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Micrometeorites, tiny extraterrestrial particles, largely produced by collisions of celestial bodies, which survived atmospheric entry, fall on Earth at a rate of 40000 tons annually, and can be retrieved on the Antarctic. Micrometeorites represent a valuable source of information on the chemical evolution of the Solar System. Their microscopic size (50-2000 mm) warrants the use of novel in situ mapping techniques of elemental analysis. Hyphenated with a low-dispersion laser ablation (LA) system, ICP-ToF-MS allows for 2D mapping at several hundred hertz, with each laser shot recorded as a single pixel, and for quasi-simultaneous rapid acquisition of almost the entire periodic table. In this contribution we present approaches for and results of high-resolution multi-element 2D mapping of micrometeorites cross-sections via LA-ICP-ToF-MS. Quantitative data for each pixel was obtained through a combination of multi-point calibration - via a set of natural glass reference materials - and 100% oxide normalization to account for ablation yield. variations. Detection limits down to a few ng/g were attained while only using a 5x5 µm square laser spot. Analysis of the elemental images of micrometeorites allows to recognize and isolate melts of different composition, complimenting petrographic observations.

We report detailed transmission electron microscope (TEM) observations of carbonates from one hydrated fine-grained Antarctic micrometeorite (H-FgMM). These carbonates show the occurrence of complex chemical variations and microstructures that provide important evidence regarding the formation and evolution of rarely analyzed H-FgMMs. The chemical variations were identified at both micrometer and nanometer scales, indicating that these carbonates formed under localized fluid conditions that suggest a variable chemical microenvironment. Individual carbonates grew from isolated reservoirs of fluid. Moreover, these carbonates contain manganese amounts almost twice as high as those measured in CM chondrites but similar to those identified in CI chondrites. Their particular compositions indicate reducing and progressively evolving conditions in the fluid from which these carbonates precipitated, probably due to water consumption during phyllosilicates formation. In addition to the compositional variability, microstructural features are pervasive in these carbonates, similar to those described in heavily shocked meteorites indicating that these carbonates were probably modified during shock processes after their formation. Since carbonates are highly susceptible to shock metamorphism, we suggest that it is essential to investigate their structure in detail before interpreting the isotopic measurements related to the time of their formation. Additionally, associated with carbonates, ubiquitous phosphates were identified in the micrometeorite analyzed. Future studies of these mineral associations will provide us further insight into the formation and evolution of asteroids, especially since they were both identified in the surface materials of Ryugu and Bennu.

The study of meteorites is gaining importance due to its importance in understanding the development of our solar system. Published data shows that out of a total of 57186 meteorites recovered from the earth, 42398 are from Antarctica. This huge contribution of Antarctic meteorites is unique to Antarctica, as it allows the preservation of meteorites and concentrates them in highly ablative stranding surfaces. These surface or meteorite traps are formed as a result of topographic obstacles to the flow of the Polar ice sheet. Large tracts of central Droning Maud Land (cDML) in East Antarctica are still unexplored for meteorites. For the benefit of the global scientific community, it is desirable to recover as many Antarctic meteorites as possible before sinking deeper into the ice as a result of the changing climate and eventually being lost to the sea. The meteorite search programme was launched in the 32nd Indian Scientific Expedition to Antarctica. The primary objective of that investigation was to conduct a preliminary survey in the vicinity of the Somovken glacier flowing between the Humboldt and the Orvin mountain ranges for the prospect of finding meteorites. The search exercise was used to gain an understanding of the science of meteorites operating in Antarctica and the meteorite concentration mechanism. It was later complimented with the micrometeorite search programme in Antarctica.

The CM chondrites are samples of primitive water-rich asteroids formed during the early solar system. They record significant interaction between liquid water and silicate rock, resulting in a mineralogy dominated by hydrated secondary phases. Their similarity to the near-Earth asteroids Bennu and Ryugu – targets of current sample return space missions – makes the analysis of CM chondrites essential to the interpretation of these enigmatic bodies. Here, we review the aqueous alteration history of the CM chondrite group. Initially, amorphous silicate, metal and sulphides within the matrix were converted into Fe-cronstedtite and tochilinite. Later, the serpentinization of refractory coarse-grained inclusions led to the addition of Mg to the fluid phase. This is reflected in the cation composition of secondary phases which evolved from Fe-rich to Mg-rich. Although most CM meteorites are classified as CM2 chondrites and retain some unaltered anhydrous silicates, a few completely altered CM1s exist (∼4.2% [Meteoritical Bulletin, 2021]). The extent of aqueous alteration can be quantified through various techniques, all of which trace the progression of secondary mineralization. Early attempts employed petrographic criteria to assign subtypes – most notably the Browning and Rubin scales have been widely adopted. Alternatively, bulk techniques evaluate alteration either by measuring the ratio of phyllosilicate to anhydrous silicate (this can be with X-ray diffraction [XRD] or infrared spectroscopy [IR]) or by measuring the combined H abundance/δD compositions. The degree of aqueous alteration appears to correlate with petrofabric strength (most likely arising due to shock deformation). This indicates that aqueous alteration may have been driven primarily by impact rather than by radiogenic heating. Alteration extent and bulk O-isotope compositions show a complex relationship. Among CM2 chondrites higher initial water contents correspond to more advanced alteration. However, the CM1s have lighter-than-expected bulk compositions. Although further analyses are needed these findings could suggest either differences in alteration conditions or initial isotopic compositions – the latter scenario implies that the CM1 chondrites formed on a separate asteroid from the CM2 chondrites. Secondary phases (primarily calcite) act as proxies for the conditions of aqueous alteration and demonstrate that alteration was prograde, with an early period at low temperatures (

