Fe‐bearing phases in a ureilite fragment from the asteroid 2008 TC3 (= Almahata Sitta meteorites): A combined Mössbauer spectroscopy and X‐ray diffraction study

The iron‐bearing phases in a ureilite fragment (AS#051) from the Almahata Sitta meteorite are studied using Mössbauer spectroscopy, X‐ray diffraction (XRD), and electron microprobe analysis (EMPA). AS#051 has a typical ureilite texture of medium‐ to coarse‐grained silicates (olivine, orthopyroxene, and pigeonite) with minor opaques (Fe‐Ni metal, troilite, and graphite). The silicate compositions, determined by EMPA, are homogeneous: olivine (Fo_90.2), orthopyroxene (En_86.3Fs_8.6Wo_5.1), and pigeonite (En_81.6Fs_8.9Wo_9.5), and are similar to those of magnesian ureilites. The modal abundance of mineral phases was determined by Rietveld refinement of the powder XRD data. The Mössbauer spectra at 295 K and 78 K are composed of two sharp well‐defined paramagnetic doublets superimposed on a well‐resolved magnetic sextet and other weak absorption features. The two paramagnetic doublets are assigned to olivine and pyroxene (orthopyroxene and pigeonite), and the ferromagnetic sextet to kamacite (magnetic hyperfine field ≈ 33.2 T), in agreement with the XRD characterization. The Mössbauer results also show the presence of small amounts of troilite (FeS) and cohenite ([Fe,Ni,Co]₃C). Using the Mössbauer data, the relative abundance of each Fe‐bearing phase is determined and compared with the results obtained by XRD.     

Magnetism and mineralogy of Almahata Sitta polymict ureilite (= asteroid 2008 TC3): Implications for the ureilite parent body magnetic field

Abstract– The Almahata Sitta meteorite is the first case of recovered extraterrestrial material originating from an asteroid that was detected in near Earth space shortly before entering and exploding in the high atmosphere. The aims of our project within the 2008 TC₃ consortium were investigating Almahata Sitta’s (AS) magnetic signature, phase composition and mineralogy, focussing on the opaque minerals, and gaining new insights into the magnetism of the ureilite parent body (UPB). We report on the general magnetic properties and behavior of Almahata Sitta and try to place the results in context with the existing data set on ureilites and ureilite parent body models. The magnetic signature of AS is dominated by a set of low‐Ni kamacites with large grain sizes. Additional contributions come from micron‐sized kamacites, suessite, (Cr) troilite, and daubreelite, mainly found in the olivine grains adjacent to carbon‐rich veins. Our results show that the paleomagnetic signal is of extraterrestrial origin as can be seen by comparing with laboratory produced magnetic records (IRM). Four types of kamacite (I–IV) have been recognized in the sample. The elemental composition of the ureilite vein metal Kamacite I (particularly Co) clearly differs from the other kamacites (II‐IV), which are considered to be indigenous. Element ratios of kamacite I indicate that it was introduced into the UPB by an impactor, supporting the conclusions of Gabriel and Pack (2009) .     

Heterogeneous distributions of amino acids provide evidence of multiple sources within the Almahata Sitta parent body,asteroid 2008 TC3

Abstract– Two new fragments of the Almahata Sitta meteorite and a sample of sand from the related strewn field in the Nubian Desert, Sudan, were analyzed for two to six carbon aliphatic primary amino acids by ultrahigh performance liquid chromatography with UV‐fluorescence detection and time‐of‐flight mass spectrometry (LC‐FT/ToF‐MS). The distribution of amino acids in fragment #25, an H5 ordinary chondrite, and fragment #27, a polymict ureilite, were compared with results from the previously analyzed fragment #4, also a polymict ureilite. All three meteorite fragments contain 180–270 parts‐per‐billion (ppb) of amino acids, roughly 1000‐fold lower than the total amino acid abundance of the Murchison carbonaceous chondrite. All of the Almahata Sitta fragments analyzed have amino acid distributions that differ from the Nubian Desert sand, which primarily contains l ‐α‐amino acids. In addition, the meteorites contain several amino acids that were not detected in the sand, indicating that many of the amino acids are extraterrestrial in origin. Despite their petrological differences, meteorite fragments #25 and #27 contain similar amino acid compositions; however, the distribution of amino acids in fragment #27 was distinct from those in fragment #4, even though both are polymict ureilites from the same parent body. Unlike in CM2 and CR2/3 meteorites, there are low relative abundances of α‐amino acids in the Almahata Sitta meteorite fragments, which suggest that Strecker‐type chemistry was not a significant amino acid formation mechanism. Given the high temperatures that asteroid 2008 TC₃ appears to have experienced and lack of evidence for aqueous alteration on the asteroid, it is possible that the extraterrestrial amino acids detected in Almahata Sitta were formed by Fischer‐Tropsch/Haber‐Bosch type gas‐grain reactions at elevated temperatures.     

