δ56Fe from the CE-5 basalt to soil primarily occurred during evaporation caused by meteorite impacts. Such isotopic variation between CE-5 basalt and soils is notably lower than what is observed for Apollo samples and reflects the low maturity of CE-5 soils. This is consistent with the low Is/FeO value constrained by magnetic approaches. Therefore, measuring the δ56Fe values of lunar soil is suitable to evaluate the degrees of maturity for lunar soils due to space weathering. In-situ analyses of δ56Fe reveal significant variations in different grains of olivine (δ56Fe: −0.57 to −0.17 ‰) and ilmenite (−0.06 to +0.42 ‰) and also in their interior (mainly for olivine). These δ56Fe variations in minerals can be ascribed to the disequilibrium crystallization of lava flow and fast cooling, which is consistent with conclusions based on petrologic observations such as its extensive differentiation and silicate liquid immiscibility. Therefore, the post-eruption processes on the lunar surface could lead to significant variations in isotopic compositions at different scales of basalts, which in turn record the history of late-stage magma evolution and space weathering on the lunar surface.

Synthesis, structural and spectroscopic characterization of defect-rich forsterite as a representative phase of Martian regolith.

Regolith draws intensive research attention because of its importance as the basis for fabricating materials for future human space exploration. Martian regolith is predicted to consist of defect-rich crystal structures due to long-term space weathering. The present report focuses on the structural differences between defect-rich and defect-poor forsterite (Mg2SiO4) - one of the major phases in Martian regolith. In this work, forsterites were synthesized using reverse strike co-precipitation and high-energy ball milling (BM). Subsequent post-processing was also carried out using BM to enhance the defects. The crystal structures of the samples were characterized by X-ray powder diffraction and total scattering using Cu and synchrotron radiation followed by Rietveld refinement and pair distribution function (PDF) analysis, respectively. The structural models were deduced by density functional theory assisted PDF refinements, describing both long-range and short-range order caused by defects. The Raman spectral features of the synthetic forsterites complement the ab initio simulation for an in-depth understanding of the associated structural defects.

Simulated Solar-wind-induced Space Weathering of Olivine Powders: Spectral Alterations in the Ultraviolet, Visible, and Near-infrared

Bombardment by solar wind ions is one of the main drivers of space weathering on airless bodies. Here, we simulate the solar-wind-driven spectral alteration of loosely packed olivine powders by irradiation with 1.2 keV helium ions (He+). We measured the reflectance spectra of the olivine powder in the ultraviolet–visible–near-infrared (UV–Vis–NIR) wavelength range (0.2–2 μm) as a function of ion fluence. In the Vis–NIR range, we observed spectral darkening, absorption band shallowing, and spectral reddening, in agreement with lunar-style space weathering and previous laboratory studies. In the UV–Vis, spectral darkening was also observed. However, a spectral bluing took place at wavelengths below 400 nm. As the simulated space weathering progressed, the spectral slopes shifted from steep-UV/shallow-NIR slopes to shallow-UV/steep-NIR slopes. Moreover, the change in the UV slope was almost 10 times larger than in the NIR, supporting the hypothesis that the UV spectral slope could be an earlier indicator of space weathering.

Infrared Colors of Small Serendipitously Found Asteroids in the UKIRT Hemisphere Survey

The UKIRT Hemisphere Survey covers the northern sky in the infrared from 0° to 60° decl. Current data releases include both J and K bands, with H-band data forthcoming. Here, we present a novel pipeline to recover asteroids from this survey data. We recover 26,138 reliable observations, corresponding to 23,399 unique asteroids, from these public data. We measure J–K colors for 601 asteroids. Our survey extends about 2 mag deeper than the Two-Micron All-Sky Survey. We find that our small inner main belt objects are less red than larger inner belt objects, perhaps because smaller asteroids are collisionally younger, with surfaces that have been less affected by space weathering. In the middle and outer main belts, we find our small asteroids to be redder than larger objects in their same orbits, possibly due to observational bias or a disproportionate population of very red objects among these smaller asteroids. Future work on this project includes extracting moving object measurements from H- and Y-band data when it becomes available.

