Abstract: Enstatite‐rich meteorites, including the aubrites, formed under conditions of very low oxygen fugacity (ƒO2: iron‐wüstite buffer −2 to −6) and thus offer the ability to study reduced magmatism present on multiple bodies in our solar system. Elemental partitioning among metals, sulfides, and silicates is poorly constrained at low ƒO2; however, studies of enstatite‐rich meteorites may yield empirical evidence of the effects of low ƒO2 on elemental behavior. This work presents comprehensive petrologic and oxygen isotopic studies of 14 aubrites, including four meteorites that have not been previously investigated in detail. The aubrites exhibit a variety of textures and mineralogy, and their elemental zoning patterns point to slow cooling histories for all 14 samples. Oxygen isotope analyses suggest that the aubrite parent bodies may be more heterogeneous than originally reported or may have experienced incomplete magmatic differentiation. Contrary to the other classified aubrites and based on textural and mineralogical observations, we suggest that the Northwest Africa 8396 meteorite shows an affinity for an enstatite chondrite parentage. By measuring major elemental compositions of silicates, sulfides, and metals, we calculate new metal–silicate, sulfide–silicate, and sulfide–metal partition coefficients for aubrites that are applicable to igneous systems at low ƒO2. The geochemical behavior of elements in aubrites, as determined using partition coefficients, is similar to the geochemical behavior of elements determined experimentally for magmatic systems on Mercury. Enstatite‐rich meteorites, including aubrites, represent valuable natural petrologic analogues to Mercury and their study could further our understanding of reduced magmatism in our solar system.

Abstract: We review constraints on the magnitude and possible causes of discrepancies, or at least major disparities, among global and near‐global data sets for lunar highland surface composition. When compared with data from other sources, reported mafic mineral abundance results from the Kaguya Spectral Profiler (Kaguya SP) spectral reflectance method for four Apollo 16 soils appear systematically low by a factor of 0.6, or an even more extreme factor (~1/3) if viewed in relation to the soils’ nonglass or CIPW mineralogy. Also, whether evaluated on a global median basis or on the basis of site‐by‐site comparison (for Apollo 16, Luna 20, and Apollo 17), the compositions found by the Kaguya SP technique show discrepancy, or at least disparity, versus other mafic abundance observations by that same factor of ~1/3. Spectral reflectance does not supply a simple bulk analysis of the target soil. The reflectance mineralogical signal is preponderantly determined by the nonglass fraction, and especially the masswise subordinate 10–20 µm grain size fraction. Literature data show that in anorthositic lunar soil, chemical composition is fractionated, more extremely anorthositic, for the nonglass component compared to the glass component. Also, the grain size fraction (10–20 μm) that most closely matches bulk reflectance has a significantly higher abundance of impact/agglutinitic glass than does the coarser material that dominates the soil mass. The Kaguya SP mafic abundance calibration needs adjustment by a factor of nearly 3 if results are to be interpreted as indicative of the mineralogy of the underlying crust. A claimed detection of several hundred lunar 500 m scale purest anorthosite (PAN; ≥98 vol% plagioclase) locales among millions of spectral reflectance observations is dubious, in part because with large data sets, compositional extremes are inevitably exaggerated as a byproduct of analytical uncertainty. Preponderance of PAN composition is rare among terrestrial layered intrusive anorthosites and is neither required nor expected for the flotation crust of a global magma ocean. Buoyant flotation and compaction would not suffice to yield pure plagioclase unless adcumulus growth was negligible, and trace element contents of ferroan anorthosites show that their mafic silicate components are for the most part of adcumulus, not “trapped melt,” derivation. A PAN‐dominated crust would imply a curiously fractionated (low) thorium/aluminum ratio for the crust, an implausibly high mantle/crust Th concentration ratio, and an oddly low Th/Al for the bulk Moon. Remote sensing techniques for planetary regolith composition are not easy to calibrate, particularly near the extremes of composition‐space and sensitivity.

Abstract: The meteorite Erg Chech (EC) 002 is the oldest felsic igneous rock from the solar system analyzed to date and provides a unique opportunity to study the formation of felsic crusts on differentiated protoplanets immediately after metal–silicate equilibration or core formation. The extinct 53Mn‐53Cr chronometer provides chronological constraints on the formation of EC 002 by applying the isochron approach using chromite, metal–silicate–sulfide, and whole‐rock fractions as well as “leachates” obtained by sequential digestion of a bulk sample. Assuming a chondritic evolution of its parent body, a 53Cr/52Cr model age is also obtained from the chromite fraction. The 53Mn‐53Cr isochron age of 1.73 ± 0.96 Ma (anchored to D'Orbigny angrite) and the chromite model age constrained between 1.46−0.68+0.78 and 2.18−1.06+1.32 Ma after the formation of calcium‐aluminium‐rich inclusions (CAIs) agree with the 26Al‐26Mg ages (anchored to CAIs) reported in previous studies. This indicates rapid cooling of EC 002 that allowed near‐contemporaneous closure of multiple isotope systems. Additionally, excess in the neutron‐rich 54Cr (nucleosynthetic anomalies) combined with mass‐independent isotope variations of 17O provides genealogical constraints on the accretion region of the EC 002 parent body. The 54Cr and 17O isotope compositions of EC 002 confirm its origin in the “noncarbonaceous” reservoir and overlap with the vestoid material Northwest Africa 12217 and anomalous eucrite Elephant Moraine 92023. This indicates a common feeding zone during accretion in the protoplanetary disk between the source of EC 002 and vestoids. The enigmatic origin of iron meteorites remains still unresolved as EC 002, which is more like a differentiated crust, has an isotope composition that does not match known iron meteorite groups that were once planetesimal cores.

