Meteorite Classes Research Articles

Hyperspectral FTIR imaging of irradiated carbonaceous meteorites

Space weathering produces variations in the optical properties of small solar system body surfaces. Laboratory studies have been performed by several groups on different meteorite classes, aimed at simulating space weathering by using ion irradiation and laser ablation. Here we present new FTIR spectral imaging data of ion irradiated carbonaceous chondrites (CV Allende and CM Murchison meteorites), simulating solar wind irradiation on primitive asteroids. Irradiation conditions (40 keV He+ and Ar+ up to 3.1016 ion/cm2) were reported in detail in previous studies. The new analyses were performed with a FTIR micro-spectrometer equipped with an FPA (Focal Plane Array) detector. Micro-reflectance spectra were acquired in the 2.5–12 μm range using a spot size of 20 μm and scanning large areas (from mm2 to cm2) of the meteorite pellets, both on virgin areas and on areas irradiated with different doses. Spectral maps allow us characterizing the heterogeneity of the meteorites at the 20 μm spatial scale. In the irradiated areas, we observe spectral modifications of both anhydrous and hydrated silicates. Spectral shifts are observed after irradiation, larger for anhydrous than hydrated silicates. Compositional heterogeneity of the pristine materials and irradiation effects are compared to each other as a function of the irradiation dose, to determine which spectral features are more sensitive to space weathering. Results are then compared with the IR spectral capabilities of instruments onboard the Hayabusa2/JAXA and OSIRIS-REx/NASA spacecrafts, to provide these missions with spectral criteria on how to distinguish space weathering from compositional heterogeneity effects at the asteroid, and in view of surface selection for sample return.

Probing the use of spectroscopy to determine the meteoritic analogues of meteors

Context. Determining the source regions of meteorites is one of the major goals of current research in planetary science. Whereas asteroid observations are currently unable to pinpoint the source regions of most meteorite classes, observations of meteors with camera networks and the subsequent recovery of the meteorite may help make progress on this question. The main caveat of such an approach, however, is that the recovery rate of meteorite falls is low (<20%), implying that the meteoritic analogues of at least 80% of the observed falls remain unknown. Aims. Spectroscopic observations of incoming bolides may have the potential to mitigate this problem by classifying the incoming meteoritic material. Methods. To probe the use of spectroscopy to determine the meteoritic analogues of incoming bolides, we collected emission spectra in the visible range (320–880 nm) of five meteorite types (H, L, LL, CM, and eucrite) acquired in atmospheric entry-like conditions in a plasma wind tunnel at the Institute of Space Systems (IRS) at the University of Stuttgart (Germany). A detailed spectral analysis including a systematic line identification and mass ratio determinations (Mg/Fe, Na/Fe) was subsequently performed on all spectra. Results. It appears that spectroscopy, via a simple line identification, allows us to distinguish the three main meteorite classes (chondrites, achondrites and irons) but it does not have the potential to distinguish for example an H chondrite from a CM chondrite. Conclusions. The source location within the main belt of the different meteorite classes (H, L, LL, CM, CI, etc.) should continue to be investigated via fireball observation networks. Spectroscopy of incoming bolides only marginally helps precisely classify the incoming material (iron meteorites only). To reach a statistically significant sample of recovered meteorites along with accurate orbits (>100) within a reasonable time frame (10–20 years), the optimal solution may be the spatial extension of existing fireball observation networks.

Color electron microprobe cathodoluminescence of Bishunpur meteorite compared with the traditional optical microscopy method

Abstract Cathodoluminescence (CL) imaging is an outstanding method for sub classification of Unequilibrated Ordinary Chondrites (UOC) - petrological type 3. CL can be obtained by several electron beam apparatuses. The traditional method uses an electron gun coupled to an optical microscope (OM). Although many scanning electron microscopes (SEM) and electron microprobes (EPMA) have been equipped with a cathodoluminescence, this technique was not fully explored. Images obtained by the two methods differ due to a different kind of signal acquisition. While in the CL-OM optical photography true colors are obtained, in the CL-EPMA the results are grayscale monochromatic electronic signals. L-RGB filters were used in the CL-EPMA analysis in order to obtain color data. The aim of this work is to compare cathodoluminescence data obtained from both techniques, optical microscope and electron microprobe, on the Bishunpur meteorite classified as LL 3.1 chondrite. The present study allows concluding that 20 KeV and 7 nA is the best analytical condition at EPMA in order to test the equivalence between CL-EPMA and CL-OM colour results. Moreover, the color index revealed to be a method for aiding the study of the thermal metamorphism, but it is not definitive for the meteorite classification.