Near-infrared spectral behavior of space-weathered olivine with varying iron content

Space weathering alters the surfaces of airless celestial bodies, thereby modifying their spectra significantly. Olivine plays a crucial role in responding to space weathering on silicate planets. However, the spectral variations that occur in olivine with varying iron content as a result of space weathering conditions remain unclear. We aim to systematically characterize the spectral variability of surface iron-rich olivine in the space weathering environments of Phobos and the Moon. We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa29, Fa50, Fa71, and Fa100). The energy levels were simulated for Phobos and the Moon. We analyzed the available near-infrared (NIR) spectroscopy. We find that olivine with higher Fe content undergoes stronger weathering under the same irradiation energy, shifting absorption centers around 1.08 mu m and 1.35 mu m to longer wavelengths. When comparing the high energy and low frequency, spectral changes are more pronounced at low energy and high frequency. The olivine with the same iron content exhibits a more noticeable shift around 1.08 mu m under various irradiation levels, while the band center around 1.35 mu m remains stable. When the same amount of radiation energy is received, changes in the spectrum are more noticeable at low energy and high impact frequency than at high energy and low impact frequency. The absorption position at $ $ 1.35 mu m is a good indicator of the Mg$\#$ value of space-weathered olivine.

Reactive Molecular Dynamics Simulations of Micrometeoroid Bombardment for Space Weathering of Asteroid (162173) Ryugu

Remote sensing observations by Hayabusa2 and laboratory measurements have revealed that the phyllosilicates on asteroid (162173) Ryugu are dehydrated/dehydroxylated due to space weathering. Reactive molecular dynamics simulations were performed to evaluate the magnitude of the dehydroxylation of Mg-rich serpentine by micrometeoroid impacts. When micrometeoroids were not coupled with interplanetary magnetic fields, serpentine could be dehydroxylated by micrometeoroids as small as 2 nm in size. In particular, ∼200 O–H bonds dissociated when the meteoroids were derived from cometary activity (the impact velocity was ∼20 km s−1). When nano-sized dust particles were accelerated to ∼300 km s−1 by the magnetic fields of solar wind plasma, the number of dissociated O–H bonds increased by 1 order of magnitude. Consequently, even 1 nm-sized dust particles can contribute to the space weathering of Ryugu. In all cases, Si–OH, H2O, and free OH were generated from the hydroxyls initially connected to Mg, which could partially offset dehydration. Despite the limitations of our computational resources, which restricted the simulation timescale to 1 ps, reactive molecular dynamic simulations demonstrated that micrometeoroid bombardment could influence the space weathering of asteroids.

Mid-infrared Measurements of Ion-irradiated Carbonaceous Meteorites: How to Better Detect Space Weathering Effects

Remote sensing study of asteroids will soon enter a new era with an increasing amount of data available thanks to the JWST, especially in the mid-infrared (MIR) range that allows identification of mineral species. It will then be possible to establish a taxonomy, as is currently available in the visible–near-infrared range, based on MIR spectral parameters. It had been previously shown that the MIR range is very sensitive to space weathering (SpWe) effects. Thus, it is crucial to determine which spectral changes are involved to disentangle initial composition from surface aging and provide tools to interpret future remote sensing data of asteroids. We present here MIR measurements of a wide variety of ion-irradiated carbonaceous chondrites as a simulation of the solar wind SpWe component. We evaluate several parameters (the Christiansen feature and Reststrahlen band positions, the width of the main Si–O band) and test different measurement conditions (ion energy and geometry of observation). We highlight a dependency of the spectral changes with the initial composition, as hydrated samples are more affected than anhydrous ones. We confirm the role of the geometry in the detection of SpWe effects as already shown in the near-infrared, with a competition effect between the depth probed by photons and the implantation depth of ions (function of the energy used). We will discuss the results in the framework of future observations and Ryugu’s and Bennu’s samples studied in the laboratory.

Space-weathered rims on lunar ilmenite as an indicator for relative exposure ages of regolith

Solar wind irradiation, as a crucial space weathering mechanism, alters the microscopic characteristics and reflectance spectrum of the lunar regolith, and its cumulative effect is strongly related to the exposure time. Ilmenite is highly resistant to solar wind irradiation. Here, we combined transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy to systematically illustrate the diverse space-weathered rims on the shoveled and drilled Chang’e-5 ilmenites resulting from varying degrees of solar wind irradiation, revealing that solar wind plays a key role in the early-stage alteration of exposed lunar soil. In addition, the space weathering microstructure observations are consistent with the model-predicted exposure history at the Chang’e-5 landing site and prove that the Chang’e-5 deep-layered drilled (~65 cm) and surface samples (<3 cm) have been exposed for a longer time than that of the intermediate-layered drilled samples. We conclude that the ilmenite rims have the potential to be an effective indicator for comparing the relative exposure ages of regolith on airless bodies.

Lunar soil record of atmosphere loss over eons.