Abstract: Chondrules are commonly surrounded by fine‐grained rims (FGRs) whose origin remains highly debated; both nebular and parent body settings are generally proposed. Deciphering their origin, however, is of fundamental importance as they could clarify the matrix–chondrule relationship and thus constrain the formation and transport conditions of chondrules in the circumsolar disk. Here, we report a systematic survey of FGRs in CO, CM, CV, and CR chondrites; we compare (i) the thickness of FGRs to the size of their host chondrules and (ii) the frequency of FGRs to the modal abundance of matrix in the respective host chondrites. Although FGRs show textural variations depending on the petrologic type of the considered chondrites, our data show a positive correlation between apparent rim thickness and the radius of the host chondrule in all chondrite groups. We also found a positive correlation between the evaluated percentages of rimmed chondrules and the modal abundance of matrix material in the chondrites. We show that this relationship could not result from parent body processes, whether matrix compaction or FGR fragmentation. Therefore, we propose that FGRs were accreted under warm conditions at the end of chondrule‐forming events. Our results thus support (i) a nebular origin for FGR, whose abundances are directly related to the abundance of available dust in regions of chondrite accretion; and (ii) the accretion of chondrites from locally formed chondrules and matrix, suggesting limited radial transport in the protoplanetary disk.

Abstract: Sample 12032,366‐18 is a 41.2 mg basaltic rock fragment collected during the Apollo 12 mission to the Moon. It is enriched in incompatible trace elements (e.g., 7 ppm Th), but does not have a bulk composition that would be considered a KREEP (enriched in potassium, rare earth elements, and phosphorous) basalt. The sample is of particular interest because it may be representative of some of the mare basalts within Oceanus Procellarum that are inferred to be Th‐rich, based on remote sensing data. The major mineral assemblage of 12032,366‐18 is pyroxene, plagioclase, olivine, and ilmenite, and the bulk composition has 4.2 wt% TiO2, 11.7 wt% Al2O3, and 0.25 wt% K2O. The sample contains regions of late‐stage crystallized minerals and glass (collectively termed mesostasis), including K‐feldspar, apatite, rare earth (RE) merrillite, ilmenite, troilite, silica, and relatively sodic plagioclase adjacent to ferroan pyroxene. The mesostasis also occurs in several areas that are highly enriched in silica and intergrown with K‐feldspar and very fine‐grained, high‐mean‐atomic‐number phases. We explore the petrology of this sample, including the origin of the Si‐K‐rich mesostasis to assess whether the mesostasis had formed by silicate liquid immiscibility (SLI). We used experiments to determine if the bulk composition of 12032,366‐18 is representative of a bulk liquid composition, how the residual liquid evolves, and to investigate the partitioning of elements between phases as the melt evolves. Experiments support that the mesostasis formed by SLI after crystallization of minerals closely matches the major‐mineral assemblage of 12032,366‐18. Experiments bracket the onset of SLI and merrillite saturation between 1024 and 1002 °C. Some high field strength elements, such as Zr and P, partition preferentially into the Fe‐rich liquid. From the experiments, we infer that the bulk composition of 12032,366‐18 represents the magma from which it crystallized. Based on the Th‐rich and KREEP‐bearing chemistry of this sample, along with experimental evidence showing that the sample is representative of a bulk liquid composition and not a cumulate, we conclude that basalt fragment 12032,366‐18 was delivered to the Apollo 12 landing site as ejecta from a distant impact and could represent an Oceanus Procellarum basalt. Missions to Oceanus Procellarum, such as Chang’E 5, have the potential to confirm whether some of those basalts are indeed enriched in Th and other incompatible trace elements as indicated by remote sensing.

