Airless Bodies Research Articles

An advanced thermal roughness model for airless planetary bodies

We present a combined reflectance and thermal radiance model for airless planetary bodies. The Hapke model provides the reflected component. The developed thermal model is the first to consistently use rough fractal surfaces, self-scattering, self-heating, and disk-resolved bolometric albedo for entire planets. We validated the model with disk-resolved lunar measurements acquired by the Chinese weather satellite Gaofen-4 at around 3.5–4.1 μm and measurements of the Diviner lunar radiometer at 8.25 μm and 25–41 μm, finding nearly exact agreement. Further, we reprocessed the thermal correction of the global lunar reflectance maps obtained by the Moon Mineralogy Mapper M3and employed the new model to correct excess thermal radiance. The results confirm the diurnal, latitudinal, and compositional variations of lunar hydration reported in previous and recent studies with other instruments. Further, we compared the model to lunar measurements obtained by the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board BepiColombo during a flyby maneuver on April 9, 2020: the measured and the modeled radiance variations across the disk match. Finally, we adapted the thermal model to Mercury for emissivity calibration of upcoming Mercury flyby measurements and in-orbit operation. Although a physical parameter must be invariant under various observation scenarios, the best lunar surface roughness fits vary between different datasets. We critically discuss possible reasons and conclude that anisotropic emissivity modeling has room for improvement and requires attention in future studies.

Evaluating the effects of space weathering on magnetite on airless planetary bodies

Magnetite is a relevant mineral component of asteroids as it has been identified in carbonaceous chondrites, on the surface of asteroid Bennu through remote sensing observations, and in samples returned from asteroid Ryugu. However, the effects of space weathering processes on magnetite have not yet been explored. To investigate how this mineral phase responds to space weathering, here we simulate micrometeoroid bombardment and solar wind irradiation of magnetite using pulsed laser and ion irradiation experiments. We performed X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and visible to near-infrared (VNIR) reflectance spectroscopy analyses to characterize the chemical, microstructural, and spectral response of magnetite to simulated space weathering. In addition, we carried out ion impact simulations using the SDTrimSP software to evaluate the calculated response of magnetite to 1 keV H+ and 4 keV He+ ions and compared these results to our XPS and TEM results. Ion irradiation simulated ∼750 years on the surface of asteroid Bennu, with a solar-wind appropriate total H:He fluence ratio (∼24). Within this time, depletion of O was observed with H+ and He+ ion irradiation, with significantly greater change via protons due to the larger fluence, where preferential sputtering promotes the formation of a metallic iron layer at the magnetite surface. This suggests that solar wind ions act as reducing agents on Fe oxides, with a fraction remaining implanted in these phases. Indeed, we observe elongated defects contained in a crystalline rim created by He+ implanted ions in the TEM. Pulsed laser irradiation, analogous to micrometeoroid impacts, generates melts on the surface of the magnetite grains. The impact melts and H+-generated metallic iron rims both result in increased VNIR spectral reflectance, but lower fluence He+ implantation has no significant spectral effect. These results suggest that space weathered magnetite could contribute to bright regions detected in remote sensing analyses of the Ryugu and Bennu surfaces by the Hayabusa2 and OSIRIS-REx missions and will contribute to the identification and interpretation of space weathered magnetite in returned samples retrieved from both asteroids.

Laboratory investigations of triboelectric charging of dust by rover wheels

The dusty plasma environments of airless bodies present challenges for both exploration and science missions to their surfaces. Objects moving on the surface, such as rover wheels, transfer charges onto regolith dust due to the triboelectric effect. The charged dust particles can readily stick to spacesuits as well as the optical, electronic, and mechanical components of equipment and instruments, causing their degradation. This effect is more pronounced in permanently shadowed regions (PSRs) of the Moon, where the charge dissipation rate is low due to a lack of ambient plasma. Here we report the first results of an experimental investigation of the tribocharging effect between dust and rover wheels. Various wheel surface materials along the triboelectric series are selected. The effects of varying wheel speeds and dust types are explored. Results indicate that charge distributions conform to the triboelectric properties of the dust and material it comes into contact with. High charges of up to ∼2 pC are measured on dust grains ∼100 μm in diameter, which is equivalent to a surface potential of few hundred volts. Results also suggest that higher wheel speeds may result in more dust-dust interactions, changing the charging behavior of the dust grains. These findings indicate a need for future studies to better understand tribocharging in various exploration scenarios in order to design effective dust mitigation strategies and methods.