We report the discovery of a unique, refractory phase-bearing micrometeorite (WF1202A-001) from the Sør Rondane Mountains, East Antarctica. A silicate-rich cosmic spherule (~400 µm) displays a microporphyritic texture containing Ca-Al-rich inclusion (CAI)-derived material (~5–10 area%), including high-Mg forsterite (Fo98-99) and enstatite (En98-99, Wo0-1). The micrometeorite also hosts a spherical inclusion (~209 µm), reminiscent of chondrules, displaying a barred olivine texture. Oxygen isotopic compositions of the micrometeorite groundmass (δ17O = –3.46‰, δ18O = 10.43‰, ∆17O = –1.96‰) are consistent with a carbonaceous chondrite precursor body. Yet, a relict forsterite grain is characterized by δ17O = –45.8‰, δ18O = –43.7‰, ∆17O = –23.1‰, compatible with CAIs. In contrast, a relict low-Ca pyroxene grain (δ17O = –4.96‰, δ18O = –4.32‰, ∆17O = –2.71‰) presumably represents a first-generation silicate grain that accreted 18O-rich gas or dust in a transient melting scenario. The spherical inclusion displays anomalous oxygen isotope ratios (δ17O = –0.98‰, δ18O = –2.16‰, ∆17O = 0.15‰), comparable to anhydrous interplanetary dust particles (IDPs) and fragments from Comet 81P/Wild2. Based on its major element geochemistry, the chondrule size, and oxygen isotope systematics, micrometeorite WF1202A-001 likely sampled a carbonaceous chondrite parent body similar to, but distinct from CM, CO, or CV chondrites. This observation may suggest that some carbonaceous chondrite bodies can be linked to comets. The reconstructed atmospheric entry parameters of micrometeorite WF1202A-001 suggest that the precursor particle originated from a low-inclination, low-eccentricity source region, most likely either the main belt asteroids or Jupiter family comets (JFCs)., SCOPUS: ar.j, info:eu-repo/semantics/published

The surface of the Moon, like that of any airless body in the Solar System, constantly experiences micrometeorite bombardment as well as the influence of solar radiation, solar wind, and other factors of outer space. As a result of the impacts of high-velocity micrometeorites over billions of years, the lunar surface silicate basis crumbles, turning into particles with a wide size distribution. Considering the explosive nature of their origin, these particles are characterized by an extremely irregular shape with sharp edges or conglomerates sintered at high temperatures or almost spherical droplets. On the illuminated side of the Moon, solar radiation, especially the ultraviolet part of its spectrum, and solar wind streams interact with the upper regolith layer, charging the regolith surface. The photoelectrons generated above the surface create, together with the charged regolith surface, a near-surface double layer. The electric field generated in this layer, as well as the particle charge fluctuations on the surface, create conditions under which electric forces may exceed the gravitational force and the van der Waals force of adhesion. As a result, micron- and submicron-sized regolith particles become capable of detaching from the surface and levitating above it. These dynamic processes cause the transport of dust particles above the lunar surface and the scattering of sunlight on these particles. Glows of this kind were observed over the lunar surface by television systems of American and Soviet landing vehicles in the early stages of lunar exploration. American astronauts who landed on the lunar surface during the Apollo program also discovered manifestations of lunar dust. It turns out that dust particles levitating over the regolith surface due to natural processes and those took off the surface due to anthropogenic factors cause many technological problems that compromise the performance of landing vehicles and their systems, hamper astronaut activity on the lunar surface, and are detrimental to their health. Based on the results of these missions, it is concluded that micron- and submicron-sized dust particles, levitating above the surface, pose a major, barely surmountable obstacle in further research and exploration of the Moon. Since then, studies of physical processes associated with the behavior of lunar dust, manifestations of its aggressive properties (toxicity), and ways to reduce the harmful effects of dust on engineering systems and on humans have become topical in theoretical and experimental research. In this review, the results of the past half century of studies on the behavior of dust particles serve as a basis to discuss the formation of the lunar regolith and the Moon’s near-surface plasma–dust exosphere under the influence of outer space factors. The causes and conditions underling the behavior of dust particles are examined as well as implications of these processes, the influence of anthropogenic factors, and possible hazards to spacecraft and engineering systems during the implementation of the currently planned programs of lunar research and exploration. The main unsolved problems are listed in studying the behavior of the dust component of the lunar regolith; ways to address the problematic issues are discussed.

The results of electron-probe and spectrophotometric study of five porous micrometeorites found on the Novaya Zemlya glacier are presented. Their reflectance spectra are compared with those of terrestrial serpentines and several carbonaceous chondrites. The possibility of discovery and identification of mineralogical absorption bands in the reflectance spectra of micrometeorite particles is demonstrated.