The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC3

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC₃ was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH^? in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC₃ consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm^?3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm^?3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC₃ in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.     

Photometric observations of Earth‐impacting asteroid 2008 TC3

Abstract– A calibrated lightcurve is presented of the near‐Earth asteroid 2008 TC₃, obtained before it impacted Earth on October 7, 2008. The asteroid was observed in unfiltered images from the end of astronomical twilight until the object entered Earth’s shadow about 2 h later. The observations covered a wide range of phase angles from 14.79° to 2.93°, during which the asteroid ranged from 82,000 km to 29,000 km distance from the observer. A method is presented for obtaining photometrically filtered brightness values for the asteroid using unfiltered imaging techniques. Over 1,700 images of the asteroid produce a lightcurve with a peak‐to‐peak variation in V of 0.76 magnitude. Analysis of the lightcurve yields values for H = 30.86 ± 0.01 and G = 0.33 ± 0.03. Combined with other constraints on the kinetic energy and diameter of the asteroid, which suggest a low 1.8 g cm^?3 density and albedo 0.05 ± 0.01, the value of H implies an asteroid of about 4.1 m in diameter, 28 m³ in volume, and 51,000 kg in mass. The determined value of G is out of range for normal, larger asteroids of albedo 0.05–0.15.     

Enstatite meteorite clasts in Almahata Sitta and other polymict ureilites: Implications for the formation of asteroid 2008 TC3 and the history of enstatite meteorite parent asteroids

The anomalous polymict ureilite Almahata Sitta (AhS) fell in 2008 when asteroid 2008 TC₃ disintegrated over Sudan and formed a strewn field of disaggregated clasts of various ureilitic and chondritic types. We studied the petrology and oxygen isotope compositions of enstatite meteorite samples from the University of Khartoum (UoK) collection of AhS. In addition, we describe the first bona fide (3.5 mm-sized) clast of an enstatite chondrite (EC) in a typical polymict ureilite, Northwest Africa (NWA) 10657. We evaluate whether 2008 TC₃ and typical polymict ureilites have a common origin, and examine implications for the history of enstatite meteorite asteroids in the solar system. Based on mineralogy, mineral compositions, and textures, the seven AhS EC clasts studied comprise one EH_a3 (S151), one EL_b3 (AhS 1002), two EH_b4-5 (AhS 2012, AhS 26), two EH_b5-6 or possibly impact melt rocks (AhS 609, AhS 41), and one EL_b6-7 (AhS 17), while the EC clast in NWA 10657 is EH_a3. Oxygen isotope compositions analyzed for five of these are similar to those of EC from non-UoK collections of AhS, and within the range of individual EC meteorites. There are no correlations of oxygen isotope composition with chemical group or subgroup. The EC clasts from the UoK collection show the same large range of types as those from non-UoK collections of AhS. The enstatite achondrite, AhS 60, is a unique type (not known as an individual meteorite) that has also been found among non-UoK AhS samples. EC are the most abundant non-ureilitic clasts in AhS but previously were thought to be absent in typical polymict ureilites, necessitating a distinct origin for AhS. The discovery of an EC in NWA 10657 changes this. We argue that the types of materials in AhS and typical polymict ureilites are essentially similar, indicating a common origin. We elaborate on a model in which AhS and typical polymict ureilites formed in the same regolith on a ureilitic daughter body. Most non-ureilitic clasts are remnants of impactors implanted at ~50–60 Myr after CAI. Differences in abundances can be explained by the stochastic nature of impactor addition. There is no significant difference between the chemical/petrologic types of EC in polymict ureilites and individual EC meteorites. This implies that fragments of the same populations of EC parent bodies were available as impactors at ~50–60 Myr after CAI and recently. This can be explained if materials excavated from various depths on EC bodies at ~50–60 Myr after CAI were reassembled into mixed layers, leaving relatively large bodies intact to survive 4 billion years. Polymict ureilites record a critical timestep in the collisional and dynamical evolution of the solar system, showing that asteroids that may have accreted at distant locations had migrated to within proximity of one another by 50–60 Myr after CAI, and providing constraints on the dynamical processes that could have caused such migrations.     

