A noble gas and cosmogenic radionuclide analysis of two ordinary chondrites from Almahata Sitta

Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (¹⁰Be, ²⁶Al, ³⁶Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm^?2, consistent with the preatmospheric size of asteroid 2008 TC₃ that produced the Almahata Sitta strewn field; (2) both have, within error, a ²¹Ne/²⁶Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic ³⁸Ar. We also discuss the approximately 3.8 Ga (⁴He) and approximately 4.6 Ga (⁴⁰Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC₃ is derived has not experienced a large break‐up event in the last 3.8 Ga.     

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.     

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.     

MicroRaman spectroscopy of diamond and graphite in Almahata Sitta and comparison with other ureilites

This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm^?1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm^?1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond‐generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G‐band peak center range of 1569.1–1577.1 cm^?1 and a G‐band FWHM range of 24.3–41.6 cm^?1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites.     

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.     

Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ¹³C from ?7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ¹³C = ca ?25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ¹⁵N values of ?53 to ?94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC₃ was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ¹³C and δ¹⁵N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.     

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.     

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.     

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) .     

Thermophysical properties of Almahata Sitta meteorites (asteroid 2008 TC3) for high‐fidelity entry modeling

Asteroid 2008 TC₃ was characterized in a unique manner prior to impacting Earth's atmosphere, making its October 7, 2008, impact a suitable field test for or validating the application of high‐fidelity re‐entry modeling to asteroid entry. The accurate modeling of the behavior of 2008 TC₃ during its entry in Earth's atmosphere requires detailed information about the thermophysical properties of the asteroid's meteoritic materials at temperatures ranging from room temperature up to the point of ablation (T ~ 1400 K). Here, we present measurements of the thermophysical properties up to these temperatures (in a 1 atm. pressure of argon) for two samples of the Almahata Sitta meteorites from asteroid 2008 TC₃: a thick flat‐faced ureilite suitably shaped for emissivity measurements and a thin flat‐faced EL6 enstatite chondrite suitable for diffusivity measurements. Heat capacity was determined from the elemental composition and density from a 3‐D laser scan of the sample. We find that the thermal conductivity of the enstatite chondrite material decreases more gradually as a function of temperature than expected, while the emissivity of the ureilitic material decreases at a rate of 9.5 × 10^?5 K^?1 above 770 K. The entry scenario is the result of the actual flight path being the boundary to the load the meteorite will be affected with when entering. An accurate heat load prediction depends on the thermophysical properties. Finally, based on these data, the breakup can be calculated accurately leading to a risk assessment for ground damage.     

Formation of diamond and lonsdaleite in ureilites by impact shock processing of graphite

The origin of diamond in ureilites has been frequently debated. We investigated carbon phase assemblages (CPAs) in five ureilitic samples of the brecciated asteroid 2008 TC₃, found within the Almahata Sitta (AHS) strewn field, by transmission electron microscopy, Raman spectroscopy, synchrotron X-ray diffraction, and cathodoluminescence. Samples MS-MU 006, MS-187, and MS-170, are of low to moderate shock degree (U-S2 and U-S3), and samples MS-MU 027 (U-S4) and MS-MU 045 (U-S5) have a higher shock degree. In MS-MU 006 and MS-187, we did not find any diamond grains. MS-170 contains disordered and distorted graphite with diamond grains up to 12 μm in size and containing inclusions of Fe,Ni-metal, FeS, Fe-phosphide, and Cr,Fe-oxide. These diamond grains formed under relatively low (5–15 GPa) shock pressures through a catalytic process in the presence of a Fe,Ni,Cr,S,P-rich melt. The highly shocked and fine-grained ureilites MS-MU 027 and MS-MU 045 have three different types of CPAs, namely a nanopolycrystalline assemblage of diamond and defect-rich diamond/lonsdaleite, disordered and distorted graphite, and polycrystalline diamond with abundant Fe-rich mineral inclusions. The CPAs that have only diamond and planar defect-rich diamond (e.g., MS-MU 027) most likely formed through martensitic transformation of graphite to diamond and lonsdaleite at >15 GPa and >2000 K. The assemblage of diamond, defect-rich diamond, and disordered and distorted graphite (e.g., MS-MU 045) formed by martensitic transformation of graphite to diamond and lonsdaleite, followed by back-transformation to disordered graphite. We did not find any conclusive evidence to support the formation of diamond grains under high static pressure.     

On the bulk density of the M-type asteroid 16 Psyche

Recent estimates of the bulk density of the largest M-type asteroid 16 Psyche (1.4 and 1.8 g/cm³) seem to be too low for asteroids of this type. The interpretation of such low densities for the asteroid 16 Psyche with its increased metal content led to estimates of its macroporosity of 70%. In the present paper, it has been shown that the extremely low density estimates are caused primarily by the usage of a very high value of the IRAS diameter of the asteroid (D = 264 km, which corresponds to an IRAS albedo of 0.10). The application of the more accurate polarimetric albedo (0.170) and diameter (213 km) of Psyche results in a bulk density of the asteroid of 3.3 ± 0.7 g/cm³. For a moderate asteroid macroporosity of 30–40%, this density even agrees with the 50% metal content and, therefore, removes both the contradiction with the radar data and the problems of the very low density and unlikely high porosity of the asteroid 16 Psyche.     