Iron meteorite bulk densities determined via 3‐D laser imaging

AbstractThis study tested the feasibility of using 3‐D laser imaging to measure the bulk density of iron meteorites. 3‐D laser imaging is a technique in which a 3‐D model of an object is built after aligning and merging individual detailed images of its surface. Assuming that the mass of the object is known, the volume of the model is calculated by software and an estimate of bulk density can be obtained by dividing mass by volume. The 3‐D laser imaging technique was used to determine the density of 46 fragments from 11 different iron meteorites. The technique proved to be robust and was applied successfully to study samples ranging close to four orders of magnitude in mass (8 g to 156 kg) and exhibiting a variety of surface textures (e.g., cracks, regmaglypts), reflectivities (e.g., polished surfaces, fusion crust, rust), and morphologies (e.g., sharp angular edges, shrapnel tendrils). Three metrics were considered to estimate the error associated with density measurements: the range accuracy of the laser camera, image alignment error, and inter‐operator variability during model building. Inter‐operator variability was the largest source of error and was highest when assembling models of samples which either lacked distinctive features or were very complex in shape. Excluding two anomalous Zagora samples where silicate inclusions might have lowered density, the densities measured using 3‐D laser imaging ranged from 6.98 to 7.93 g cm−3, consistent with previous studies. There is overlap between bulk density and iron meteorite class, and therefore bulk density cannot be used in isolation as a classification criterion. It is a good indicator, however, of weathering effects and of the potential presence of low‐density inclusions.

The surface composition of Ceres from the Dawn mission

Ceres’ surface composition is of special interest because it is a window into the interior state and the past evolution of this dwarf planet. Disk-integrated telescopic spectral observations indicated that Ceres’ surface is hydroxylated, similar to but not exactly the same as some of the carbonaceous chondrite classes of meteorites. Furthermore, Ceres’ bulk density is low, suggesting significant water content. The Dawn mission in orbit around Ceres provided a new and much larger set of observations on the mineralogy, molecular and elemental composition, and their distributions in association with surface features and geology. The set of articles contained in this special issue is the first treatment of the entire surface composition of Ceres using the complete High Altitude Mapping Orbit (HAMO) Dawn Ceres data set and the calibrations from all the Dawn instruments. Most articles here treat the different geologic quadrangles of Ceres within the context of the entire body. There also are articles that treat global or technical topics. As a whole, these articles provide a current and comprehensive view of Ceres’ surface composition. Ceres’ surface composition shows a fairly uniform and widespread distribution of NH4- and Mg-phyllosilicates and carbonates, mixed with a dark component and with some exposures of salts and water-ice on Ceres’ surface, all indicative of the presence of aqueous alteration processes that involved the entire dwarf planet. There is also likely some contamination by low velocity infall, as seen on Vesta, but it is more difficult to distinguish this infall from native Ceres material, unlike for the Vesta case. This article introduces and provides the context for the following papers, presents a summary of the various findings, and integrates them into some general conclusions.

Bulk chemical compositions of Antarctic meteorites in the NIPR collection

Bulk chemical compositions of meteorites were traditionally analyzed by wet chemical analysis, and NIPR has data for 1162 meteorites as of September 2017. We discuss the classification of meteorites on the basis of these data. Chondrite data are distributed in an anomalously wide range of compositions on the Urey-Craig diagram. One of the reasons for such wide distribution is terrestrial weathering producing Fe2O3–bearing phases from Fe-Ni metal and sulfides. Another important factor affecting the bulk compositional data is brecciation. Our observations indicate that many brecciated chondrites contain anomalously abundant opaque minerals, or are depleted in them, resulting in unusual compositions. In case of enstatite and some carbonaceous chondrites, the bulk compositions are distributed in wider ranges than reported before. The bulk compositions of HED meteorites are consistent with their mineralogy and classification. Our study suggests that wet chemical data are still significant for the meteorite classification. However, petrographic observation is indispensable for evaluating the bulk chemistry and classification of meteorites.