Based on sediment-dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H-chondrite dominance is similar to that recorded within an enigmatic 3He anomaly (70, 27, and 3%) in the Turonian, but different from just before this 3He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K-Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H-chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian 3He anomaly, resulting in an H-chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor. (Less)

The first known asteroid with the orbit inside that of Venus is 2020~AV$_{2}$. This may be the largest member of a new population of small bodies with the aphelion smaller than 0.718~au, called Vatiras. The surface of 2020~AV$_{2}$ is being constantly modified by the high temperature, by the strong solar wind irradiation that characterizes the innermost region of the Solar system, and by high-energy micrometeorite impacts. The study of its physical properties represents an extreme test-case for the science of near-Earth asteroids. Here, we report spectroscopic observations of 2020~AV$_{2}$ in the 0.5-1.5~$\mu m$ wavelength interval. These were performed with the Nordic Optical Telescope and the William Herschel Telescope. Based on the obtained spectra, we classify 2020~AV$_{2}$ as a Sa-type asteroid. We estimate the diameter of this Vatira to be $1.50_{-0.65}^{+1.10}$ km by considering the average albedo of A-type and S-complex asteroids ($p_V=0.23_{-0.08}^{+0.11}$), and the absolute magnitude (H=$16.40\pm0.78$ mag). The wide spectral band around 1~$\mu m$ shows the signature of an olivine rich composition. The estimated band centre $BIC = 1.08 \pm 0.02~\mu m$ corresponds to a ferroan olivine mineralogy similar to that of brachinite meteorites., Comment: Accepted for publication in MNRAS. 10 pages, 5 figures

The shock-explosive processing of rocks by micrometeorite and meteorite bombardment on the surface of the Moon and other airless bodies of the solar system is a dominant geological and geochemical process that produces the loose layer of regolith. The paper presents results of modeling the process of a micrometeorite impact using a millisecond laser. The targets in the experiment were a sample of basalt of composition similar to those of basalts at Moon’s mare areas and a sample of basalt glass obtained by melting this basalt. At the laser “impact”, products ejected from the crater (glass spherules, droplets of various shapes, and condensates) and melting products in the crater were studied by electron microscopy, electron microprobe analysis with energy-dispersive spectrometers, X-ray diffraction, and X-ray fluorescence analysis. It has been shown that the glasses (melt drops) obtained in the experiment with the basalt target are characterized by a heterogeneous distribution of chemical composition. They were mixed in variable proportions as a result of melting the original minerals of the basalt. The spherules that have undergone significant evaporation differentiation in the experiment with a basalt target (crystalline basalt) make up about 25%. When the target was basalt glass, the proportion of the spherules (that have undergone profound evaporative differentiation) was ~90%. The glass was depleted in high- and medium-volatility components (Na2O, K2O, FeO, etc.). The most strongly differentiated glasses corresponded to high-alumina HASP glasses found on the Moon (Al2O3 > 34 wt %, SiO2 < 32 wt %).

Tomkins et al. [A. G. Tomkins et al., Nature 533, 235–238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O2 in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O2 must have been near present-day levels (∼21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O2. But data from shock tube experiments suggest that CO2 itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO2/N2 ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N2, like today, the lower limit on the CO2 mixing ratio is ∼0.23. This would produce a mean surface temperature of ∼300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN2 was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ∼291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower—closer to 0.6 bar—for CO2 to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO2 and surface pressure; and could help resolve discrepancies between climate models and existing CO2 proxies such as paleosols.

Space weathering (SW) processes including solar wind irradiation and micrometeorite bombardment produce nano-phase iron (npFe) and amorphous layers that redden and darken the visible and near-infrared (VNIR) reflectance spectra of the surface of airless bodies. To understand how SW may affect the directional reflectance and polarization properties of regolith layers, we carried out pulsed laser irradiation experiments to simulate micrometeorite bombardment process on olivine (OL), orthopyroxene (OPX), anorthosite (AN), ilmenite (ILM), and JSC-1A Lunar Regolith Simulant (LRS). All samples have been equalized with a size distribution of (

Here we present results from pulsed laser irradiation of CI and CM simulant samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. For this study, we focused on determining what type of alteration occurs in the 3-micron absorption region, as this region will be critical to ascertain compositional information of the surface regolith of hydrated asteroids. Generally, using entirely in situ spectral analysis, we find that the laser produces similar effects in both samples. Specifically, irradiation causes the blue spectral slope to decrease until it is relatively flat and that the sample darkens initially with laser irradiation but brightens back to about half of its original level by the end of the irradiation. Furthermore, we also find that laser irradiation causes the band depth on the 3-micron absorption band to increase by as much as 30%, yet the shape of the entire absorption band does not change and the band minima of the 2.72 micron shifts less than 0.001 micron after laser irradiation. The constancy of the latter two parameters, which will be most critical to compositional analysis, suggests that this spectral region could be very useful to determine the asteroid composition on surfaces on hydrated asteroids that have undergone extensive aqueous alteration even if the surface had been subject to a significant amount of space weathering. Whether the same conclusion will be generally applicable to other surfaces containing minerals with a wide range of aqueous alteration is currently unclear but will be tested in future studies.