Micro-meteoroids impact vaporization as source for Ca and CaO exosphere along Mercury's orbit

The study of the micro-meteoroid environment is relevant to planetary science and space weathering of airless bodies, as the Moon or Mercury. In fact, the meteoroids hit directly the surfaces producing impact debris and vapor, thus contributing to shape the exosphere of the planet. This work is focused on the study and modelling of the Mercury's Ca exosphere formation through the process of Micro-Meteoroids Impact Vaporization (MMIV). The MESSENGER/NASA mission provided measurements of Mercury's Ca exosphere, allowing the study of its configuration and its seasonal variations. The observed Ca exhibited very high energies, with a scale height consistent with a temperature > 50,000 K, originated mainly on the dawn-side of the planet. It was suggested that the originating process is due to MMIV, but previous estimations were not able to justify the observed intensity and energy. We investigate the possible pathways to produce the high energy observed in the Ca exosphere and discuss about the generating mechanism. The most likely origin may be a combination of different processes involving the release of atomic and molecular surface particles. We use the exospheric Monte Carlo model by Mura et al. (2007) to simulate the 3-D spatial distribution of the Ca-bearing molecule and atomic Ca exospheres generated through the MMIV process, and we show that their morphology and intensity are consistent with the available MESSENGER observations if we consider a cloud quenching temperature < 3750 K. The results presented in this paper can be useful in the exospheric studies and in the interpretation of active surface release processes, as well as in the exosphere observations planning for the ESA-JAXA BepiColombo mission that will start its nominal mission phase in 2026.

In-situ measurement of hydrogen on airless planetary bodies using laser-induced breakdown spectroscopy

Remote-sensing observations on the surface of airless bodies, such as the Moon and asteroids, have confirmed the presence of hydrogen-bearing materials. However, their spatial distributions at small scales (mm–m) and depth profiles have great uncertainties. In-situ analyses of hydrogen-bearing materials with laser-induced breakdown spectroscopy (LIBS) have been proposed to resolve these problems, as the footprint of LIBS ablation is small (≲1 mm) and can penetrate into the subsurface by excavating the surface layer. Nevertheless, the measurement accuracy of hydrogen with LIBS on airless and hydrous planetary bodies has not been evaluated because it requires extensive calibration using hydrogen-rich geologic materials under a high-vacuum condition. In addition, whether hydrogen occurs as hydroxyl or ice has been difficult to ascertain via LIBS analysis because molecular information is typically lost in the ablation plasma. To resolve these problems, we conducted two experiments. First, compressed powders of rocks were measured by LIBS under vacuum (<3 × 10−2 Pa) to evaluate the calibration accuracies and detection limits in rocks and compacted soils on airless bodies. Several geostandards including basalts and feldspars were doped with various concentrations of hydroxyls (Mg(OH)2 and Ca(OH)2) to prepare hydrogen-rich samples up to 15 wt% in H2O-equivalent concentration (wt%H2O). Our results show that the hydrogen concentration can be accurately calibrated from the LIBS spectra by using multivariate models or matrix-matched calibration curves (i.e., calibration using samples with comparable bulk elemental abundances), facilitating the correction of significant matrix effects observed in the intensities of the 656 nm Hα line. We obtained a measurement accuracy of ±0.9 wt%H2O in the 0–12 wt%H2O range via matrix-matched calibration. This level of accuracy is sufficient for many planetary and resource exploration applications, such as designing hardware and operation for mining water on the Moon. We estimate the 2σ limit of detection (LOD) to be 0.4 wt%H2O based on the average of all samples, although better LODs were obtained for some individual matrix (e.g., 0.2 wt%H2O for basalt/feldspar–Ca(OH)2 mixtures). Such LOD shows that exploitable ice on the Moon can be detected with 2σ confidence by LIBS. Second, we demonstrate that the molecular structure of hydrogen can be distinguished by operating LIBS in tandem with heating lasers. In this method, the samples are heated prior to LIBS analysis using a continuous-wave laser with adjusted fluence and duration. Our results indicate that ice and hydroxyl can be distinguished because the Hα lines of ice-bearing samples decrease after the laser heating due to sublimation, but those of hydroxyl-bearing samples are retained. In addition, we report an enhancement of hydrogen emission from loose powders, suggesting that hydrogen in lunar soils may be measured with higher sensitivity. The results of this study show that LIBS is a versatile and powerful tool for accurate stand-off measurement of hydrogen-bearing materials on airless planetary bodies.