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.     

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.     

A possible YORP effect on C and S Main Belt Asteroids

A rotating frequency analysis in a previous paper, showed that two samples of C and S-type asteroids belonging to the Main Belt, but not to any families, present two different values for the transition diameter to a Maxwellian distribution of the rotation frequency, respectively 48 and 33 km. In this paper, after a more detailed statistical analysis, aiming to verify that the result is physically relevant, we found a better estimate for the transition diameter, respectively D_C = 44 ± 2 km and D_S = 30 ± 1 km. The ratio between these estimated transition diameters, D_C/D_S = 1.5 ± 0.1, can be supported with the help of the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, although other physical causes cannot be completely ruled out.In this paper we have derived a simple scaling law for YORP which, taking into account the different average heliocentric distance, the bulk density, the albedo and the asteroid “asymmetry surface factor”, has enabled us to reasonably justify the ratio between the diameters transition of C-type and S-type asteroids. The same scaling law can be used to estimate a new ratio between the bulk densities of S and C asteroids samples (giving ρ_S/ρ_C ≈ 2.9 ± 0.3), and can explain why the asteroids near the transition diameter have about the same absolute magnitude. For C-type asteroids, using the found density ratio and other estimates of S-type density, it is also possible to estimate an average bulk density equal to 0.9 ± 0.1 g cm⁻³, a value compatible with icy composition. The suggested explanation for the difference of the transition diameters is a plausible hypothesis, consistent with the data, but it needs to be studied more in depth with further observations.     

The thermal and physical characteristics of the Gao-Guenie (H5) meteorite

Measurements of the bulk density, grain density, porosity, and magnetic susceptibility of 19 Gao-Guenie H5 chondrite meteorite samples are presented. We find average values of bulk density 〈ρ_bulk〉=3.46±0.07 g/cm³, grain density 〈ρ_grain〉=3.53±0.08 g/cm³, porosity 〈P(%)〉=2.46±1.39, and bulk mass magnetic susceptibility 〈log χ〉=5.23±0.11. Measurements of the specific heat capacity for a 3.01-g Gao-Guenie sample, a 61.37-g Gao-Guenie sample, a 62.35-g Jilin H5 chondrite meteorite sample, and a 51.37-g Sikhote-Alin IIAB Iron meteorite sample are also presented. Temperature interpolation formula are further provided for the specific heat capacity, thermal conductivity, and thermal diffusivity of the 3.01-g Gao-Guenie sample in the temperature range 300<T (K)<800. We briefly review the possible effects of the newly deduced specific heat and thermal conductivity values on the ablation of meteoroids within the Earth's atmosphere, the modeling of asteroid interiors and the orbital evolution of meteoroids through the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect.     

Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe-Ni metal

The crystal structures of orthopyroxene (En_86.3Fs_8.6Wo_5.1, space group Pbca) and pigeonite (En_81.7Fs_8.8Wo_9.5, space group P2₁/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single-crystal X-ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single-crystal diffraction data. The results indicate a rather disordered intracrystalline Fe²⁺-Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe-Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si-bearing kamacite, and arise from two different nearest-neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)₃C], schreibersite [(Fe,Ni)₃P], and an Fe-oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.     

Radar observations and the shape of near-Earth asteroid 2008 EV5

We observed the near-Earth ASTEROID 2008 EV5 with the Arecibo and Goldstone planetary radars and the Very Long Baseline Array during December 2008. EV5 rotates retrograde and its overall shape is a 400 ± 50 m oblate spheroid. The most prominent surface feature is a ridge parallel to the asteroid’s equator that is broken by a concavity about 150 m in diameter. Otherwise the asteroid’s surface is notably smooth on decameter scales. EV5’s radar and optical albedos are consistent with either rocky or stony-iron composition. The equatorial ridge is similar to structure seen on the rubble-pile near-Earth asteroid (66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid in the past. We interpret the concavity as an impact crater. Shaking during the impact and later regolith redistribution may have erased smaller features, explaining the general lack of decameter-scale surface structure.     

Modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby

The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury’s intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P_sw = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 R_M³ − nT and an offset of 0.18 R_M to the north of the equator, where R_M is Mercury’s radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north-south asymmetry in exospheric sodium in the neutral tail.     

Lunar atmospheric H2 detections by the LAMP UV spectrograph on the Lunar Reconnaissance Orbiter

We report on the detection of H₂ as seen in our analysis of twilight observations of the lunar atmosphere observed by the LAMP instrument aboard NASA’s Lunar Reconnaissance Orbiter. Using a large amount of data collected on the lunar atmosphere between September 2009 and March 2013, we have detected and identified, the presence of H₂ in the native lunar atmosphere, for the first time. We derive a surface density for H₂ of 1.2 ± 0.4 × 10³ cm⁻³ at 120 K. This is about 10 times smaller than originally predicted, and several times smaller than previous upper limits from the Apollo era data.