One of the signs of “space weathering” on the Moon, including the effect of the solar wind and micrometeorite bombardment, is the formation of nanophase metallic iron (np-Fe0) in the lunar regolith, which is observed in condensate films on the surface of regolith mineral grains and in agglutinate glasses. The results of pulsed laser experiments simulating a micrometeorite “impact” on a basalt target are reported. Iron nanoglobules structured into chains and most likely marking the front of the shock wave were discovered in its melt products. The experiments clearly showed that np-Fe0 may be formed without participation of a reducing agent, such as implanted hydrogen ions of the solar wind, as well as bypassing the process of iron condensation from shock-generated vapor. The similar np-Fe0 chain structures were detected in the impact glass of the lunar regolith.

We have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the Frasnian-Famennian stratotype section at Coumiac in southern France, providing the first insights on the types of micrometeorites and meteorites that fell on Earth at this time. The data can test whether the small cluster of roughly coeval, large impact structures is related to an asteroid breakup and shower with possible bearings also on the late 26 Devonian biodiversity crisis. A total of ~180 extraterrestrial spinel grains (>32 microns) were recovered from 957 kg of rock. Noble-gas measurements of individual grains show high solar-wind content, implying an origin from decomposed micrometeorites. Element analyses indicate a marked dominance of ordinary chondritic over achondritic grains, similar to the recent flux. The relation between H, L and LL meteorites is ~29-58-13%, similar to the late Silurian flux, ~31-63-6%, but different from the distribution, ~45-45-10%, in the recent and the Cretaceous flux. Our data show no indication of a generally enhanced late Devonian micrometeorite flux that would accompany an asteroid shower. However, in a single limestone bed that formed immediately before the Upper Kellwasser horizon, that represents the main end-Frasnian species-turnover event, we found an enrichment of ~10 ordinary chondritic grains (>63 microns) per 100 kg of rock, compared to the ~1-3 grains per 100 kg that characterize background. The anomalously abundant grains are of mixed H, L and LL types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2.4 Ma Earth-orbit eccentricity cycles at the onset of the Upper Kellwasser event. In the present solar system the dust accretion at Earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud. Besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late Silurian and Devonian, in contrast to the mid-Ordovician, when achondritic meteorites that are rare on Earth today were common, followed by the influx of a flood of debris related to the breakup of the L-chondrite parent body. Our accumulated data for six time windows through the Phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid-Ordovician. We note that the sources in the asteroid belt of the H and L3 meteorites, the two most common types of meteorites today and through much of the Phanerozoic, remain elusive.

The spectral slopes of S and Q-type asteroids are altered by the weathering of their surfaces due to solar wind interactions and micrometeorite impacts, as well as any processes that work to remove that weathered material. These processes of space weathering and asteroid resurfacing compete with each other to determine the spectral slope of each asteroid, with space weathering raising the spectral slope and resurfacing lowering it. By considering the distribution of spectral slopes with respect to orbital location and size, we can determine which potential resurfacing processes are the most dominant. In this study, we show that the spectral slopes of S and Q-type Near-Earth Asteroids (NEAs) decrease with decreasing perihelion, but only for perihelia q ≲ 0.9 AU. We then use this distribution of spectral slopes vs. perihelion as a constraint for modeling two proposed resurfacing mechanisms. For both models, we numerically integrate the orbits of NEAs over their lifetimes and track their spectral slopes. First, we model the resurfacing of asteroids due to close encounters with the terrestrial planets and found that no combination of parameters could explain the spectral slope vs. perihelion trend at q ≲ 0.9 AU. We also model the resurfacing of asteroids due to thermally induced surface degradation, by assuming a power law relationship between the resurfacing timescale and the solar distance. For a space weathering timescale of an asteroid on a circular orbit at 1 AU of τ S W 0 ≲ 5 Myr, we find a range of resurfacing timescales and power law exponents that generate a spectral slope vs. perihelion distribution consistent with observations. We discuss that previous studies supporting the resurfacing of asteroids due to close encounters could be a result of confounding variables, as the correlation of Q-type asteroids' past orbits with the terrestrial planets are confounded by the higher fraction of Q-type asteroids at low perihelia, which could be created by another mechanism. We also discuss previous studies that qualitatively support the process of thermally induced surface degradation at low perihelia, and conclude that thermal processes are the most consistent explanation for resurfacing of asteroids with low perihelia q ≲ 0.9 AU.

We performed pulsed-laser irradiation of a chip of the CM2 Murchison carbonaceous chondrite meteorite to simulate micrometeorite impacts on carbonaceous asteroids. Optical reflectance spectroscopy and transmission electron microscopy were performed to characterize the unirradiated and irradiated samples and vapor and melt deposits collected on a glass slide ∼7 mm from the surface of the sample. The spectrum of the deposit on the glass slide shows a red slope between 0.35–2.5 µm, while the irradiated surface of the meteorite shows only slight darkening over the same spectral range. We identified predominant melt products and vesiculated textures in the glass slide deposit, in the fine-grained matrix of the meteorite, and in individual mineral phases of the meteorite chip. Extracted focused ion beam (FIB) sections from the matrix material, an olivine grain, a pentlandite grain, and from the glass slide deposit were analyzed by scanning transmission electron microscopy (STEM). Microstructural and chemical analyses based on the STEM observations show widespread melting and the formation of Fe-bearing nanoparticles (including prevalent Fe–Ni–sulfides) across the surface of the meteorite. The section extracted from the glass slide revealed nanoparticles embedded in a chemically and microstructurally complex deposit, which likely formed as a result of both melting and vaporization processes. These analyses reveal a significantly more compositionally diverse population of nanoparticles compared to what is observed in lunar or ordinary chondritic space weathered samples. We discuss the implications these results have for the space weathering of carbonaceous asteroids and their importance for understanding the surface processes on primitive bodies.