Space Weathering of Genesis Mission Solar-wind Collectors with Inferences for Weathering on Airless Bodies

Samples from the Genesis Solar Wind Sample Return (NASA Discovery 5) are a unique opportunity to study the initial stages of space weathering, i.e., the physical and chemical effects of solar-wind irradiation. Arrays of collectors containing multiple materials were each exposed to a different solar-wind regime (fast, slow, bulk, or coronal mass ejection) at the L1 point for long durations (years). Materials exposed to the solar wind included metals, semiconductors, and insulators. Although the time of exposure was obviously short relative to samples having extraterrestrial origins, optical properties, surface chemical properties, and matrix structure have changed in many collectors due to exposure to solar wind. The thickness of amorphous zones, where present, appears to correspond with the depth of the peak of the solar-wind H distribution in each regime. Damage from high-energy particles was negligible because the collectors were 700 μm or less in thickness and shielded from the back by the spacecraft. Micrometeorite impacts and sputtering were also negligible because of the short exposure times. Our current results are preliminary: We hope future workers will extend this study both to support Genesis characterization efforts and to further understand space-weathering processes on a geologic timescale.

Molecular Transport Conditions Required for the Formation of Penitentes on Airless, Ice‐Covered Worlds, With Specific Application to Europa, Enceladus, and Callisto

AbstractThe surface morphology of airless, ice‐covered moons of the outer solar system, such as Europa, Enceladus, and Callisto, is not well known at centimeter‐ to meter‐scales. Ice and snow erode differently on such worlds in part because sublimation is the dominant process. On Earth, ice penitentes have been observed in sublimation‐driven environments, and may provide a guide for similar formations on ice‐covered worlds. Penitentes are blade‐like snow features observed on Earth in high‐altitude, low‐latitude snowfields. Models of penitente formation on Earth break down within the free‐molecular regime of airless bodies, leaving a major gap in understanding whether such morphologies can form on their surfaces. To investigate the morphologic evolution of icy bodies, we developed a Sublimation Monte Carlo (SMC) model that enables a numerical approach to modeling exosphere‐surface interactions at free‐molecular conditions. The SMC model uses Monte Carlo tracking of molecules emitted from the surface to determine the net molecular interchange that drives surface changes. We validated results against experiments, matching the evolution of pre‐formed penitentes as they receded in height and became less pronounced. Our results reveal the importance of molecular redeposition on topology, indicating that the stable morphology of isothermal topographies is a planar morphology on regions of net sublimation, regardless of initial surface shape. A study of parametrically varying temperature profiles for sinusoidal penitentes resulted in the following requirement for penitente growth: the trough temperature must exceed the peak temperature by a threshold value, which notably depends on the surface aspect ratio and peak temperature.