The neodymium stable isotope composition of the silicate Earth and chondrites

The non-chondritic neodymium (Nd) 142Nd/144Nd ratio of the silicate Earth potentially provides a key constraint on the accretion and early evolution of the Earth. Yet, it is debated whether this offset is due to the Earth being formed from material enriched in s-process Nd isotopes or results from an early differentiation process such as the segregation of a late sulfide matte during core formation, collisional erosion or a some combination of these processes. Neodymium stable isotopes are potentially sensitive to early sulfide segregation into Earth's core, a process that cannot be resolved using their radiogenic counterparts. This study presents the first comprehensive Nd stable isotope data for chondritic meteorites and terrestrial rocks. Stable Nd measurements were made using a double spike technique coupled with thermal ionisation mass spectrometry. All three of the major classes of chondritic meteorites, carbonaceous, enstatite and ordinary chondrites have broadly similar isotopic compositions allowing calculation of a chondritic mean of δ146/144Nd = −0.025 ± 0.025‰ (±2 s.d.; n=39). Enstatite chondrites yield the most uniform stable isotope composition (Δ146/144Nd = 26 ppm), with considerably more variability observed within ordinary (Δ146/144Nd = 72 ppm) and carbonaceous meteorites (Δ146/144Nd = 143 ppm). Terrestrial weathering, nucleosynthetic variations and parent body thermal metamorphism appear to have little measurable effect on δ146/144Nd in chondrites. The small variations observed between ordinary chondrite groups most likely reflect inherited compositional differences between parent bodies, with the larger variations observed in carbonaceous chondrites being linked to varying modal proportions of calcium–aluminium rich inclusions. The terrestrial samples analysed here include rocks ranging from basaltic to rhyolitic in composition, MORB glasses and residual mantle lithologies. All of these terrestrial rocks possess a broadly similar Nd isotope composition giving an average composition for the bulk silicate Earth of δ146/144Nd = −0.022 ± 0.034‰ (n=30). In the samples here magmatic differentiation appears to only have an effect on stable Nd in highly evolved magmas with heavier δ146/144Nd values observed in samples with >70 wt% SiO2. The average stable Nd isotope composition of chondrites and the bulk silicate Earth are indistinguishable at the 95% confidence level. However, mantle samples do possess variable stable Nd isotope compositions (Δ146/144Nd = 75 ppm) with an average δ146/144Nd value of −0.008‰. If these heavier values represent the true composition of pristine mantle then it is not possible to completely rule out some role for core formation in accounting for some of the offset between the mantle and chondrites. Overall, these results indicate that the mismatch of 142Nd between the Earth and chondrites is best explained by a higher proportion of s-process Nd in the Earth, rather than partitioning into sulfide or S-rich metal in the core.

Elemental composition analysis of stony meteorites discovered in Phitsanulok, Thailand

A meteorite is a fragment of pure stone, iron or the mixture of stony–iron. The falling of meteorites into Earth’s surface is part of Earth’s accretion process from dust and rocks in our solar system. When these fragments come close enough to the Earth to be attracted by its gravity, they may fall into the Earth. Following the detection of objects that fall from the sky onto a home in Phitsanulok in June 27, the meteorites were analyzed by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS) instruments. The results from SEM/EDS analysis show that the meteorites are mainly composed of Fe-Ni and Fe-s. The meteorite is Achondrite, a class of meteorite which does not contain Chondrule. The meteorites in this work are thought to be part of a large asteroid.

Reconciliation of the excess 176Hf conundrum in meteorites: Recent disturbances of the Lu-Hf and Sm-Nd isotope systematics

The long-lived 176Lu-176Hf and 147Sm-143Nd radioisotope systems are commonly used chronometers, but when applied to meteorites, they can reveal disturbances. Specifically, Lu-Hf isochrons commonly yield dates up to ∼300Myr older than the solar system and varying initial 176Hf/177Hf values. We investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites. Meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible. Minerals scatter away from isochron regressions for both meteorite classes, with low-Hf phases such as plagioclase and olivine typically being most displaced above (or left of) reference isochrons. Relatively Hf-rich pyroxene is less disturbed but still to the point of steepening Lu-Hf errorchrons. Using our Lu-Hf and Sm-Nd data, we tested various Hf and Lu redistribution scenarios and found that decoupling of Lu/Hf from 176Hf/177Hf must postdate the accumulation of significant radiogenic 176Hf. Therefore early irradiation or diffusion cannot explain the excess 176Hf. Instead, disturbed meteorite isochrons are more likely caused by terrestrial weathering, contamination, or common laboratory procedures. The partial dissolution of phosphate minerals may predominantly remove rare earth elements including Lu, leaving relatively immobile and radiogenic Hf behind. Robust Lu-Hf (and improved Sm-Nd) meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic Hf and silicate-hosted terrestrial contaminants without disturbing parent-daughter ratios.