185 fine-grained micrometeorites (FgMMs) from the Novaya Zemlya micrometeorite collection were studied to elucidate the sources of cosmic dust. The compositions of the micrometeorites were compared with those of carbonaceous chondrites and cosmic spherules. FgMMs were weakly or moderately heated when decelerated in the atmosphere, and some of them thereby retained their original textures (unmelted FgMMs), while others consist mostly of porous degassing products of phyllosilicates (scoriaceous FgMMs). According to the composition of their matrix, we divide the FgMMs into two groups: high- and low-Fe. The textures and compositions of the FgMMs are generally similar to those of the matrix of carbonaceous chondrites. The FgMM group with high-Fe matrix corresponds to the composition of CI, CM, CO, and CR chondrites, which may have been a source of these meteorites. Conversely, the low-Fe group of FgMMs differs from chondrites of these groups and is likely related to the carbonate-poor matrix of the Tagish Lake carbonaceous chondrite or is presented by a material that is still absent from meteorite collections. We have compared the compositions of the FgMMs with the compositions of various types of cosmic spherules (CSs), which were produced by melting of micrometeoroids (cosmic dust particles) at their deceleration in the atmosphere. Both original and published data on CS compositions are used. Porphyritic and barred textural varieties of CSs could have been produced from micrometeoroids similar in the compositions to the high-Fe FgMMs, whereas low-Fe micrometeoroids might have been precursors of cryptocrystalline and glass CSs.

During the Apollo era of lunar exploration, mysterious albedo patterns, called swirls, captured the imagination of the scientific community. A key aspect of this interest was due to the discovery that the swirls are associated with localized relatively strong remnant magnetic fields. Analysis of returned soil samples revealed that solar wind, galactic cosmic rays, and micrometeorite impacts change the albedo of surface soil grains, a process known as space weathering that reduces regolith reflectance over time. Thus it was natural to invoke local magnetic structures as shields that retard space weathering resulting in relatively less space weathering of the regolith in regions with the strongest localized magnetic fields. However, the origins of the magnetic anomalies and associated swirls remain enigmatic to this day. We designed the Swirl CubeSat to determine the nature of remnant lunar magnetic fields and investigate their role in moderating space weathering of the regolith and assess their suitability for radiation protection of surface assets. Swirl has one focused observational objective: characterize the magnetic field associated with the Reiner Gamma Swirl (RGS) at sub-kilometer spatial sampling, with 0.5 nTesla accuracy and 100 m spatial precision. In the Swirl mission concept, the Swirl spacecraft, a 6U CubeSat, would deploy as a secondary payload from a vehicle on a deep space trajectory. A series of maneuvers would then place the spacecraft in a low orbit that would then be modified to have a periapse of 5–10 km for thirty orbits passing over RGS. Prime Swirl observations were designed during these low-altitude passes and consist of high-resolution vector magnetic field measurements and monochrome navigation imaging.

Extraterrestrial dust that reaches the Earth’s surface has shown to represent the diverse types of samples from different precursors, namely, asteroid complexes and cometary bodies from the solar system. A substantial amount of this dust that strikes the upper atmosphere is believed to have been lost due to frictional heating with air molecules. Cosmic spherules that are melted particles are some of the widely recognized micrometeorites that survived this catastrophic entry process; however, their primordial characteristics are altered from their precursors making it difficult to identify the precursors. An individual peculiar spherule MS-I35-P204 recovered from the Antarctica blue ice has been identified. The spherule has been segregated using magnetic separation method, mounted in epoxy, and examined using SEM, subsequently analysed under electron microprobe. It is surrounded by a thin magnetite rim, and also holds a single kamacite bead that protrudes out at its top. The interior mineralogy mostly constitutes of a bulk pyroxene normative glass (MnO>2wt%) with several vesicles. The rare mineral phase is a skeletal aggregate of free silica, bearing Fe nuggets embedded in a glass. An isolated narrow lath of forsterite appears to be chondritic and is observed as relict grain that is associated with an anomalous low Ca pyroxene (MnO ~1.3 wt%, FeO~13 wt%). Earlier, free silica has been reported in some chondritic meteorites particularly the Enstatite and Ordinary group, and also in some carbonaceous chondrites such as CM, CR, CH, and K. It profoundly forms a pod that encloses the ferromagnesian silicate in silica-bearing chondrules. The unusual mineral assemblage seen in this spherule thereby appears to constrain probably the unique type of its contributor which need to be studied.