A laboratory study of the phase ratio imagery method

As a new planetary remote sensing tool, the phase ratio imagery calculates the ratio of images taken at different phase angles and may suppress surface albedo variations and enhance surface texture features. This technique has been used in the study of surface structure of airless bodies such as the Moon and Mercury. To understand the effectiveness of the method, we carried out laboratory phase ratio measurements on eight planetary analog materials including four pure minerals olivine, orthopyroxene, labradorite, ilmenite and four mixtures, the lunar regolith simulant JSC-1A, the lunar highland simulant, the Martian soil simulant, and the CI asteroid simulant, all in two size distributions, 0–45 μm and 90–105 μm. For each sample, the phase ratio A(α1)/A(α2) is obtained by measuring the reflectance at two phase angles α1 and α2 with α1<α2 at two radiation wavelengths, 633 nm and 905 nm. The results show that: (1) The particle size distributions can be differentiated by measuring the phase ratio A(α1)/A(α2), and in order to increase the discriminative power of the particle size distribution, the value of (α1-α2) should be as large as possible. (2) For pure minerals, larger grains have smaller phase ratio values, because larger grains of pure minerals are more forward scattering, leading to larger A(α2) and thus smaller phase ratio. For mixtures with simulated agglutinates that hold minerals together as composite particles, larger grains have higher phase ratios because they are less forward scattering due to multiple internal reflections and hence more absorptions. Since real planetary regoliths are likely dominated by composite particles with agglutinates, it is expected that larger grains would have larger phase ratio values.

Unveiling the mechanics of lunar regolith erosion through analysis of CE-4 and CE-5 landing images and fluid simulation

During landing, the plume from the lander erodes the regolith on the lunar surface, thus destroying the nature of this surface. Landing also provides an opportunity to extract the mechanical properties of the lunar regolith in situ and to study wind erosion on airless bodies such as the Moon. Our goal in this study is to quantify the interparticle force of the lunar regolith, the erosion depth, and other parameters and to test the reliability of the plume erosion model. The erosion depth provides the necessary reference information for the precise interpretation of scientific results obtained from returned samples. We measure the total mass of the lunar regolith mobilized by the plume during the Moon landing. This information is also helpful for future lunar missions. With high-quality data from the Chang’E-5 (CE-5) and Chang’E-4 (CE-4) missions, we measure the erosion depth and the total mass and combine the results with computational fluid dynamics (CFD) to extract the interparticle force. We then test the plume erosion model according to the results from image measurement and CFD and propose a new formula with which to calculate the threshold friction velocity at which plume erosion is initiated. This calculation shows that the interparticle force for a 4-μm-diameter particle is 3.38–16.1 nN. The results also show that the CE-5 landing plume stripped away a 1.2-cm-deep layer of regolith, creating a ≈10-m-diameter crater on the lunar surface, and dispersed ≈441 kg of lunar regolith. Any analysis of a CE-5 drilling sample that is sensitive to the regolith depth must consider this 1.2-cm-thick eroded layer. When a plume erodes the lunar surface, the minimum shear stress required is much less than that predicted by the erosion model, which can no longer be used to predict whether erosion will occur.

Establishing a Best Practice for SDTrimSP Simulations of Solar Wind Ion Sputtering

Solar wind (SW) ion irradiation on airless bodies can play an important role in altering their surface properties and surrounding exosphere. Much of the ion sputtering data needed for exosphere studies come from binary collision approximation sputtering models such as TRansport of Ions in Matter and its more recent extension, SDTrimSP. These models predict the yield and energy distribution of sputtered atoms, along with the depth of deposition and damage of the substrate, all as a function of the incoming ion type, impact energy, and impact angle. Within SDTrimSP there are several user-specific inputs that have been applied differently in previous SW ion sputtering simulations. These parameters can influence the simulated behavior of both the target and sputtered atoms. Here, we have conducted a sensitivity study into the SDTrimSP parameters in order to determine a best practice for simulating SW ion impacts onto planetary surfaces. We demonstrate that ion sputtering behavior is highly sensitive to several important input parameters including the ion impact angle and energy distribution and the ejected atom surface binding energy. Furthermore, different parameters can still result in similarities in the total sputtering yield, potentially masking large differences in other sputtering-induced behaviors such as the elemental yield, surface concentration, and damage production. Therefore, it is important to consider more than just the overall sputtering behavior when quantifying the importance of different parameters. This study serves to establish a more consistent methodology for simulations of SW-induced ion sputtering on bodies such as Mercury and the Moon, allowing for more accurate comparisons between studies.