Internal structure and physical properties of the Asteroid 2008 TC3 inferred from a study of the Almahata Sitta meteorites

The density measurements of Almahata Sitta ureilites reveal a bulk density of ∼3.1 g/cm³. This value, together with the 2008 TC₃ asteroid shape model and albedo, was used to estimate the asteroid’s mass. Based on the study of recovered meteorites and atmospheric entry observations Asteroid 2008 TC₃ is compositionally heterogeneous and of low mechanical strength. Thus we consider the presence of significant macroporosity likely, lowering asteroid’s bulk density compared to that of the Almahata Sitta ureilites. Most realistic albedos lie in a range of 0.09-0.2 and the presence of significant macroporosity leads to mass estimates below 20 × 10³ kg, which is lower than previously estimated. The presence of a non-ureilitic fraction and space weathering may affect the albedo and also influence the mass estimates. However, from current data it is not possible to quantify this effect.     

40Ar/39Ar ages of L4, H5, EL6, and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008 TC3)

The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC₃, including ureilitic (70%–80%) and diverse non-ureilitic materials. We determined the ⁴⁰Ar/³⁹Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS-D (EL6), as well as a clast of ureilitic trachyandesite MS-MU-011, also known as ALM-A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the ⁴⁰Ar/³⁹Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published ⁴⁰Ar/³⁹Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum-rich inclusion and is consistent with the interpretation that 2008 TC₃ was a polymict ureilite. With allowance for its 10-Ma uncertainty, the 4549-Ma ⁴⁰Ar/³⁹Ar age of ALM-A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative ³⁹Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of ⁴⁰Ar over the last 4.5 Ga. The other five such samples lost some ⁴⁰Ar at estimated times no earlier than 3800–4500 Ma bp . Clustering of ages in the low-temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic-ray exposure ages based on ³⁸Ar concentrations average 17 ± 5 Ma but may include some early irradiation.     

Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite

Asteroid 2008 TC₃ (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth's atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre‐atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC₃ probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC₃ samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC₃ was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re‐evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium‐aluminum‐rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC₃. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti‐type chondrites were implanted by low‐velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth‐crossing orbits. 2008 TC₃ comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC₃ and other polymict ureilites.     

Cosmic‐ray exposure ages of six chondritic Almahata Sitta fragments

The Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3‐5, CB, LL4/5, R‐like). The cosmic‐ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS‐179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short‐lived radionuclides in fragment MS‐179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS‐179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous ²¹Ne production‐rate, or, perhaps most likely, MS‐179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS‐179 had an extraordinarily high activity of neutron‐capture ³⁶Cl, ~600 dpm kg^?1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl‐bearing mineral lawrencite.     

Astrometric and photometric studies of the asteroid 2008 TC3

Near real time astrometric and photometric observations of the asteroid 2008 TC3, discovered 19 hours before it fell to Earth in the area of northern Sudan, were conducted on the night of October 6–7, 2008, using an automated telescope ZA-320M of the Pulkovo Observatory. In the interval of 4 h, 270 observations in the integral band of the telescope were performed, which was about one-third of all global observations of the asteroid. Based on the analysis of all cases, physical parameters of the asteroid were assessed. The estimates of the absolute magnitude of the asteroid (M _V = 30.6 ± 0.4 μm), its size (4.8 ± 0.8 m), and weight (131 ± 5 t) were obtained. A frequency analysis of the observational series was conducted, which helped to detect the periodicity in the brightness variation of the asteroid. The elements of the heliocentric orbit of the asteroid were refined. The trajectory of the asteroid, taking into account the atmospheric drag and nonsphericity of the Earth, was simulated.     

Wüstite in the fusion crust of Almahata Sitta sulfide‐metal assemblage MS‐166: Evidence for oxygen in metallic melts

Meteorite fusion crusts form during the passage of a meteoroid through the Earth's atmosphere and are highly oxidized intergrowths as documented by the presence of e.g., oxides. The porous and irregular fusion crust surrounding the Almahata Sitta sulfide‐metal assemblage MS‐166 was found highly enriched in wüstite (Fe_1‐xO). Frictional heating of the outer portions of the assemblage caused partial melting of predominantly the Fe‐sulfide and minor amounts of the outer Ni‐rich portions of the originally zoned metal in MS‐166. Along with melting significant amounts of oxygen were incorporated into the molten fusion crust and mainly FeS was oxidized and desulfurized to form wüstite. Considerable amounts of FeS were lost due to ablation, whereas the cores of the large metal grains appear largely unmelted leaving behind metal grains and surrounding wüstite‐rich material (matte). Metal grains along with the surrounding matte typically form an often highly porous framework of globules interconnected with the matte. Although textures and chemical composition suggest that melting of Fe,Ni metal occurred only partially (Ni‐rich rims), there is a trace elemental imprint of siderophile element partitioning influenced by oxygen in the metallic melt as indicated by the behavior of W and Ga, the two elements significantly affected by oxygen in a metallic melt. It is remarkable that MS‐166 survived the atmospheric passage as troilite inclusions in iron meteorites are preferentially destroyed.     