The Moon has a tenuous atmosphere produced by space weathering. The short-lived nature of the atoms surrounding the Moon necessitates continuous replenishment from lunar regolith through mechanisms such as micrometeorite impacts, ion sputtering, and photon-stimulated desorption. Despite advances, previous remote sensing and space mission data have not conclusively disentangled the contributions of these processes. Using high-precision potassium (K) and rubidium (Rb) isotopic analyses of lunar soils from the Apollo missions, our study sheds light on the lunar surface-atmosphere evolution over billions of years. The observed correlation between K and Rb isotopic ratios (δ 87Rb = 0.17 δ 41K) indicates that, over long timescales, micrometeorite impact vaporization is the primary source of atoms in the lunar atmosphere.

Photometric Properties within the Reiner Gamma Swirl: Constraining Formation Mechanisms

The area in the Reiner Gamma swirl studied by Weirich et al. for topographic correlations also displays correlations with the Hapke-model-derived single-scattering albedo, surface roughness, and particle scattering properties with swirl unit. The correlations with single-scattering albedo associate compositional variations in plagioclase and FeO content with swirl unit. The correlations with photometric surface roughness show a rougher surface on-swirl, implying a potentially more porous surface on-swirl compared to off-swirl. This suggests the variations in single-scattering albedo are dominated by the compositional differences and not structural differences, such as compaction. Grain-size differences could still contribute to the albedo variations. Differences in particle scattering properties between on-swirl and off-swirl are counter-indicative of the trend expected from variations in space weathering, unless there is a process to initiate either size sorting or compositional differences. The photometric properties point to a complex interaction of multiple processes to form the swirl units, not a singular dominant process. Variations in weathering, dust mobilization and entrapment, and impact modification may all play a key role.

Impact-dispersed Fe–Fe1−xS core–shell particles in Chang’e-5 lunar soil impact glass

Lunar soil undergoes space weathering and accumulates optically active opaque particles (OAOpq) of different sizes, resulting in a darkening or red shift of the reflectance spectrum. The surfaces of weakly weathered objects exhibit spectral characteristics of strong weathering; these mechanisms are still unclear. The causes of OAOpq in lunar soil are complex, especially for submicrometer particles, which account for the largest mass proportion. We found ubiquitous impact-dispersed Fe–Fe1-xS core–shell particles in Chang’e-5 lunar soil impact glass and splatter. The crystal structure, particle size distribution, and chemical composition of OAOpq in the impact glass indicate that these OAOpq consist of sulfides or metals from multiple sources. Thermodynamic evidence, diffusion behavior, and particle dispersion characteristics indicate that impact dispersion is the most likely formation mechanism of these OAOpq. The proposed impact dispersion provides a reason for the large number of OAOpq and the limited products for in situ reactions. This process explains why lunar soil with a low degree of weathering exhibits substantial spectral modification properties. The results provide insights into space weathering of the lunar surface and also imply that impact-dispersed OAOpq may be the primary modification type on asteroid surfaces. The unique chemical properties of Fe–Fe1-xS OAOpq also indicate that the lunar regolith has the potential for resource utilization.

Elemental differentiation and isotopic fractionation during space weathering of Chang’E-5 lunar soil