Space weathering effects and potential spectral alteration on Phobos and the Moon: Clues from the Fe content of olivine

Context. Olivine responds to space weathering in the fastest and most profound way, which results in significant space weathering spectral alteration effects (SWSAEs) on airless silicate bodies. Although Mg-rich olivine (Fa10) has been subjected to extensive studies, SWSAEs of Fe-rich (Fa# &gt; 20) or Fa-dominant (Fa# ⩾ 50) olivine are still poorly understood. Aims. We aim to systematically characterize the space weathering effects and the associated spectral alterations of Fe-rich olivine on the surface of Phobos and the Moon. Methods. We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa29, Fa50, Fa71, and Fa100) with energy levels simulated for Phobos and the Moon and analyzed the irradiated olivine for microscopic characteristics and near-infrared (NIR) and Raman spectroscopy. Results. Micron-level thick alteration layers are found in Fa100 compared to those hundreds of nanometers thick in Fa29, Fa50, and Fa71. With increasing irradiation energy levels and Fa# values, nanophase iron (np-Fe0) particles increase in size but decrease in quantity. The np-Fe0 formed via in situ decomposition are ubiquitously present, while those formed via vapor deposition are primarily found in Fa29 but rarely in Fa# ⩾ 50. The size fraction of intermediate (10–40 nm) and large (40–60 nm) np-Fe0 proportionally increases with Fa# values. The NIR spectra of weathered olivine show darkening over reddening in most cases, but Fa100 under the most irradiated condition shows brightening-reddening spectral effects. The Raman spectra of weathered olivine show a reduction in intensity without peak shifts. Conclusions. The Fa# values of olivine are a more critical factor in controlling the SWSAEs on Phobos than those on the Moon. If Phobos and Deimos contain substantial Fe-rich or Fa-dominant olivine, similar to Mars, thick alteration rims and large np-Fe0 formed via space weathering may cause darkening-reddening and potentially brightening-reddening spectral effects on the Martian moons.

Simulation of Space Weathering on Asteroid Spectra through Hydrogen Ion Irradiation of Meteorites

Space weathering can be defined as the combination of physical and chemical changes that occur in material exposed to an interplanetary environment on the surface of airless bodies. This process results in alterations in material spectroscopic features. Eventually, it can lead to misinterpretation of remotely sensed data in the visible–near-infrared wavelength range. This study simulates the solar wind effect on asteroid spectra through low-energy 1 keV H+ irradiation of meteorite pressed-powder samples under three fluences, 2 × 1017, 5 × 1017, and 1 × 1018 H+ cm−2, and evaluates changes associated with reflectance spectra. The meteorites subjected to the study are Bjurböle (L/LL4), Avanhandava (H4), and Luotolax (howardite). The most prominent changes in all three meteorites are (1) a decrease of 550 nm reflectance, (2) reddening in the 1 μm region, and (3) a monotonous decrease in absorption band strengths in Bjurböle. No significant changes were observed in the 2 μm region. The results imply that at short timescales (102–103 yr), radiation damage as amorphization and vesicle formation caused by low-energy solar wind is the dominant space weathering factor in all three meteorite compositions, causing spectral changes predominantly in the 1 μm region.

Effects of Albedo on the MIR Emissivity Spectra of Silicates for Lunar Comparison

AbstractWe used laboratory analysis to investigate the effect of mineral albedo, as defined at 750 nm, on the midinfrared emissivity spectra of silicates under lunar environment conditions. Optical darkening has long been recognized as an effect of space weathering on the visible‐to‐near‐infrared spectra of the Moon. However, space weathering has not been as thoroughly investigated in the mid‐infrared. Because mid‐infrared spectra are strongly influenced by the anisothermality of the lunar surface environment, it is likely that any darkening effects of space weathering would also change the thermal gradient in heavily space weathered lunar regolith. To isolate this variable, we added nanophase carbon to particulate samples of forsterite, augite, and anorthite to achieve a range of albedo samples and measured their midinfrared spectra under lunar environment conditions within the Planetary and Asteroid Regolith Spectroscopy Environment Chamber at Stony Brook University. We observe a shift in the Christiansen Feature maximum to longer wavelengths and decreasing spectral contrast with decreasing albedo. These shifts are well correlated with the observation of space weathering effects on the Diviner Lunar Radiometer Experiment compositional data, and point to the need for further investigation into the effects of space weathering on the midinfrared spectra of airless bodies.