The prospect of high‐precision Pb isotopic dating of meteorites

Abstract— The radiogenic ²⁰⁷Pb/²⁰⁶Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi‐stage evolution of U‐Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is ±0.5–1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U‐Pb system in meteorites.     

Si‐bearing metal and niningerite in Almahata Sitta fine‐grained ureilites and insights into the diversity of metal on the ureilite parent body

A detailed mineralogical and chemical study of Almahata Sitta fine‐grained ureilites (MS‐20, MS‐165, MS‐168) was performed to shed light on the origin of these lithologies and their sulfide and metal. The Almahata Sitta fine‐grained ureilites (silicates <30 μm grain size) show textural and chemical evidence for severe impact smelting as described for other fine‐grained ureilites. Highly reduced areas in Almahata Sitta fine‐grained ureilites show large (up to ～1 mm) Si‐bearing metal grains (up to ～4.5 wt% Si) and niningerite [Mg_>0.5,(Mn,Fe)_<0.5S] with some similarities to the mineralogy of enstatite (E) chondrites. Overall, metal grains show a large compositional variability in Ni and Si concentrations. Niningerite grains probably formed as a by‐product of smelting via sulfidation. The large Si‐Ni variation in fine‐grained ureilite metal could be the result of variable degrees of reduction during impact smelting, inherited from coarse‐grained ureilite precursors, or a combination of both. Large Si‐bearing metal grains probably formed via coalescence of existing and newly formed metal during impact smelting. Bulk and in situ siderophile trace element abundances indicate three distinct populations of (1) metal crystallized from partial melts in MS‐20, (2) metal resembling bulk chondritic compositions in MS‐165, and (3) residual metal in MS‐168. Almahata Sitta fine‐grained ureilites developed their distinctive mineralogy due to severe reduction during smelting. Despite the presence of E chondrite and ureilite stones in the Almahata Sitta fall, a mixing relation of E chondrites or their constituents and ureilite material in Almahata Sitta can be ruled out based on isotopic, textural, and mineral‐chemical reasons.     

Thermal modeling of shock melts in Martian meteorites: Implications for preserving Martian atmospheric signatures and crystallization of high‐pressure minerals from shock melts

The distribution of shock melts in four shergottites, having both vein and pocket geometry, has been defined and the conductive cooling time over the range 2500 °C to 900 °C calculated. Isolated 1 mm² pockets cool in 1.17 s and cooling times increase with pocket area. An isolated vein 1 × 7 mm in Northwest Africa (NWA) 4797 cools to 900 °C in 4.5 s. Interference between thermal haloes of closely spaced shock melts decreases the thermal gradient, extending cooling times by a factor of 1.4 to 100. This is long enough to allow differential diffusion of Ar and Xe from the melt. Small pockets (1 mm²) lose 2.2% Ar and 5.2% Xe during cooling, resulting in a small change in the Ar/Xe ratio of the dissolved gas over that originally trapped. With longer cooling times there is significant fractionation of Xe from Ar and the Ar/Xe ratio increases rapidly. The largest pockets show less variation of Ar/Xe and likely preserve the original trapped gas composition. Considering all of the model calculations, even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse, i.e., >0.01 s. The crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite.     

Numerical modeling of asteroid survivability and possible scenarios for the Morokweng crater‐forming impact

The fate of the impactor is an important aspect of the impact‐cratering process. Defining impactor material as surviving if it remains solid (i.e., does not melt or vaporize) during crater formation, previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity, impact angle (with respect to the horizontal), and target density. Here, we show that in addition to these, impactor survivability depends on the porosity and shape of the impactor. Increasing impactor porosity decreases impactor survivability, while prolate‐shaped (polar axis > equatorial axis) impactors survive impact more so than spherical and oblate‐shaped (polar axis < equatorial axis) impactors. These results are used to produce a relatively simple equation, which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters. By applying our findings to the Morokweng crater‐forming impact, we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the Morokweng crater. In addition to previous suggestions of a low‐velocity and/or high‐angled impact, this work suggests that an elongated and/or low porosity impactor may also help explain the anomalously high survivability of the Morokweng impactor.     