Neon produced by solar cosmic rays in ordinary chondrites

AbstractSolar‐cosmic‐ray‐produced Ne (SCR‐Ne), in the form of low cosmogenic 21Ne/22Ne ratios (21Ne/22Necos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR‐Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low 3He/21Ne ratios, the low 4He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low 21Ne/22Necos in the range 0.711–0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to ~20% of SCR‐Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR‐Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR‐Ne in rare meteorites, although we cannot exclude that SCR‐Ne is more common in Martian meteorites than it is in small ordinary chondrites.

Revising the shock classification of meteorites

AbstractThe current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock‐forming minerals such as olivine and plagioclase. The S6 stage (55–90 GPa; 850–1750 °C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high‐pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high‐pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high‐pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable high‐pressure phases. High‐pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s−1) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.

Olivine dissolution in molten silicates: An experimental study with application to chondrule formation

AbstractMg‐rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low‐Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high‐temperature experiments (1450–1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X‐ray microtomography. Results indicate that forsterite dissolution in chondrule‐like melts is a very fast process with rates that range from ~5 μm min−1 to ~22 μm min−1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas‐melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg‐rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica‐poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low‐Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule.

Cluster analysis on the bulk elemental compositions of Antarctic stony meteorites

Remote sensing observations by recent successful missions to small bodies have revealed the difficulty in classifying the materials which cover their surfaces into a conventional classification of meteorites. Although reflectance spectroscopy is a powerful tool for this purpose, it is influenced by many factors, such as space weathering, lighting conditions, and surface physical conditions (e.g., particle size and style of mixing). Thus, complementary information, such as elemental compositions, which can be obtained by X-ray fluorescence (XRF) and gamma-ray spectrometers (GRS), have been considered very important. However, classifying planetary materials solely based on elemental compositions has not been investigated extensively. In this study, we perform principal component and cluster analyses on 12 major and minor elements of the bulk compositions of 500 meteorites reported in the National Institute of Polar Research (NIPR), Japan database. Our unique approach, which includes using hierarchical cluster analysis, indicates that meteorites can be classified into about 10 groups purely by their bulk elemental compositions. We suggest that Si, Fe, Mg, Ca, and Na are the optimal set of elements, as this set has been used successfully to classify meteorites of the NIPR database with more than 94% accuracy. Principal components analysis indicates that elemental compositions of meteorites form eight clusters in the three-dimensional space of the components. The three major principal components (PC1, PC2, and PC3) are interpreted as (1) degree of differentiations of the source body (i.e., primitive versus differentiated), (2) degree of thermal effects, and (3) degree of chemical fractionation, respectively.

S-Process Os isotope enrichment in ureilites by planetary processing

Ubiquitous nucleosynthetic isotope anomalies relative to the terrestrial isotopic composition in Mo, Ru, and other elements are known from both bulk chondrites and differentiated meteorites, but Os isotope ratios reported from such meteorites have been found to be indistinguishable from the terrestrial value. The carriers of s- and r-process Os must thus have been homogeneously distributed in the solar nebula. As large Os isotope anomalies are known from acid leachates and residues of primitive chondrites, the constant relative proportions of presolar s- and r-process carriers in such chondrites must have been maintained during nebular processes. It has long been assumed that partial melting of primitive chondrites would homogenize the isotopic heterogeneity carried by presolar grains. Here, ureilites, carbon-rich ultramafic achondrites dominantly composed of olivine and low-Ca pyroxene, are shown to be the first differentiated bulk Solar System materials for which nucleosynthetic Os isotope anomalies have been identified. These anomalies consist of enrichment in s-process Os heterogeneously distributed in different ureilites. Given the observed homogeneity of Os isotopes in all types of primitive chondrites, this Os isotope variability among ureilites must have been caused by selective removal of s-process-poor Os host phases, probably metal, during rapid localized melting on the ureilite parent body. While Mo and Ru isotope anomalies for all meteorites measured so far exhibit s-process deficits relative to the Earth, the opposite holds for the Os isotope anomalies in ureilites reported here. This might indicate that the Earth preferentially accreted olivine-rich restites and inherited a s-process excess relative to smaller meteorite bodies, consistent with Earth's high Mg/Si ratio and enrichment of s-process nuclides in Mo, Ru, and Nd isotopes. Our new Os isotope results imply that caution must be used when applying nucleosynthetic isotope anomalies as provenance indicators between different classes of meteorites.