A solar wind-derived water reservoir on the Moon hosted by impact glass beads

The past two decades of lunar exploration have seen the detection of substantial quantities of water on the Moon’s surface. It has been proposed that a hydrated layer exists at depth in lunar soils, buffering a water cycle on the Moon globally. However, a reservoir has yet to be identified for this hydrated layer. Here we report the abundance, hydrogen isotope composition and core-to-rim variations of water measured in impact glass beads extracted from lunar soils returned by the Chang’e-5 mission. The impact glass beads preserve hydration signatures and display water abundance profiles consistent with the inward diffusion of solar wind-derived water. Diffusion modelling estimates diffusion timescales of less than 15 years at a temperature of 360 K. Such short diffusion timescales suggest an efficient water recharge mechanism that could sustain the lunar surface water cycle. We estimate that the amount of water hosted by impact glass beads in lunar soils may reach up to 2.7 × 1014 kg. Our direct measurements of this surface reservoir of lunar water show that impact glass beads can store substantial quantities of solar wind-derived water on the Moon and suggest that impact glass may be water reservoirs on other airless bodies.

Low sulfide concentration in Mercury’s smooth plains inhibits hollows

MESSENGER (Mercury, Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury led to the discovery of hollows. These geological landforms have no close counterpart on other airless silicate bodies. Multispectral images and geochemical measurements by MESSENGER suggest that hollows are formed by the loss of volatile-bearing minerals. We investigated the mineralogical composition of the hollows using near-ultraviolet to near-infrared spectra obtained by MESSENGER. We compared reflectance spectra of hollows with laboratory spectra of Mercury's analogs: sulfides, chlorides, silicates, and graphite. The best candidates to reproduce the curvature of the hollow spectra are calcium sulfide, magnesium sulfide, and sodium sulfide. In addition, we performed spectral modeling with spectra obtained at the highest spectral and spatial resolution within the hollows. Our results show that the enrichment of sulfides in hollow material is up to two times higher than the sulfide concentration derived from chemical measurements of Mercury's high-reflectance smooth plains. This result explains the small percentage of hollows found within these plains.

Spectral effects of regolith porosity in the Mid-IR – Pyroxene

The regolith porosity on airless bodies in the Solar System has a known effect on remotely acquired spectra in the mid-infrared (MIR; 5–35 μm). Previous experiments quantifying this effect have focused on olivines. Here we report systematic laboratory experiments designed to quantify the effect of regolith porosity on the MIR spectra of pyroxene, an important rock forming mineral on Earth, the Moon, and throughout the Solar System. Specifically, we have measured MIR spectra of eight natural pyroxene minerals and mixtures over three particle size fractions (< 20 μm, 20–45 μm, and 45–63 μm) with varying degrees of porosity. Our results indicate that 0% regolith porosity spectra are dominated by surface scattering, 90% regolith porosity spectra are dominated by volume scattering, and the transition between the two regimes is gradual. Comparing selected spectra to the Trojan asteroid (624) Hektor, we estimate the regolith porosity of its surface to be >80%, and that the surface contains a significant fraction of amorphous material. Overall, these results are quite similar to our previous olivine findings, supporting the conclusion that regolith porosity strongly affects the shape and subsequent interpretation of MIR spectra of silicate-rich surfaces.