To investigate the chemical variation during space weathering of young mare basalts, here we report elemental, radiogenic Sr-Nd and stable Fe-Mg-Ca isotopic data of Chang’E-5 sieved soils and breccias. From the coarse fraction to the fine one, the sieved soils display increasing Al2O3 (10.34 wt%–13.36 wt%) and Sr (248 ppm–307 ppm) but decreasing FeO (23.50 wt%–20.22 wt%), MgO (6.88 wt%–5.78 wt%), FeO/Al2O3 (2.27–1.51) and MgO/Al2O3 (0.67–0.43). The contents of rare earth elements (except Eu) and high field strength trace elements do not vary with particle size but correlate with P2O5 contents. Given the limited contribution from contamination by meteorites and exotic materials ejected far away from the landing site, these elemental variations can be explained by differential comminution and distribution behaviors of plagioclase and mesostasis phases. These sieved soils yield a Sm-Nd isochron age (1.84 ± 0.83 Ga) comparable to that of basaltic clasts obtained by U-Pb dating (∼2.0 Ga). However, their Rb-Sr isotopic system is disturbed as indicated by their relatively homogeneous 87Sr/86Sr (0.701425–0.701592) despite variable Rb/Sr (0.017–0.028). These results suggest the Sm-Nd isotopic system is more robust to impact disturbance during space weathering compared to the Rb-Sr isotopic system. Given that the bulk soil still plots on the 2.03 Ga Rb-Sr reference isochron from the pristine plagioclases in CE-5 basalts, this disturbance did not affect the Rb-Sr isotopic system on the bulk scale. The CE-5 bulk soil has higher Mg# (33.6), 87Rb/86Sr (0.06) and present-day 87Sr/86Sr (0.701542) than the mean composition of reported basaltic clasts (Mg#: ∼28; 87Rb/86Sr: ∼0.038; 87Sr/86Sr: ∼0.700941), possibly implying that the bedrocks in CE-5 landing site consist of multiple magma pulses. The δ56Fe (0.122 ± 0.002 ‰ to 0.199 ± 0.008 ‰) and δ26Mg (−0.204 ± 0.016 ‰ to −0.109 ± 0.006 ‰) of sieved CE-5 soils increase with decreasing particle sizes but their δ44/42Ca (0.38 ± 0.04 ‰ to 0.44 ± 0.02 ‰) are relatively homogeneous. Mass balance modelling indicates that differential comminution has limited influence on the Fe-Mg-Ca stable isotopic compositions. We further dismiss the role of solar-wind sputtering, as Ca and Mg are more susceptible to sputtering and thus would be expected to show larger isotope fractionations compared to Fe, which is inconsistent with the observations. Free evaporation may explain the elevated δ56Fe and δ26Mg in fine fractions at given very limited depletion in FeO and MgO. The observed positive correlation between δ56Fe and δ26Mg, however, is much steeper than the slope expected for free evaporation, indicating also other mechanisms (e.g., Fe-Mg inter-diffusion). Since the CE-5 soil has a unique composition compared with Apollo and Luna soils, the chemical differentiation identified in this study provides new insights for establishing a connection between the chemistry and reflectance spectral properties of lunar soil. Our combined Fe-Mg-Ca isotopic study also provides a paradigm to distinguish the role of solar-wind sputtering and impact evaporation, and shows that the inter-particle diffusion process may be an important mechanism for the isotope fractionation among lunar soil components.

Past orbital evolution and its effects on the surface of (162173) Ryugu

ABSTRACT The Japanese probe Hayabusa 2 recently visited the NEA (162173) Ryugu. This mission provided valuable information about the asteroid, including in situ images and a surface sample brought back to the Earth. The data suggests that Ryugu has likely been affected by space weathering, but the underlying cause of the surface modification remains unclear. This paper aims to shed light on the problem by providing new information about the possible past orbital path of Ryugu. We used a procedure to estimate how Ryugu has dynamically evolved since it entered the NEAs’ region and whether this can explain the observed data. We have conducted numerical integrations of the N-body gravitational problem. The system comprises the Sun, the Solar system’s eight planets, and a group of 29 000 test particles initially placed in the main asteroidal belt (MAB) near the ν6 secular resonance. We then tracked the particles that left the MAB and eventually arrived close to Ryugu’s current orbit. Our analysis showed that out of 307 clones of Ryugu, only three came very close to the Sun at a distance of less than 0.1 au. Consequently, it is unlikely that Ryugu has undergone significant surface temperature variations. We also computed that ∼70 per cent of the clones have spent more than 5 Myr at an equivalent distance of 1 au from the Sun. This result suggests that Ryugu has been exposed to significant amounts of solar radiation over a long period, which may have resulted in space weathering effects on its surface.

Laboratory Spectral Characterization of Ribbeck Aubrite: Meteorite Sample of Earth-impacting Near-Earth Asteroid 2024 BX1

Characterization of near-Earth objects (NEOs) is critical for Earth-impact hazard assessment. Particularly crucial to our physical understanding of NEOs are laboratory spectral measurements of meteorites as they are the best and most widely available analog materials, barring sample return missions. However, most meteorites do not have direct orbital links to specific asteroids, making it challenging to identify their source body in the NEO or main-belt asteroid populations. Near-Earth asteroid (NEA) 2024 BX1 was discovered on 2024 January 20 at 21:48 UTC from MPC code K88, impacting the Earth (west of Berlin, Germany) 165 minutes later. The incoming bolide was observed by multiple meteor cameras, which enabled successful reconstruction of its exo-atmospheric orbit and quick recovery. We present results from laboratory spectral characterization of the Ribbeck meteorite in the UV–mid-infrared wavelengths (0.2–14.2 μm) over seven grain size bins (<45 μm–slab). Our results suggest that Ribbeck has spectral properties consistent with enstatite achondrite (aubrite) meteorites. Our grain-size spectral analysis shows that albedo and spectral slope decrease as grain size increases. In addition, increasing grain size also shifts the taxonomic type in the Bus–DeMeo system from Xn to B types, suggesting the limitations of taxonomy in classifying small, regolith-free NEAs. We also present results of our comparison between Ribbeck data and spectra of E types in the main-belt and NEA populations. Principal component analysis of our Ribbeck samples shows variations parallel to the α line, which can be confused with space weathering in PC space.