Abstract: We report the concentrations and isotope ratios of light noble gases (He, Ne, Ar) in 10 small basalt fragments derived from lunar regolith soils at the Apollo 12 landing site. We use cosmic ray exposure (CRE) and shielding condition histories to consider their geological context. We have devised a method of using cosmogenic Ne isotopes to partition the CRE history of each sample into two stages: a duration of “deep” burial (shielding of 5–500 g cm−2) and a duration of near‐surface exposure (shielding of 0 g cm−2). Three samples show evidence of measurable exposure at the lunar surface (durations of between 6 ± 2 and 7 ± 2 Myr). The remaining seven samples show evidence of a surface residence duration of less than a few hundred thousand years prior to collection. One sample records a single‐stage CRE age range of between 516 ± 36 and 1139 ± 121 Myr, within 0–5 g cm−2 of the lunar surface. This is consistent with derivation from ballistic sedimentation (i.e., local regolith reworking) during the Copernicus crater formation impact at ~800 Myr. The remaining samples show CRE age clusters around 124 ± 11 Myr and 188 ± 15 Myr. We infer that local impacts, including Surveyor crater (180–240 Ma) and Head crater (144 Ma), may have brought these samples to depths where the cosmic ray flux was intense enough to produce measurable cosmogenic Ne isotopes. More recent small impacts that formed unnamed craters may have exhumed these samples from their deep shielding conditions to the surface (i.e., ~0–5 g cm−2) prior to collection from the lunar surface during the Apollo 12 mission.

Abstract: The 350–2500 nm reflectance spectra of five enstatite achondrites (aubrites), five metal‐rich chondrites (CBa, CBb, CH/CBb, and ungrouped), and seven sulfide mineral samples (three troilites, pyrrhotite, pentlandite, a mixture of pentlandite and chalcopyrite, and oldhamite) have been measured to search for spectral parameters that may offer insight into the surface composition of so‐called “spectrally featureless” asteroids. Spectral data were acquired from powders, slabs, and hand samples. Aubrites exhibit high reflectance, generally positive slopes at visible wavelengths, and low‐to‐negative infrared slopes, consistent with E‐/Xe‐type asteroids. The metal‐rich chondrites exhibit low reflectance, moderate visible slopes, and low near‐infrared slopes, somewhat consistent with M−/X‐complex asteroids. The metal‐rich chondrites exhibit absorption features at ~900 nm arising from Fe2+‐bearing silicates. Sulfides exhibit low to moderate reflectance and high visible and near‐infrared slope, intermediate to the T‐ and L‐type asteroids. The D‐type asteroids, which have high visible and near‐infrared slopes, are not well‐matched by sulfides. Spectral data of the largest M−/X‐type asteroid, (16) Psyche, are consistent with both powder from the Isheyevo CH/CBb chondrite and powder of meteoritic troilite. The data presented here will support interpretation of data returned from future spacecraft missions to “spectrally featureless” asteroids, like the Psyche, Lucy, and DART/Hera missions.

Abstract: On June 1, 2019, just before 7:30 p.m. local time, the Desert Fireball Network (DFN) detected a −9.3 magnitude fireball over South Australia near the Western Australia border. The event was observed by six fireball observatories, and lasted for 5 s. One station was nearly directly underneath the trajectory, greatly constraining the trajectory solution. This trajectory's backward numerical integrations indicate that the object originated from the outer main belt with a semimajor axis of 2.75 au. A light curve was also extracted and showed that the body experienced very little fragmentation during its atmospheric passage. A search campaign was conducted with several DFN team members and other volunteers. One 42 g fragment was recovered within the predicted fall area based on the dark flight model. Based on measurements of short‐lived radionuclides, the fragment was confirmed to be a fresh fall. The meteorite, Arpu Kuilpu, has been classified as an H5 ordinary chondrite. This marks the fifth fall recovered in Australia by the DFN, and the smallest meteoroid (≃2 kg) to ever survive entry and be recovered as a meteorite.

Abstract: Using high‐resolution atomic force microscopy (AFM) with CO‐functionalized tips, we atomically resolved individual molecules from Murchison meteorite samples. We analyzed powdered Murchison meteorite material directly, as well as processed extracts that we prepared to facilitate characterization by AFM. From the untreated Murchison sample, we resolved very few molecules, as the sample contained mostly small molecules that could not be identified by AFM. By contrast, using a procedure based on several trituration and extraction steps with organic solvents, we isolated a fraction enriched in larger organic compounds. The treatment increased the fraction of molecules that could be resolved by AFM, allowing us to identify organic constituents and molecular moieties, such as polycyclic aromatic hydrocarbons and aliphatic chains. The AFM measurements are complemented by high‐resolution mass spectrometry analysis of Murchison fractions. We provide a proof of principle that AFM can be used to image and identify individual organic molecules from meteorites and propose a method for extracting and preparing meteorite samples for their investigation by AFM. We discuss the challenges and prospects of this approach to study extraterrestrial samples based on single‐molecule identification.