Quantifying the effects of submicroscopic metallic iron on VIS–NIR spectra of lunar soils

Metallic iron (Fe0) particles with sizes ranging from a few nanometers to the submicroscopic scale and formed by space weathering are specific components of lunar soil. Previous studies have suggested that the iron significantly alters the optical properties of lunar soil. For example, nanophase metallic iron (npFe0) causes both reddening and darkening of the lunar soil spectrum, and submicroscopic metallic iron (SMFe) only causes darkening. Here, we prepared SMFe particles with an average size of ∼180 nm embedded within melt glasses through carbothermal reduction experiments to analogize agglutinated glasses in the lunar soil. We evaluated the effect of SMFe content on visible and near-infrared (VIS–NIR) reflectance spectra of these lunar soil samples simulants. The spectral data show that SMFe content plays a key role in the optical properties of samples, including the average reflectance in the VIS-NIR range (400–2150 nm), and the absorption depth at ∼2 μm. A small amount (0.05 wt%) of SMFe mainly causes significant spectral darkening, and the average reflectance is reduced by 50% when the SMFe content rises to 0.36 wt%. Both the average reflectance and the absorption depth at ∼2 μm show a negative correlation with the SMFe content. We developed a quantitative model relating the spectral characteristics and the SMFe abundance based on experimental results. Thus, the SMFe contents play a key role in altering spectral characteristics of airless bodies during remote sensing spectroscopic detection.

Subsolidus breakdown of armalcolite: Constraints on thermal effects during shock lithification of lunar regolith

Abstract Shock lithification of regolith breccias is a ubiquitous process on the surfaces of airless planetary bodies and may induce thermal effects, including melting on regolith breccia minerals. However, potential thermal effects on lithic and mineral clasts in regolith breccias have seldom been quantitatively constrained. Here, we report two types of micro-textures of armalcolite [(Mg,Fe2+)Ti2O5] in an Mg-suite lithic clast from lunar regolith breccia meteorite Northwest Africa 8182. One type of armalcolite contains oriented fine-grained ilmenite grains; the other occurs as an aggregate of ilmenite, rutile, spinel, and loveringite. We propose that the two types of micro-textures formed through subsolidus breakdown of armalcolite by different processes. The formation of ilmenite inclusions in armalcolite is related to slow cooling after the solidification of its source rock, whereas the ilmenite-rutile-spinel-loveringite aggregates probably formed during the shock lithification event of NWA 8182. The results indicate that the temperature at the margin of lithic clasts could be raised up to at least 600 °C during strong shock lithification of lunar regolith and has profound thermal effects on the mineralogical and isotopic behaviors of lithic and mineral fragments in lunar regolith breccias.

Possible Formation Mechanism of Lunar Hematite

Hematite, a ferric mineral with diagnostic features in the visible and infrared spectral range, has recently been discovered in the polar regions of the Moon by the Chandrayaan-1 Moon Mineralogy Mapper (M3). The oxygen involving the oxidization process producing lunar hematite is supposed to originate from the Earth’s upper atmosphere, and hematite with different ages may have preserved information on the oxygen evolution of the Earth’s atmosphere in the past billions of years. The discovery of lunar hematite may provide insight into the understanding of the oxidation products on the Moon and other airless bodies. In this work, we analyze hematite abundance distribution in the lunar polar regions, showing that the content of hematite on the lunar surface increases with latitude, and is positively correlated with surface water abundance. We suggest that the latitude dependence of hematite is derived from the latitude dependence of water, which indicates that water may play an essential role in the formation of hematite. The correlation between hematite and the optical maturity parameter (OMAT) was analyzed and a significant positive correlation was observed, which suggests that the hematite in the polar regions is the result of gradual and persistent oxidation reactions. In addition, based on the analysis of oxygen particles in the Earth wind, it was found that O+ and O2+ are much more abundant, suggesting that low-energy O+ or O2+ ions escaping from the upper atmosphere of the Earth may play a crucial role in the formation of hematite in the lunar polar regions.

Detection of ferric iron in an exsolved lunar pyroxene using electron energy loss spectroscopy (EELS): Implications for space weathering and redox conditions on the Moon

AbstractTo shed light on the mechanism of formation of nanophase iron particles (npFe) in space‐weathered materials from airless bodies, we analyzed exsolved and unexsolved space‐weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy‐dispersive X‐ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space‐weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe0, Fe2+, and Fe3+ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe2+ and Fe3+ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe3+. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe2+ and Fe3+ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe3+ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe3+‐rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.