Types Of Meteorites Research Articles

Meteorite reconnaissance in Saudi Arabia

AbstractMeteorite searches in Saudi Arabia between 2008 and 2014 yielded 46 meteorites from the Yabrin area (23°N 49°E), 35 meteorites from the Rub' al‐Khali sand desert (19°–20°N, 48°–51°E), and 1 meteorite from Al Haddar. No meteorites were found near Hafar al Batin (29°N 45°E). The 82 new meteorites represent ~57 falls comprising 43 ordinary chondrites, 4 carbonaceous chondrites, 2 enstatite chondrites, 3 ureilites, 3 eucrites, 1 acapulcoite, and 1 lunar meteorite. The median of 31 14C terrestrial ages is 6.2 ka, significantly younger than the Oman population (19.5 ka, n = 128). A further assessment of terrestrial 14C contamination is advised by a 11–15 ka 14C terrestrial age of heavily weathered meteorite Khawr al Fazra 014, geology indicating a terrestrial age >100 ka. Find densities of 0.4–2.8 km−2 for Yabrin and the western Rub’ al‐Khali are similar to ~0.5 km−2 observed in Oman. Higher find densities of ~135 km−2 (29 km−2 for masses >10 g) exist on small Pleistocene outcrops in blowouts in the south‐central Rub’ al‐Khali: 21 unpaired meteorites (four >10 g) were found in 11 blowouts with a combined area of 0.14 km2. The Rub’ al‐Khali meteorites show a relatively high degree of weathering (median W 3.6; 2.5 for Yabrin), low median mass (4.3/138 g), and a high H/L ratio (2.3/1.1). The high density of small meteorites is explained by prolonged sand protection and recent deflation. The high meteorite density and relatively high proportion of rare meteorite types render the Rub’ al‐Khali blowouts an interesting target for future exploration.

Nature of late accretion to Earth inferred from mass-dependent Ru isotopic compositions of chondrites and mantle peridotites

Elevated abundances of highly siderophile elements in Earth's mantle are thought to reflect the late accretion of primitive material after the cessation of core formation, but the origin of this material, and whether or not it can be linked to specific types of meteorites remain debated. Here, mass-dependent Ru isotopic data for chondrites and terrestrial peridotites are reported to evaluate the chemical nature and type of the late-accreted material. After correction for nucleosynthetic Ru isotope anomalies, enstatite, ordinary and carbonaceous chondrites all have indistinguishable mass-dependent Ru isotopic compositions. Thus, neither distinct formation conditions in the solar nebula nor parent body processes resulted in significant mass-dependent Ru isotope fractionation. All five terrestrial peridotites analyzed have mass-dependent Ru isotopic compositions that are indistinguishable from each other and from the composition of chondrites. The chondritic mass-dependent Ru isotopic composition of Earth's mantle is difficult to reconcile with prior suggestions that the late accretionary assemblage was a mixture of chondrites with a chemically evolved metal component. Although this mixture can reproduce the suprachondritic Ru/Ir inferred for Earth's mantle, it consistently predicts a heavy Ru isotopic composition of Earth's mantle with respect to chondrites. This is because metal components with elevated Ru/Ir are also enriched in heavy Ru isotopes, resulting from isotope fractionation during core crystallization. Thus, if late accretion involved impacts of differentiated protoplanetary bodies, then the projectile cores must have been either homogenized upon impact, or added to Earth's mantle completely, because otherwise Earth's mantle would have inherited a non-chondritic mass-dependent Ru isotopic composition from the unrepresentative sampling of core material.

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.

Meteorites hint at crowded early solar system

Tiny diamonds from a rare type of meteorite are evidence for a lost planet between the size of Mercury and Mars, according to new research (Nat. Commun. 2018, DOI: 10.1038/s41467-018-03808-6). The findings support a proposed picture of a crowded early solar system where collisions between planetlike bodies were common. In 2008, a car-sized asteroid exploded over Sudan, scattering meteorites known as Almahata Sitta across the Nubian Desert. Many of the meteorites belong to an unusual class called ureilites thought to come from a single, long-since-destroyed body in our solar system referred to as the ureilite planetary body (UPB). Ureilites contain a mix of iron and magnesium silicate minerals along with graphite and small diamonds. Scientists previously hypothesized three ways diamonds in ureilites could form: The shock that demolished the UPB could have transformed graphite into diamond; vapor deposition of hydrocarbon gases could have produced the minerals; or they could have

The Prevailing Catalytic Role of Meteorites in Formamide Prebiotic Processes.

Meteorites are consensually considered to be involved in the origin of life on this Planet for several functions and at different levels: (i) as providers of impact energy during their passage through the atmosphere; (ii) as agents of geodynamics, intended both as starters of the Earth’s tectonics and as activators of local hydrothermal systems upon their fall; (iii) as sources of organic materials, at varying levels of limited complexity; and (iv) as catalysts. The consensus about the relevance of these functions differs. We focus on the catalytic activities of the various types of meteorites in reactions relevant for prebiotic chemistry. Formamide was selected as the chemical precursor and various sources of energy were analyzed. The results show that all the meteorites and all the different energy sources tested actively afford complex mixtures of biologically-relevant compounds, indicating the robustness of the formamide-based prebiotic chemistry involved. Although in some cases the yields of products are quite small, the diversity of the detected compounds of biochemical significance underlines the prebiotic importance of meteorite-catalyzed condensation of formamide.