Abstract: Martian meteorites are rare; therefore, the discovery of new meteorites has the potential to significantly expand our current understanding of Mars. In this study, we describe four new shergottites, all found within the past 5 yr, in Northwest Africa (NWA): NWA 10441, NWA 10818, NWA 11043, and NWA 12335. To determine the geochemical and mineralogical composition of these new shergottites, a number of traditional and nontraditional analytical techniques were utilized, such as high‐resolution X‐ray computed tomography (for 3‐D modal abundance determination) and electron backscattered diffraction (for identification of shock features). This enabled a comprehensive, nondestructive investigation of the in situ and bulk characteristics of these meteorites. From the results, we confirm the preliminary classifications of NWA 10441 and NWA 12335 as basaltic (diabasic), and NWA 10818 and NWA 11043 as poikilitic, shergottites. Chondrite‐normalized rare earth element (REE) patterns of shergottites distinguish likely source reservoirs in the Martian mantle. NWA 10441 and NWA 12335 have bulk enriched REE patterns. NWA 10818 has an intermediate REE pattern, being slightly depleted in the light REE. Although published data for bulk rock REE in NWA 11043 indicate an enriched pattern, here we show that targeted in situ analyses of unaltered minerals reveal an intermediate REE pattern, suggesting that terrestrial weathering combined with shock processes experienced by these meteorites on ejection may affect the bulk analysis. Extensive fracturing in NWA 11043 likely acted as conduits for terrestrial alteration products. We suggest that in situ spot checking of REE in meteorites will constrain any weathering effect on the REE pattern of the bulk rock.

Abstract: Meteorites are prone to errestrial weathering not only after their fall on the Earth’s surface but also during storage in museum collections. To study the susceptibility of this material to weathering, weathering experiments were carried out on polished sections of the H5 chondrite Asuka 10177. The experiments consisted of four 100‐days cycles during which temperature and humidity varied on a twelve hours basis. The first alteration cycle consisted of changing the temperature from 15 to 25 °C; the second cycle consisted of modifying both humidity and temperature from 35 to 45% and 15 to 25 °C, respectively; the third cycle consisted of varying the humidity level from 40 to 60%; and the fourth cycle maintained a fixed high humidity of 80%. Weathering products resulting from the experiments were identified and characterized using scanning electron microscopy–energy dispersive spectroscopy and Raman spectroscopy. Such products were not observed at the microscopic scale after the first cycle of alteration. Conversely, products typical of the corrosion of meteoritic FeNi metal were observed during scanning electron microscope surveys after all subsequent cycles. Important increases in the distribution of weathering products on the samples were observed after cycles 2 and 4 but not after cycle 3, suggesting that the combination of temperature and humidity fluctuations or high humidity (>60%) alone is most detrimental to chondritic samples. Chemistry of the weathering products revealed a high degree of FeNi metal corrosion with a limited contribution of troilite corrosion. No clear evidence of mafic silicate alteration was observed after all cycles, suggesting that postretrieval alteration remains limited to FeNi metal and to a lesser extent to troilite.

Abstract: Asteroids and comets are thought to form in the inner and outer solar systems, respectively. Chondritic porous and smooth interplanetary dust particles (CP IDPs and CS IDPs, respectively) in the stratosphere are regarded as dust grains from comets and hydrated asteroids, respectively. Here, we describe an Antarctic micrometeorite (AMM) composed of lithologies of both CP and CS IDPs. In addition to the CS IDP‐like compact lithology that experienced severe aqueous alteration, the CP IDP‐like porous lithology shows evidence of very weak aqueous alteration. The structure of the organic matter in the porous lithology varies from that in the CP IDPs to aromatic‐rich organic matter. In contrast, the structure of the organic matter in the compact lithology is homogenous, which is consistent with higher degrees of aqueous alteration. Its structure is more similar to that of CP IDPs and Wild 2 samples than that of meteoritic insoluble organic matter, suggesting that the compact lithology formed from the porous lithology. Some CP IDPs are related to cometary dust streams, such as those originating from 26P/Grigg‐Skjellerup. In addition, the presence of this AMM indicates an additional origin of the CP IDPs and their equivalent AMMs. The mineralogy and organic chemistry of this AMM suggest that its parent body was composed of the same building blocks as those of the comets, and later experienced incomplete aqueous alteration. The AMM probably formed as microbreccia in the regolith layer composed of materials from a CP IDP‐like crust and a hydrated interior.

Abstract: Paqueite (Ca3TiSi2[Al,Ti,Si]3O14; IMA 2013‐053) and burnettite (CaVAlSiO6; IMA 2013‐054) are new refractory minerals, occurring as euhedral to subhedral crystals within aluminous melilite in A‐WP1, a type A Ca‐Al‐rich inclusion, and CGft‐12, a compact type A (CTA) from the Allende CV3 carbonaceous chondrite. Type paqueite from A‐WP1 has an empirical formula of (Ca2.91Na0.11)Ti4+Si2(Al1.64Ti4+0.90Si0.24V3+0.12Sc0.07Mg0.03)O14, with a trigonal structure in space group P321 and cell parameters a = 7.943 Å, c = 4.930 Å, V = 269.37 Å3, and Z = 1. Paqueite’s general formula is Ca3TiSi2(Al,Ti,Si)3O14 and the endmember formula is Ca3TiSi2(Al2Ti)O14. Type burnettite from CGft‐12 has an empirical formula of Ca1.01(V3+0.56Al0.25Mg0.18)(Si1.19Al0.81)O6. It assumes a diopside‐type C2/c structure with a = 9.80 Å, b = 8.85 Å, c = 5.36 Å, β = 105.6°, V = 447.7 Å3, and Z = 4. Burnettite’s general formula is Ca(V,Al,Mg)AlSiO6 and the endmember formula is CaVAlSiO6. Paqueite and burnettite likely originated as condensates, but the observed grains may have crystallized from local V‐rich melts produced during a later thermal event. For CGft‐12, the compositions of paqueite, clinopyroxene, and perovskite suggest that type As drew from two distinct populations of grains. Hibonite grains drew from multiple populations, but these were well mixed and not equilibrated prior to incorporation into type A host melilite.