A propensity for n‐ω‐amino acids in thermally altered Antarctic meteorites

Abstract– Asteroids and their fragments have impacted the Earth for the last 4.5 Gyr. Carbonaceous meteorites are known to contain a wealth of indigenous organic molecules, including amino acids, which suggests that these meteorites could have been an important source of prebiotic organic material during the origins of life on Earth and possibly elsewhere. We report the detection of extraterrestrial amino acids in thermally altered type 3 CV and CO carbonaceous chondrites and ureilites recovered from Antarctica. The amino acid concentrations of the thirteen Antarctic meteorites ranged from 300 to 3200 parts‐per‐billion (ppb), generally much less abundant than in amino acid‐rich CI, CM, and CR carbonaceous chondrites that experienced much lower temperature aqueous alteration on their parent bodies. In contrast to low‐temperature aqueously altered meteorites that show complete structural diversity in amino acids formed predominantly by Strecker–cyanohydrin synthesis, the thermally altered meteorites studied here are dominated by small, straight‐chain, amine terminal (n‐ω‐amino) amino acids that are not consistent with Strecker formation. The carbon isotopic ratios of two extraterrestrial n‐ω‐amino acids measured in one of the CV chondrites (δ¹³C approximately ?25‰) are consistent with ¹³C‐depletions observed previously in hydrocarbons produced by Fischer‐Tropsch type reactions. The predominance of n‐ω‐amino acid isomers in thermally altered meteorites hints at cosmochemical mechanisms for the preferential formation and preservation of a small subset of the possible amino acids.     

Vesta and the HED meteorites: Mid‐infrared modeling of minerals and their abundances

Abstract— We demonstrate that the use of an established spectral deconvolution algorithm with mid‐infrared spectral libraries of mineral separates of varying grain sizes is capable of identifying the known mineral compositions and abundances of a selection of howardite, eucrite, and diogenite (HED) meteorite samples. In addition, we apply the same technique to mid‐infrared spectral emissivity measurements of Vesta that have been obtained from Cornell's Mid‐Infrared Asteroid Spectroscopy (MIDAS) Survey and the Infrared Space Observatory (ISO). Each Vesta measurement was made over a different range of longitudes. Our spectral deconvolution results to the Vesta spectra corroborate that Vesta's surface is howardite or eucrite‐like in composition and heterogeneous across its surface. The spectral fits produced by the linear deconvolution algorithm yields good results for the HED samples of known composition, thus giving us a high degree of confidence that our results for Vesta are valid.     

Density,porosity, mineralogy,and internal structure of cosmic dust and alteration of its properties during high‐velocity atmospheric entry

X‐ray microtomography (XMT), X‐ray diffraction (XRD), and magnetic hysteresis measurements were used to determine micrometeorite internal structure, mineralogy, crystallography, and physical properties at μm resolution. The study samples include unmelted, partially melted (scoriaceous), and completely melted (cosmic spherules) micrometeorites. This variety not only allows comparison of the mineralogy and porosity of these three micrometeorite types but also reveals changes in meteoroid properties during atmospheric entry at various velocities. At low entry velocities, meteoroids do not melt and their physical properties do not change. The porosity of unmelted micrometeorites varies considerably (0–12%) with one friable example having porosity around 50%. At higher velocities, the range of meteoroid porosity narrows, but average porosity increases (to 16–27%) due to volatile evaporation and partial melting (scoriaceous phase). Metal distribution seems to be mostly unaffected at this stage. At even higher entry velocities, complete melting follows the scoriaceous phase. Complete melting is accompanied by metal oxidation and redistribution, loss of porosity (1 ± 1%), and narrowing of the bulk (3.2 ± 0.5 g cm^?3) and grain (3.3 ± 0.5 g cm^?3) density range. Melted cosmic spherules with a barred olivine structure show an oriented crystallographic structure, whereas other subtypes do not.     

MUSES‐C target asteroid (25143) 1998 SF36: A reddened ordinary chondrite

Abstract— Near‐Earth asteroid (25143) 1998 SF36 is a planned target for the Japanese MUSES‐C sample return mission. High signal‐to‐noise and relatively high‐resolution (50 Å) visible and near‐infrared spectroscopic measurements obtained during this asteroid's favorable 2001 apparition reveal it to have a red‐sloped S(IV)‐type spectrum with strong 1 and 2 μm absorption bands analogous to those measured for ordinary chondrite meteorites. This red slope, which is the primary spectral difference between (25143) 1998 SF36 and ordinary chondrite meteorites, is well modeled by the spectrum of 0.05% nanophase iron (npFe⁰) proposed as a weathering mechanism by Pieters et al. (2000). Asteroid 1998 SF36 appears to have a surface composition corresponding to that of ordinary chondrite meteorites and is most similar in spectral characteristics and modeled olivine/pyroxene content to the LL chondrite class.