By analyzing the trajectories of ionized constituents of the lunar exosphere in time-varying electromagnetic fields, we can place constraints on the composition, structure, and dynamics of the lunar exosphere. Heavy ions travel slower than light ions in the same fields, so by observing the lag between field rotations and the response of ions from the lunar exosphere, we can place constraints on the composition of the ions. Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) provides an ideal platform to utilize such an analysis, since its two-probe vantage allows precise timing of the propagation of field discontinuities in the solar wind, and its sensitive plasma instruments can detect the ion response. We demonstrate the utility of this technique by using fully time-dependent charged particle tracing to analyze several minutes of ion observations taken by the two ARTEMIS probes ~3000-5000 km above the dusk terminator on 25 January 2014. The observations from this time period allow us to reach several interesting conclusions. The ion production at altitudes of a few hundred kilometers above the sunlit surface of the Moon has an unexpectedly significant contribution from species with masses of 40 amu or greater. The inferred distribution of the neutral source population has a large scale height, suggesting that micrometeorite impact vaporization and/or sputtering play an important role in the production of neutrals from the surface. Our observations also suggest an asymmetry in ion production, consistent with either a compositional variation in neutral vapor production or a local reduction in solar wind sputtering in magnetic regions of the surface.

We analysed the heterogenous fine-grained (sub-μm) matrix of a small (58 × 93 μm), unmelted and minimally heated (

Alteration processes on asteroid and comet surfaces, such as thermal fracturing, (micrometeorite) impacts or volatile outgassing, are complex mechanisms that form diverse surface morphologies and roughness on various scales. These mechanisms and their interaction may differ on the surfaces of different bodies. Asteroid Ryugu and comet 67P/Churyumov–Gerasimenko, both, have been visited by landers that imaged the surfaces in high spatial resolution. We investigate the surface morphology and roughness of Ryugu and 67P/Churyumov–Gerasimenko based on high-resolution in situ images of 0.2 and 0.8 mm pixel resolution over an approximately 25 and 80 cm wide scene, respectively. To maintain comparability and reproducibility, we introduce a method to extract surface roughness descriptors (fractal dimension, Hurst exponent, joint roughness coefficient, root-mean-square slope, hemispherical crater density, small-scale roughness parameter, and Hapke mean slope angle) from in situ planetary images illuminated by LEDs. We validate our method and choose adequate parameters for an analysis of the roughness of the surfaces. We also derive the roughness descriptors from 3D shape models of Ryugu and orbiter camera images and show that the higher spatially resolved images result in a higher roughness. We find that 67P/Churyumov–Gerasimenko is up to 6 per cent rougher than Ryugu depending on the descriptor used and attribute this difference to the different intrinsic properties of the materials imaged and the erosive processes altering them. On 67P/Churyumov–Gerasimenko sublimation appears to be the main cause for roughness, while on Ryugu micrometeoroid bombardment as well as thermal fatigue and solar weathering may play a significant role in shaping the surface.

The annual flux of extraterrestrial material on Earth is largely dominated by sub-millimetre particles. The mass distribution and absolute value of this cosmic dust flux at the Earth's surface is however still uncertain due to the difficulty in monitoring both the collection efficiency and the exposure parameter (i.e. the area-time product in m2.yr). In this paper, we present results from micrometeorite collections originating from the vicinity of the CONCORDIA Station located at Dome C (Antarctica), where we performed several independent melts of large volumes of ultra-clean snow. The regular precipitation rate and the exceptional cleanliness of the snow from central Antarctica allow a unique control on both the exposure parameter and the collection efficiency. A total of 1280 unmelted micrometeorites (uMMs) and 808 cosmic spherules (CSs) with diameters ranging from 30 to 350 μm were identified. Within that size range, we measured mass fluxes of 3.0 μg.m−2.yr−1 for uMMs and 5.6 μg.m−2.yr−1 for CSs. Extrapolated to the global flux of particles in the 12-700 μm diameter range, the mass flux of dust at Earth's surface is 5 , 200 ± 1200 1500 tons.yr−1 ( 1 , 600 ± 500 and 3 , 600 ± 700 1000 tons.yr−1 of uMMs and CSs, respectively). We indicate the statistical uncertainties expected for collections with exposure parameters in the range of 0.1 up to 105 m2.yr. In addition, we estimated the flux of altered and unaltered carbon carried by heated and un-heated particles at Earth's surface. The mass distributions of CSs and uMMs larger than 100 μm are fairly well reproduced by the CABMOD-ZoDy model that includes melting and evaporation during atmospheric entry of the interplanetary dust flux. These numerical simulations suggest that most of the uMMs and CSs originate from Jupiter family comets and a minor part from the main asteroid belt. The total dust mass input before atmospheric entry is estimated at 15,000 tons.yr−1. The existing discrepancy between the flux data and the model for uMMs below 100 μm suggests that small fragile uMMs may evade present day collections, and/or that the amount of small interplanetary particles at 1 AU may be smaller than expected.

Iron-rich micrometeorites from a 2.7 billion-year-old limestone could constrain atmospheric CO2 levels on the early Earth., Earth’s atmospheric composition during the Archean eon of 4 to 2.5 billion years ago has few constraints. However, the geochemistry of recently discovered iron-rich micrometeorites from 2.7 billion–year–old limestones could serve as a proxy for ancient gas concentrations. When micrometeorites entered the atmosphere, they melted and preserved a record of atmospheric interaction. We model the motion, evaporation, and kinetic oxidation by CO2 of micrometeorites entering a CO2-rich atmosphere. We consider a CO2-rich rather than an O2-rich atmosphere, as considered previously, because this better represents likely atmospheric conditions in the anoxic Archean. Our model reproduces the observed oxidation state of micrometeorites at 2.7 Ga for an estimated atmospheric CO2 concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO2 level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.