Asteroid reflectance spectra from Gaia DR3: Near-UV in primitive asteroids

Context. In the context of charge-coupled devices (CCDs), the ultraviolet (UV) region has mostly remained unexplored after the 1990s. Gaia DR3 offers the community a unique opportunity to explore tens of thousands of asteroids in the near-UV as a proxy of the UV absorption. This absorption has been proposed in previous works as a diagnostic of hydration, organics, and space weathering. Aims. In this work, we aim to explore the potential of the NUV as a diagnostic region for primitive asteroids using Gaia DR3. Methods. We used a corrective factor over the blue part of Gaia spectra to erase the solar analog selection effect. We identified an artificial relation between the band noise and slope and applied a signal-to-noise ratio (S/N) threshold for Gaia bands. Meeting the quality standards, we employed a Markov chain Monte Carlo (MCMC) algorithm to compute the albedo threshold, maximizing primitive asteroid inclusion. Utilizing one- and two-dimensional (1D and 2D) projections, along with dimensionality-reduction methods (such as PCA and UMAP), we identified primitive asteroid populations. Results. We uncovered: (a) the first observational evidence linking UV absorption to the 0.7 µm band, tied to hydrated iron-rich phyllosilicates; and (b) a 2D space revealing a split in C-type asteroids based on spectral features, including UV absorption. The computed average depth (3.5 ± 1.0 %) and center (0.70 ± 0.03 µm) of the 0.7 µm absorption band for primitive asteroids observed with Gaia is in agreement with the literature values. Conclusions. In this paper, we shed light on the importance of the UV absorption feature to discriminate among different mineralogies (i.e., iron-rich phyllosilicates vs. iron-poor) or to identify taxonomies that are conflated in the visible (i.e., F-types vs. B-types). We have shown that this is a promising region for diagnostic studies of the composition of primitive asteroids.

Global Inversion of Lunar Surface Oxides by Adding Chang’e-5 Samples

The chemical distribution on the lunar surface results from the combined effects of both endogenic and exogenic geological processes. Exploring global maps of chemical composition helps to gain insights into the compositional variation among three major geological units, unraveling the geological evolution of the Moon. The existing oxide abundance maps were obtained from spectral images of remote sensing and geochemical data from samples returned by Apollo and Luna, missing the chemical characteristics of the Moon’s late critical period. In this study, by adding geochemical data from Chang’e (CE)-5 lunar samples, we construct inversion models between the Christiansen feature (CF) and oxide abundance of lunar samples using the particle swarm optimization–extreme gradient boosting (PSO-XGBoost) algorithm. Then, new global oxide maps (Al2O3, CaO, FeO, and MgO) and Mg# with the resolution of 32 pixels/degree (ppd) were produced, which reduced the space weathering effect to some extent. The PSO-XGBoost models were compared with partial least square regression (PLSR) models and four previous results, indicating that PSO-XGBoost models possess the capability to effectively describe nonlinear relationships between CF and oxide abundance. Furthermore, the average contents of our results and the Diviner results for 21 major maria demonstrate high correlations, with R2 of 0.95, 0.82, 0.95, and 0.86, respectively. In addition, a new Mg# map was generated, which reveals different magmatic evolutionary processes in the three geologic units.

Incipient space weathering on asteroid 162173 Ryugu recorded by pyrrhotite

AbstractRegolith samples returned from asteroid 162173 Ryugu by the Hayabusa2 mission provide direct means to study how space weathering operates on the surfaces of hydrous asteroids. The mechanisms of space weathering, its effects on mineral surfaces, and the characteristic time scales on which alteration occurs are central to understanding the spectroscopic properties and the taxonomy of asteroids in the solar system. Here, we investigate the behavior of the iron monosulfides mineral pyrrhotite (Fe1−xS) at the earliest stages of space weathering. Using electron microscopy methods, we identified a partially exposed pyrrhotite crystal that morphologically shows evidence for mass loss due to exposure to solar wind ion irradiation. We find that crystallographic changes to the pyrrhotite can be related to sulfur loss from its space‐exposed surface and the diffusive redistribution of resulting excess iron into the interior of the crystal. Diffusion profiles allow us to estimate an order of magnitude of the exposure time of a few thousand years consistent with previous estimates of space exposure. During this interval, the adjacent phyllosilicates did not acquire discernable damage, suggesting that they are less susceptible to alteration by ion irradiation than pyrrhotite.