SEEDING THE PREGENETIC EARTH: METEORITIC ABUNDANCES OF NUCLEOBASES AND POTENTIAL REACTION PATHWAYS

Carbonaceous chondrites are a class of meteorite known for having a high content of water and organics. In this study, abundances of the nucleobases, i.e., the building blocks of RNA and DNA, found in carbonaceous chondrites are collated from a variety of published data and compared across various meteorite classes. An extensive review of abiotic chemical reactions producing nucleobases is then performed. These reactions are then reduced to a list of 15 individual reaction pathways that could potentially occur within meteorite parent bodies. The nucleobases guanine, adenine and uracil are found in carbonaceous chondrites in the amounts of 1$-$500 ppb. It is currently unknown which reaction is responsible for their synthesis within the meteorite parent bodies. One class of carbonaceous meteorites dominate the abundances of both amino acids and nucleobases$-$the so-called CM2 (e.g. Murchison meteorite). CR2 meteorites (e.g. Graves Nunataks) also dominate the abundances of amino acids, but are the least abundant in nucleobases. The abundances of total nucleobases in these two classes are $330 \pm250$ ppb and $16 \pm13$ ppb respectively. Guanine most often has the greatest abundances in carbonaceous chondrites with respect to the other nucleobases, but is 1$-$2 orders of magnitude less abundant in CM2 meteorites than glycine (the most abundant amino acid). Our survey of the reaction mechanisms for nucleobase formation suggests that Fischer-Tropsch synthesis (i.e. CO, H$_2$ and NH$_3$ gases reacting in the presence of a catalyst such as alumina or silica) is the most likely for conditions that characterize early states of planetesimals.

Near-infrared spectroscopy of 3:1 Kirkwood Gap asteroids III

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the composition and meteorite affinities of a subset of fourteen asteroids in/near the 3:1 Kirkwood Gap. Eight asteroids were identified as having either one or two absorption features, while six were deemed featureless. The compositional analysis of Asteroids (355) Gabriella and (1447) Utra reveal Fs and Fa values which are consistent with values for the L-type ordinary chondrites (Fs19–22 and Fa22–26). The location of these two bodies with respect to each other and to the previously identified L-chondrite parent body Asteroid (1722) Goffin, suggests a small L-chondrite genetic family. These results support the model that the L-chondrites come from an asteroid family rather than from a single object. Asteroids (1368) Numidia, (1587) Kahrstadt, (1854) Skvortsov, (2497) Kulikovskij, and (5676) Voltaire were analyzed and determined to have “basaltic” silicate mineralogies similar to those of the HED (howardite–eucrite–diogenite) meteorite group. In particular, we found that the compositions of (1368), (1587) and (1854) are consistent with olivine-orthopyroxenitic diogenites, while (2497) and (5676)’s compositions are consistent with harzburgitic diogenites. The Band I and Band II absorption feature depths are much shallower than seen in diogenite spectra, typically ∼70% depth (Burbine, T.H. et al. [2000]. Forging asteroid–meteorite relationships through reflectance spectroscopy. Lunar Planet. Sci. XXXI. Abstract 1844). The nature of the weak features seen in the asteroid spectra when compared to measured band depths of in situ diogenite samples indicate an additional mechanism(s) acting to weaken the features, most likely space weathering. The aforementioned five asteroids are plausible sources for the olivine-orthopyroxenitic diogenites and harzburgitic diogenites, and very well may be fragments of Vesta. Asteroid (46) Hestia is an interesting object whose surface minerals may be consistent with a CR2 chondrite; however, the unique spectrum deserves further study in the future. Featureless Asteroids (248) Lameia, (1960) Guisan, (3345) Tarkovskij and (6212) 1993 MS1 surface materials are likely organic assemblages consistent with the Type 1 or 2 carbonaceous chondrite meteorite class; however specific terrestrial meteorite analog could not be identified. The spectra of Asteroids (3228) Pire and (3999) Aristarchus are consistent with each other and have been assigned to the Eulalia by Walsh et al. (Walsh, K.J. et al. [2013]. Icarus 225, 283–297). Spectrally they are similar to (495) in terms of blue-slope and albedo (Fieber-Beyer, S.K., et al. [2012]. Icarus 221, 593–602), thus increasing our confidence the three bodies are truly related dynamically and genetically. By extrapolation and due to their location adjacent to the 3:1 Kirkwood Gap, (3228) and (3999) are plausible sources of the CV3OXB carbonaceous chondrites.