Abstract: Meteor impacts can induce unique pressure‐dependent structural changes in minerals due to the propagation of shock waves. Plagioclase—ubiquitous throughout the Earth’s crust, extraterrestrial bodies, and meteorites—is commonly used for reconstructing the impact history and conditions of the parent bodies. However, there have been unresolved inconsistencies in the interpretation of shock transformations across previous studies: The pressure at which amorphization begins and the process by which it occurs is the subject of ongoing debate. Here, we utilize time‐resolved in situ X‐ray diffraction (XRD) to probe the phase transformation pathway of plagioclase during shock compression at a sub‐nanosecond timescale. Direct amorphization begins at pressures much lower than what was previously assumed, just above the Hugoniot elastic limit of 5 GPa, with full amorphization to a high‐density amorphous phase, observed at 32(10) GPa and 20 ns. Upon release, the material partially recrystallizes back into the original structure, demonstrating a memory effect.

Abstract: The current meteorite taxonomy, a result of two centuries of meteorite research and tradition, entangles textural and genetic terms in a less than consistent fashion, with some taxa (like “shergottites”) representing varied lithologies from a single putative parent body while others (like “pallasites”) subsume texturally similar objects of multifarious solar system origins. The familiar concept of “group” as representative of one primary parent body is also difficult to define empirically. It is proposed that the classification becomes explicitly binominal throughout the meteorite spectrum, with classes referring to petrographically defined primary rock types, whereas groups retain a genetic meaning, but no longer tied to any assumption on the number of represented parent bodies. The classification of a meteorite would thus involve both a class and a group, in a two‐dimensional fashion analogous to the way Van Schmus and Wood decoupled primary and secondary properties in chondrites. Since groups would not substantially differ, at first, from those in current use de facto, the taxonomic treatment of “normal” meteorites, whose class would bring no new information, would hardly change. Yet classes combined with high‐ or low‐level groups would provide a standardized grid to characterize petrographically and/or isotopically unusual or anomalous meteorites—which make up the majority of represented meteorite parent bodies—for example, in relation to the carbonaceous/noncarbonaceous dichotomy. In the longer term, the mergers of genetically related groups, a more systematic treatment of lithology mixtures, and the chondrite/achondrite transition can further simplify the nomenclature.

Abstract: Primitive achondrites of the acapulcoite–lodranite clan (ALC) are residues of partial melting that displays a continuum of thermal metamorphism and partial melting most likely set by burial depth within an internally heated, primordial acapulcoite–lodranite parent body (ALPB). New major and trace element data from eight ALC meteorites and the application of several thermometric methods suggest that the ALPB was affected by partial differentiation disrupted by rapid cooling from peak, magmatic temperatures. Application of rare earth element‐in‐two‐pyroxene thermometry recovers temperatures of 1125–1250 °C for lodranites, while two‐pyroxene solvus and Ca‐in‐olivine thermometry recover lower temperatures for ALC meteorites (941–1114 °C and 686–850 °C, respectively). Major and trace element disequilibrium in acapulcoite and transitional groups provides evidence for cryptic melt infiltration and melt rock reaction within these layers of the ALPB. From lodranites, we determined rapid cooling rates of ~1 to ~26 °C yr−1 from peak temperatures, consistent with collisional fragmentation of the parent body during differentiation. After this initial period of rapid cooling, cooling rates decreased by two to four orders of magnitude through Ca‐in‐olivine closure temperatures (~750 °C). We hypothesize that the primordial ALPB possessed an onion shell‐type layered structure that was disrupted by collisional breakup during partial differentiation. Thermal modeling suggests that ALC samples originate from ~300 m to ~10 km radius collisional fragments that cooled rapidly over time scales of several to ~20,000 yr, then reaccreted to form a slower cooling, second‐generation rubble‐pile asteroid. The source of ALC meteorites is a second‐generation (or later) rubble‐pile body of S‐type spectral class located near the Jupiter 3:1 mean motion resonance in the Main Belt of asteroids.

Abstract: To date, over 500 short-period rocky planets with equilibrium temperatures above 1500 K have been discovered. Such planets are expected to support magma oceans, providing a direct interface between the interior and atmosphere. This provides a unique opportunity to gain insight into their interior compositions through atmospheric observations. A key process in doing such work is the vapor outgassing from the lava surface. LavAtmos is an open-source code that calculates the equilibrium chemical composition of vapor above a melt for a given composition and temperature. Results show that the produced output is in good agreement with the partial pressures obtained from experimental laboratory data as well as with other similar codes from literature. LavAtmos allows for the modeling of vaporisation of a wide range of different mantle compositions of hot-rocky exoplanets. In combination with atmospheric chemistry codes, this enables the characterization of interior compositions through atmospheric signatures.