We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating ∼15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo91)) and a natural pyroxene (Bamble orthopyroxene (En87)) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERTIS experiment onboard the BepiColombo mission. For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, ∼450 nm in olivine, 100-250 nm in pyroxene, as well as in developed “nanostratigraphy” of laser-ablated material, like nanophase iron (npFe). In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites.

We present the first reconstruction of the micrometeorite flux to Earth in the Silurian Period. We searched 321 kg of condensed, marine limestone from the Late Silurian Cellon section, southern Austria, for refractory chrome-spinel grains from micrometeorites that fell on the ancient sea floor. A total of 155 extraterrestrial spinel grains (10 grains >63 μm, and 145 in the 32-63 μm fraction) were recovered. For comparison, we searched 102 kg of similar limestone from the mid-Ordovician Komstad Formation in southern Sweden. This limestone formed within ~1 Ma after the breakup of the L chondrite parent body (LCPB) in the asteroid belt. In the sample we found 444 extraterrestrial spinel grains in the >63 μm fraction, and estimate a content of at least 7000 such grains in the 32-63 μm fraction. Our results show that in the late Silurian, ~40 Ma after the LCPB, the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude, almost down to background levels. Among the ordinary chondrites, the dominance of L-chondritic micrometeorites has waned off significantly, from >99% in the post-LCPB mid-Ordovician to ~60% in the Late Silurian, with ~30% H-, and ~10% LL-chondritic grains. In the Late Silurian, primitive achondrite abundances are similar to today's value, contrasting to the much higher abundances observed in pre-LCPB mid-Ordovician sediments. (Less)

Radar-bright deposits at the poles of Mercury are located in permanently shadowed regions, which provide thermally stable environments for hosting and retaining water ice on the surface or in the near subsurface for geologic timescales. While the areal distribution of these radar-bright deposits is well characterized, their thickness, and thus their total mass and volume, remain poorly constrained. Here we derive thickness estimates for selected water-ice deposits using small, simple craters visible within the permanently shadowed, radar-bright deposits. We examine two endmember scenarios: in Case I, these craters predate the emplacement of the ice, and in Case II, these craters postdate the emplacement of the ice. In Case I, we find the difference between estimated depths of the original unfilled craters and the measured depths of the craters to find the estimated infill of material. The average estimated infilled material for 9 craters assumed to be overlain with water ice is ∼ 41 − 14 + 30 m, where 1−σ standard error of the mean is reported as uncertainty. Reported uncertainties are for statistical errors only. Additional systematic uncertainty may stem from georeferencing the images and topographic datasets, from the radial accuracy of the altimeter measurements, or from assumptions in our models including (1) ice is flat in the bowl-shaped crater and (2) there is negligible ice at the crater rims. In Case II, we derive crater excavation depths to investigate the thickness of the ice layer that may have been penetrated by the impact. While the absence of excavated regolith associated with the small craters observed suggests that impacts generally do not penetrate through the ice deposit, the spatial resolution and complex illumination geometry of images may limit the observations. Therefore, it is not possible to conclude whether the small craters in this study penetrate through the ice deposit, and thus Case II does not provide a constraint on the ice thickness. For Mercury's polar deposits, we argue that Case I of the small craters predating the emplacement of the ice deposits is more likely, given other geologic evidence that suggests that these ice deposits are relatively young. Using the ice thickness estimates from Case I to calculate the total amount of water ice currently contained in Mercury's polar deposits results in a value of ∼10 14 –10 15 kg. This is equivalent to ∼100–1000 km 3 ice in volume. This volume of water ice is consistent with delivery via micrometeorite bombardment, Jupiter-family comets, or potentially a single impactor.

Space weathering is an important process on the Moon and other airless celestial bodies. The most common space weathering effects are amorphization of the top surface of soil grains and formation of nanophase iron particles (npFe) within the partially amorphous rims. Hence, space weathering significantly affects optical properties of the surface of the Moon and other airless celestial bodies. Transmission electron microscope (TEM) analysis of Apollo 15 soil grains displays npFe (≤5 nm in size) embedded in the space-weathered rim (∼60 nm in thickness) of a pyroxene grain, consistent with previous studies. In contrast, submicron-sized fragments that adhere to the pyroxene grain show distinct space weathering features. Silicon oxide nanoparticles (npSiOx) were observed with npFe in a submicron-sized Mg–Fe silicate fragment. This is the first discovery of npSiOx as a product of space weathering. The npSiOx and the coexisting npFe are ∼10–25 nm in size, significantly larger than the typical npFe in the space weathered rim of the pyroxene grain. The coexisting npSiOx and npFe were probably formed directly in micrometeorite shock-induced melt, instead of in a solar-wind generated vapor deposit or irradiated rim. This new observation will shed light on space weathering processes on the Moon and airless celestial bodies.