The composition of Vesta from the Dawn mission

Vesta’s surface composition has been of special interest since early, disk-integrated telescopic spectral observations indicated that it is basaltic, differentiated and similar to the HED (howardite–eucrite–diogenite) class of meteorites. The Dawn mission, orbiting Vesta, provided a large and varied set of unique observations on the detailed mineralogy, molecular and elemental composition, and their distributions in association with surface features and geology. The set of articles contained in this special issue is the first treatment of the entire surface composition of Vesta using the complete Dawn Vesta data set and the calibrations from the entire campaign. Most articles treat a region of Vesta within the context of the entire body, but there are several articles that treat global or technical topics. As a whole, these articles provide a current and comprehensive view of Vesta’s composition using all the relevant data that is available. Vesta’s surface composition is consistent with the upper layer being created by igneous processes, while a more mafic lithology generally associated with a mantle is surprisingly limited. There is evidence of contamination by low velocity infall of several types of objects: dark hydrated/hydroxylated material, and probably Fe/Mg silicates differing from Vesta’s. Isolated blocks of differing compositions, seen especially in crater walls, could indicate incomplete melting and mixing during the differentiation process, and retention of some evidence of the original building blocks of the accreted Vesta. This lead article introduces and provides the context for the following articles, presents a summary of the various findings, and integrates them into overall conclusions.

NEW INSIGHT INTO THE SOLAR SYSTEM'S TRANSITION DISK PHASE PROVIDED BY THE METAL-RICH CARBONACEOUS CHONDRITE ISHEYEVO.

Many aspects of planet formation are controlled by the amount of gas remaining in the natal protoplanetary disks (PPDs). Infrared observations show that PPDs undergo a transition stage at several megayears, during which gas densities are reduced. Our Solar System would have experienced such a stage. However, there is currently no data that provides insight into this crucial time in our PPD's evolution. We show that the Isheyevo meteorite contains the first definitive evidence for a transition disk stage in our Solar System. Isheyevo belongs to a class of metal-rich meteorites whose components have been dated at almost 5 Myr after formation of Ca, Al-rich inclusions, and exhibits unique sedimentary layers that imply formation through gentle sedimentation. We show that such layering can occur via the gentle sweep-up of material found in the impact plume resulting from the collision of two planetesimals. Such sweep-up requires gas densities consistent with observed transition disks (10-12-10-11 g cm-3). As such, Isheyevo presents the first evidence of our own transition disk and provides new constraints on the evolution of our solar nebula.

Constraining the source regions of lunar meteorites using orbital geochemical data

AbstractLunar meteorites provide important new samples of the Moon remote from regions visited by the Apollo and Luna sample return missions. Petrologic and geochemical analysis of these meteorites, combined with orbital remote sensing measurements, have enabled additional discoveries about the composition and age of the lunar surface on a global scale. However, the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite. Here, we investigate the link between meteorite and source region on the Moon using the Lunar Prospector gamma ray spectrometer remote sensing data set for the elements Fe, Ti, and Th. The approach has been validated using Apollo and Luna bulk regolith samples, and we have applied it to 48 meteorites excluding paired stones. Our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites. Basaltic and intermediate Fe regolith breccia meteorites are found to have the best constrained potential launch sites, with some impact breccias and pristine mare basalts also having reasonably well‐defined potential source regions. Launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element, such as Mg, will probably be required to identify potential source regions for these.

Potentially hazardous Asteroid 2007 LE: Compositional link to the black chondrite Rose City and Asteroid (6) Hebe

The research is an integrated effort beginning with telescopic observations and extending through detailed mineralogical characterizations to provide constraints on the albedo, diameter, composition, and meteorite affinity of near-Earth object-potentially hazardous asteroid (NEO-PHA 2007 LE). Results of the analysis indicate a diameter of 0.56kilometers (km) and an albedo of 0.08. 2007 LE exhibits a 1-μm absorption feature without a discernible Band II feature. Compositional analysis of 2007 LE reveal Fs17 and Fa19 values, which are consistent with the Fa and Fs values for the H-type ordinary chondrites (Fs14.5–18 and Fa16–20) and of Asteroid (6) Hebe (Fs17 and Fa15). Spectroscopically, 2007 LE does not appear like the average H-chondrite spectra, exhibiting a reddened spectrum and subdued absorption feature. Further investigation of the meteorite classes yielded a black chondrite, Rose City, which is both similar in mineralogy and spectrally to PHA 2007 LE. Dynamical analysis could not directly link the fall of the Rose City meteorite to 2007 LE. As it stands, 2007 LE and Rose City have a compositional link, and both could come from the same parent body/possible family, one known source of the H chondrites is (6) Hebe.