Abstract: Carbon aggregates from two differently shocked ureilites were analyzed to gain insight into the shock transformation of graphite to diamond in ureilites, which happened when the ureilite parent body (UPB) was most likely destroyed by massive impact events. We present data for carbon aggregates from the highly shocked (U‐S6) Northwest Africa (NWA) 6871 and the medium shocked (U‐S3) NWA 3140. Both samples contain abundant carbon aggregates which were analyzed by X‐ray diffraction and micro‐Raman spectroscopy revealing the presence of close associations of (compressed) nanographite, micro‐ and nanodiamond, as well as Fe‐rich phases. Graphite and diamond in NWA 6871 show shock indicators that are absent in NWA 3140. Based on Raman geothermometry on graphite, we calculated mean temperatures of 1368 ± 120 °C and 1370 ± 120 °C for NWA 3140 and NWA 6871, respectively. For comparison, a geothermometer based on the partitioning of Cr between olivine and low‐Ca pyroxene was applied on NWA 3140, which yielded a temperature of only 1215 ± 16 °C. The graphite‐based temperatures are the highest reported for graphite in ureilites so far and exceed calculated magmatic temperatures for ureilites from silicate‐ and chromite‐based geothermometers. Graphite temperatures fall into the temperature field of catalytic diamond synthesis, which supports the hypothesis of direct transformation from graphite to diamond upon shock. Although the temperatures estimated seem to be independent of the shock degree, they can be ascribed to the shock event that destroyed the UPB.

Abstract: This paper presents an attempt to reconstruct the Campo del Cielo (CdC) impact event, that is, to estimate the preatmospheric mass and velocity of the iron meteoroid and pre‐impact parameters of its fragments allowing formation of funnels and impact craters. The goal of this study is to improve the understanding of the effects small‐scale iron meteoroids can have on the Earth's surface. We model the meteoroid's atmospheric flight taking deceleration, ablation, and fragmentation into account, and then compare the results with available observations. We found that a fragment's velocity near the surface should be <1 km s−1 in order to form a funnel with an intact meteorite inside. The estimates of preatmospheric (at an altitude of 100 km) parameters of the CdC impact event are as follows: minimal mass of 7500–8500 t, which corresponds to a diameter range of 12.2–12.8 m; maximum entry angle above the atmosphere of ~16.5° and velocities of 14.5–18.4 km s−1, which is close to the one most frequently reached by Near‐Earth objects (NEOs). Near the surface, the largest fragments with a mass of 400–1500 t and velocities of 4–7 km s−1 form impact craters whereas fragments with a mass <31 t and velocities <1 km s−1 form funnels. Masses <3 t are not included in our simulations. Their total mass is 280–460 t at the point of disruption but <110 t on the Earth's surface. These numerous small fragments are dispersed over a large area and are very popular among meteorite hunters and dealers. In spite of all the observed crater location/size data and impactor velocity limits from the models, there are far more free parameters than constraints. As a result, any values for preatmospheric mass, velocity, and entry angle are merely representative or limitative as opposed to true values.

Abstract: Enriched shergottites contain interstitial Si‐rich mesostasis; however, it is unclear whether such mesostasis is formed by impact or magmatic processes. We use laser ablation multicollector inductively coupled plasma mass spectrometry U–Pb measurements of minerals within the interstitial Si‐rich mesostasis and of merrillite within the coarse‐grained groundmass of Martian‐enriched gabbroic shergottite Northwest Africa (NWA) 6963. The date derived of tranquillityite, Cl‐apatite, baddeleyite, and feldspar from the Si‐rich mesostasis is 172.4 ± 6.1 Ma, and the derived merrillite date is 178.3 ± 10.6 Ma. We conclude, based on textural observation, that merrillite is a late magmatic phase in NWA 6963, that it was not produced by shock, and that its U–Pb‐system was not reset by shock. The indistinguishable dates of the gabbroic merrillite and the minerals within the Si‐rich mesostasis in NWA 6963 indicate that the Si‐rich mesostasis represents a late‐stage differentiated melt produced in the final phase of the magmatic history of the gabbroic rock and not a shock melt. This can likely be transferred to similar Si‐rich mesostases in other enriched shergottites and opens the possibility for investigations of Si‐rich mesostasis in enriched shergottites to access their magmatic evolution. Our results also provide a crystallization age of 174 ± 6 Ma (weighted average) for NWA 6963.