Without the protection of the atmosphere, the soils on lunar surfaces undergo a series of optical, physical, and chemical changes during micrometeorite bombardment. To simulate the micrometeorite-bombardment process and analyze the impact characteristics, four types of rocks, including terrestrial basalt and anorthosite supposed to represent lunar rock, an H-type chondrite (the Huaxi ordinary chondrite) and an iron meteorite (the Gebel Kamil iron meteorite) supposed to represent micrometeorite impactors, are irradiated by a nanosecond-pulse laser in a high vacuum chamber. Based on laser irradiation experiments, the laser pits are found to be of different shapes and sizes which vary with the rock type. Many melt and vapor deposits are found on the mineral surfaces of all the samples, and nanophase iron (npFe) or Fe-Ni alloy particles are typically distributed on the surfaces of ilmenite, kamacite or other minerals near kamacite. By analyzing the focused ion beam (FIB) ultrathin slices of laser pits with a transmission electron microscope (TEM), the results show that the subsurface structures can be divided into three classes and that npFe can be easily found in Fe-bearing minerals. These differences in impact characteristics will help determine the source material of npFe and infer the type of micrometeorite impactors. During micrometeorite bombardment, in the mare regions, the npFe are probably produced simultaneously from lunar basalt and micrometeorites with iron-rich minerals, while the npFe in the highlands regions mainly come from micrometeorites.

As the youngest features on Ganymede, ray craters are useful in revealing the sources of recent impactors and surface modification processes on the satellite. We examine craters with D > 10 km on Ganymede from images obtained by the Voyager and Galileo spacecraft to identify ray craters and study their spatial distributions. Furthermore, we carefully select images of appropriate solar and emission angles to obtain unbiased ray crater densities. As a result, we find that the density of large ray craters (D > 25 km) on the bright terrain exhibits an apex-antapex asymmetry, and its degree of asymmetry is much lower than the theoretical estimation for ecliptic comets. For large craters (D > 25 km), ecliptic comets ought to be less important than previously assumed, and a possible explanation is that nearly isotropic comets may play a more important role on Ganymede than previously thought. We also find that small ray craters (10 km 10 km) on the dark terrain show no apex-antapex asymmetry. We interpret that the distribution difference between the terrain types comes from preferential thermal sublimation on the dark terrain, while the distribution difference between large and small ray craters suggests that rays of small craters are more readily erased by some surface modification processes, such as micrometeorite gardening.

The term “space weathering” refers to processes that include changes in the physical, chemical, mineralogical, and spectral properties of the surface of asteroids, comets, and some planets and their satellites, such as the bombardment by micrometeorites, solar wind ions, and cosmic rays. In this study, we focus on micrometeorite impacts, which may be the primary contributor to the annual mass flow of material that reaches the surface of such bodies. Studying the processes and effects associated with micrometeorite impacts is fundamental for understanding the evolution of the solar system and its components. From an experimental point of view, it is typically assumed that micrometeorite impacts may be simulated by ns-pulsed lasers and, indeed, many experimental studies have been performed based on such assumption. These studies have the common main goal to understand how micrometeorite impacts may change the physical-chemical and spectral properties of the bombarded surfaces. However, here we perform the first experimental study dedicated to the morphological characterization of the impact craters created by ns-pulsed laser ablation, in order to determine how well ns-pulsed lasers simulate the crater morphology of natural micrometeorite impacts. For this purpose, the laser ablation technique was applied to three different silicates: feldspar, quartz, and jadeite. For each of these minerals, two ablation scenarios have been considered: in air and in water. The craters formed by ns-pulsed laser ablation were characterized, from the morphological point of view, using a profilometer. Using this data we estimated the depth:diameter ratio of each crater. The comparison with literature data shows that the simple craters formed by ns-pulsed laser ablation closely resemble craters formed by natural micrometeorite impacts. In other words, from a morphological point of view, ns-pulsed laser ablation is appropriate for the simulation of micrometeorite impacts. We additionally verified that the value of the depth:diameter ratio does not depend, within errors, on the total number of laser pulses or the repetition frequency, at least within the ranges covered in these experiments: i) between 1 and 1200 laser pulses and ii) between 1 and 10 Hz.

We examined the influence of micrometeorite bombardment simulated with an excimer laser, which irradiated mixtures of minerals with an energy density between 1.9 J/cm2 and 3.3 J/cm2. As starting material for the mixtures, we selected a natural olivine (Fo91) and a natural pyroxene (En87). Olivine and pyroxene are important rock-forming minerals of Earth's upper mantle and also occur on other terrestrial planetary bodies. The minerals were used to prepare a compositional range of mixtures that were pressed into pellets and examined especially by diffuse reflectance mid-infrared spectroscopy to identify changes caused by micrometeorite impacts as one tracer of space weathering on airless bodies such as Mercury or Moon. Spectral mid-IR measurements were carried out on untreated and treated samples in high vacuum. For comparison, the untreated samples are investigated in low vacuum under various phase angles. Especially the spectral variation between 7 μm–14 μm is of interest as the most important mid-IR features of rock-forming minerals as the Christiansen feature, Reststrahlen bands, and Transparency feature are located in this spectral region. As a result, we found that the mid-IR spectra of the powder pellets show signs of agglutination after irradiation. Here, olivine demonstrates a larger effect on irradiation than pyroxene, whereby pyroxene in mixtures can mask the presence of olivine in the mid-IR spectra after micrometeorite bombardment. We also observed a shift of the CF to longer wavelengths (shorter wavenumbers) after irradiation, and from high to low vacuum, a modification more prominent in olivine than in pyroxene. Therefore, this study shows that the identification and quantification of minerals in mixtures solely on the base of the CF location of the respective material can lead to an overestimation of modal abundances, which is important for the interpretation of data coming back from space missions, e.g., from the MERTIS experiment onboard BepiColombo.