Abstract: Through the investigation of terrestrial ages of meteorites from Oman, we aim to better understand the time scales of meteorite accumulation and erosion in Oman and the meteorite flux in the past. Here, we present 14C and 14C‐10Be terrestrial ages of seven ordinary chondrite strewn fields and two unpaired single meteorites from the Sultanate of Oman. After critical evaluation of multiple data for each strewn field, we propose “best estimate terrestrial ages,” typically based on 14C/10Be. For objects for which complex irradiation histories are known or suspected, terrestrial ages were calculated solely using 14C. The best estimate strewn field ages range from 8.1 ± 3.0 ka (SaU 001) to 35.2 ± 5.1 ka (Dho 005). Including two previously dated strewn fields, the mean and median age of nine Oman strewn fields is 15.9 ± 12.3 and 13.6 ka, respectively. The new data show a general good agreement with data previously obtained in a different laboratory, and we observe a similar general correlation between strewn field ages and mean weathering grade as in previous work based on individual meteorites. Weathering degree W4 is reached for dated samples after 20–35 ka. While the age statistics of strewn fields does not show the previously observed lack of young events, the low abundance of young (0–5 ka) individual meteorites as compared with older (~20 ka) meteorites is confirmed by our data and remains unexplained.

Abstract: Our goal was to devise a bridge between shock determinations of asteroid regolith grains by standard light optical petrography, synchrotron X‐ray diffraction (SXRD), and electron backscattered diffraction (EBSD). We determined the optimal conditions under which to measure the shock stage of olivine crystals in astromaterial grains by EBSD. We applied this EBSD procedure to the shock stage determination of four regolith grains from asteroid Itokawa, returned to earth by the Hayabusa spacecraft. Interpretation of these data required a parallel examination of three ordinary chondrite standards that exhibited shock histories ranging from stage 2 to stage 4, using all three techniques. Standard light optical petrography indicated shock stage of S2/3 for the 24 Itokawa grains analyzed. SXRD results for seven Itokawa grains indicate a shock stage of S2. EBSD maps of four Itokawa grains indicate shock stage S3. Thus, the different techniques indicate slightly different shock stages, probably due to small sampling populations for EBSD and SXRD. We therefore recommend that significantly more than seven regolith grains should be separately analyzed by any shock determination technique, probably between 10 and 20. In any case, Itokawa regolith grains have been shocked to stage S2/3, or approximately 5–10 GPa. Finally, we investigated the crystallinity of one Itokawa olivine by SXRD, determining that the 5–10 GPa shock it had experienced did not appreciably alter the size of the unit cell, contrary to some previous suggestions.

Abstract: The early solar system was a dynamic period during which the formation of early solids set into motion the process of planet building. Although both astrophysical observations and theoretical modeling demonstrate the presence of widespread transport of material, we lack concrete quantitative constraints on timings, distances, and mechanisms thereof. To trace these transport processes, one needs objects of known early formation times and these objects would need to be distributed throughout parent bodies with known accretion times and distances. Generally, these criteria are met by “regular” (i.e., non–fractionated and unidentified nuclear and excluding hibonite‐rich) Ca‐Al‐rich inclusions (CAIs) as these objects formed very early and close to the young Sun and contain distinctive nucleosynthetic isotope anomalies that permit provenance tracing. However, nucleosynthetic isotopic signatures of such refractory inclusions have so far primarily been analyzed in chondritic meteorites that formed within ~4 AU from the Sun. Here, we investigate Ti isotopic signatures of four refractory inclusions from the ungrouped carbonaceous chondrite WIS 91600 that was previously suggested to have formed beyond ~10 AU from the Sun. We show that these inclusions exhibit correlated excesses in 50Ti and 46Ti and lack large Ti isotopic anomalies that would otherwise be indicative of more enigmatic refractory materials with unknown formation ages. Instead, these isotope systematics suggest the inclusions to be genetically related to regular CAIs commonly found in other chondrites that have a broadly known formation region and age. Collectively, this implies that a common population of CAIs was distributed over the inner ~10 AU within ~3.5 Myr, yielding an average (minimum) speed for the transport of millimeter‐scale material in the early solar system of ~1 cm s−1.

Abstract: Soluble organic matter analyses of astromaterials can provide valuable information on the chemistry of our solar system and the processes that occur within it. The surface of the Earth, however, is a significant source of organic compounds due to the presence of life; this environment represents a major source of potential contamination for recently fallen meteorites. Here, we analyze select stones of the CM2 Aguas Zarcas carbonaceous chondrite, which fell on April 23, 2019, in Aguas Zarcas, San Carlos county, Alajuela province, Costa Rica, with the goal of determining the complement of intrinsic and contaminant soluble organic matter. The specimens were collected pre- and post-rainfall, days to weeks after the stones fell on the Earth. Through gas chromatography-mass spectrometry analysis of soluble organic matter in dichloromethane and hot water extracts of meteorite powders, we differentiate between extraterrestrial and contaminant sources for each organic compound detected. In this study, N-tert-butyldimethylsilyl- N-methyltrifluoroacetamide (MTBSTFA) was used to derivatize the hot water extracts to test out its “one-pot” extraction capabilities. The majority of the detectable organic compounds are contaminants and can be explained as being sourced from the terrestrial surface onto which the meteorite fell. Our results have implications for how environmental factors, such as land use and rainfall events in this case, can impact the intrinsic organics in carbonaceous chondrites.

Abstract: The Earth's atmosphere is impacted daily by both meteoroids and artificial objects. Calibrated observations of the emitted light at sufficiently high sampling rates can enable or improve the estimation of impactor attributes such as size, cohesion, trajectory, and composition, but are difficult to obtain owing to the unpredictability, brevity, and high dynamic (brightness) range of impacts. Ground‐based camera systems have successfully monitored small regions of the atmosphere at video frame rates and with limited radiometric capabilities, but most impacts occur over the 70% of the Earth's surface covered by water and are therefore missed by these networks. The Geostationary Lightning Mapper (GLM) instruments aboard Geostationary Operational Environmental Satellites 16 and 17 provide near‐hemispherical coverage at 500 frames per second. These data have been shown to contain the signatures of many independently confirmed impacts, often from both viewing angles simultaneously, and constitute an observational resource that is currently unparalleled in the public domain. NASA's Asteroid Threat Assessment Project has implemented an automated impact detection pipeline that processes data from GLM daily. Given a detected impact, the GLM data contain a wealth of information for use in quantitative follow‐up analyses. However, impact events differ from lightning in ways that violate key assumptions built into GLM's design. The result is that GLM's onboard processing introduces errors into pixel observations of impact events and the calibrated energies near the periphery of the detector may be substantially overestimated. We present methods for mitigating these and other issues to produce a data product more suitable for impact analyses than the existing GLM lightning product.

Abstract: In order to understand the effects of the earliest fluid‐assisted hydration processes on asteroids, we performed one hydrothermal experiment using three different reactants (FeO‐rich amorphous silicates, iron metal powder, and water) at conditions informed by our current state of knowledge of asteroidal alteration. This experiment provides, for the first time, clear evidence that the growth of fayalite can occur during hydrothermal alteration, as described previously in meteorites. These newly formed fayalite crystals are elongated and porous, similar to the ones described in CV3, CK, and ordinary chondrites. The results show that (1) fayalite could form even if chemical equilibrium was not reached in the experiment, at a water to rock mass ratio (0.4 W/R at the beginning of the experiment) higher than the values calculated to be thermodynamically viable at equilibrium (W/R > 0.2); (2) the composition and the texture of the reactants changed during the hydrothermal alteration process, suggesting that the reactants, especially the amorphous silicates, underwent dissolution and reprecipitation; (3) fayalite can form at low temperature (220 °C), which is at the transition between hydrothermal alteration and fluid‐assisted metamorphism in chondrites. The results are consistent with previous mineralogical observations and thermodynamic models, which suggest that fayalite crystals are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and disequilibrium assemblages that compose the pristine materials that condensed in the early solar nebula. This experiment suggests that two variables play a very important role in the formation of fayalite during the hydrothermal growth (W/R mass ratio and the fluid composition). These results are similar to the recent observations of the fine‐grained matrix of ordinary chondrites.

Abstract: Ordinary chondrites account for the majority of the described meteorites on Earth. To expand the toolbox of analytical techniques available to describe such specimens, this study evaluates the application of a previously described fayalite determination method by X‐ray diffraction (XRD) to equilibrated ordinary chondrites. A suite of ordinary chondrites, ranging from petrologic type 4 to 6, and types H, L, and LL were analyzed by both XRD and electron probe microanalysis. A comparison of the results shows good agreement between the two methods with an R2 of 0.95 and better agreement for homogenous ordinary chondrites above petrographic grade 4. The differences between the two methods can largely be attributed to analytical uncertainty, as well as differences between point and bulk sampling techniques. These differences were used to identify two polymict breccia samples, Peace River and Northwest Africa 10946. Of note is the effect of exposure of the ordinary chondrites to room temperature and humidity conditions after sample preparation (powdering) and the impact on measured fayalite content by XRD. As such, it is recommended that XRD analyses of meteorites be performed immediately after sample preparation.

Abstract: An unusual variety of impact‐related diamond from the Popigai impact structure—yakutites—is characterized by complementary methods including optical microscopy, X‐ray diffraction, radiography and tomography, infrared, Raman, and luminescence spectroscopy providing structural information at widely different scales. It is shown that relatively large graphite aggregates may be transformed to diamond with preservation of many morphological features. Spectroscopic and X‐ray diffraction data indicate that the yakutite matrix represents bulk nanocrystalline diamond. For the first time, features of two‐phonon IR absorption spectra of bulk nanocrystalline diamond are interpreted in the framework of phonon dispersion curves. Luminescence spectra of yakutite are dominated by dislocation‐related defects. Optical microscopy supported by X‐ray diffraction reveals the presence of single crystal diamonds with sizes of up to several tens of microns embedded into nanodiamond matrix. The presence of single crystal grains in impact diamond may be explained by chemical vapor deposition–like growth in a transient cavity and/or a seconds‐long compression stage of the impact process due to slow pressure release in a volatile‐rich target. For the first time, protogenetic mineral inclusions in yakutites represented by mixed monoclinic and tetragonal ZrO2 are observed. This implies the presence of baddeleyite in target rocks responsible for yakutite formation.