Do L chondrites come from the Gefion family?

Ordinary chondrites (H, L, and LL chondrites) are the most common type of meteorites comprising 80 per cent of the meteorites that fall on Earth. The source region of these meteorites in the main asteroid belt has been a basis of considerable debate in the small bodies community. L chondrites have been proposed to come from the Gefion asteroid family, based on dynamical models. We present results from our observational campaign to verify a link between the Gefion asteroid family and L chondrite meteorites. Near-infrared spectra of Gefion family asteroids (1839) Ragazza, (2373) Immo, (2386) Nikonov, (2521) Heidi, and (3860) Plovdiv were obtained at the NASA Infrared Telescope Facility (IRTF). Spectral band parameters including band centres and the band area ratio were measured from each spectrum and used to constrain the composition of these asteroids. Based on our results, we found that some members of the Gefion family have surface composition similar to that of H chondrites, primitive achondrites, and basaltic achondrites. No evidence was found for L chondrites among the Gefion family members in our small sample study. The diversity of compositional types observed in the Gefion asteroid family suggests that the original parent body might be partially differentiated or that the three asteroids with non-ordinary chondrite compositions might be interlopers.

Reply to the comment by M. Genge and M. Van Ginneken on paper entitled “Unmelted cosmic metal particles from the Indian Ocean”

Reply to the comment by M. Genge and M. Van Ginneken on paper entitled “Unmelted cosmic metal particles from the Indian Ocean”

Australasian microtektites: Impactor identification using Cr, Co and Ni ratios

Impactor identification is one of the challenges of large-scale impact cratering studies due to the dilution of meteoritic material in impactites (typically < 1 wt%). The nature of the impactor that generated the Australasian tektite/microtektite strewn field, i.e., the largest Cenozoic strewn field (∼15% of the Earth's surface), the youngest (∼0.78 Myr old) on Earth, and the only one without an associated impact crater so far, is an outstanding issue. We identify a chondritic impactor signature in 77 Australasian microtektites (size range: ∼200–700 µm) from within 3000 km from the hypothetical impact location in Indochina (∼17°N, 107°E) based on variations of Cr, Co and Ni interelement ratios in a Co/Ni vs Cr/Ni space (46 microtektites analyzed in this work by Laser Ablation-Inductively Coupled Plasma -Mass Spectrometry and 31 from literature by means of Neutron Activation Analyses with Cr, Co and Ni concentrations up to ∼370, 50 and 680 µg/g, respectively). Despite substantial overlap in Cr/Ni versus Co/Ni composition for several meteorite types with chondritic composition (chondrites and primitive achondrites), regression calculation based on ∼85% of the studied microtektites best fit a mixing line between crustal compositions and an LL chondrite. However, due to some scatter mainly in the Cr versus Ni ratios in the considered dataset, an LL chondrite may not be the best fit to the data amongst impactors of primitive compositions. Eight high Ni/Cr and five low Ni/Cr outlier microtektites (∼15% in total) deviate from the above mixing trend, perhaps resulting from incomplete homogenization of heterogeneous impactor and target precursor materials at the microtektite scale, respectively.Together with previous evidence from the ∼35 Myr old Popigai impact spherules and the ∼1 Myr old Ivory Coast microtektites, our finding suggests that at least three of the five known Cenozoic distal impact ejecta were generated by the impacts of large stony asteroids of chondritic composition, and possibly of ordinary chondritic composition. The impactor signature found in Australasian microtektites documents mixing of target and impactor melts upon impact cratering. This requires target-impactor mixing in both the two competing models in literature for the formation of the Australasian tektites/microtektites: the impact cratering and low-altitude airburst plume models.

Laboratory spectroscopy of meteorite samples at UV-vis-NIR wavelengths: Analysis and discrimination by principal components analysis

Meteorite samples are measured with the University of Helsinki integrating-sphere UV-vis-NIR spectrometer. The resulting spectra of 30 meteorites are compared with selected spectra from the NASA Planetary Data System meteorite spectra database. The spectral measurements are transformed with the principal component analysis, and it is shown that different meteorite types can be distinguished from the transformed data. The motivation is to improve the link between asteroid spectral observations and meteorite spectral measurements.

Laboratory mid-IR spectra of equilibrated and igneous meteorites. Searching for observables of planetesimal debris

Meteorites contain minerals from Solar System asteroids with different properties (like size, presence of water, core formation). We provide new mid-IR transmission spectra of powdered meteorites to obtain templates of how mid-IR spectra of asteroidal debris would look like. This is essential for interpreting mid-IR spectra of past and future space observatories, like the James Webb Space Telescope. First we present new transmission spectra of powdered ordinary chondrite, pallasite and HED meteorites and then we combine them with already available transmission spectra of chondrites in the literature, giving a total set of 64 transmission spectra. In detail we study the spectral features of minerals in these spectra to obtain measurables used to spectroscopically distinguish between meteorite groups. Being able to differentiate between dust from different meteorite types means we can probe properties of parent bodies, like their size, if they were wet or dry and if they are differentiated (core formation) or not.We show that the transmission spectra of wet and dry chondrites, carbonaceous and ordinary chondrites and achondrite and chondrite meteorites are distinctly different in a way one can distinguish in astronomical mid-IR spectra. Carbonaceous chondrites type < 3 (aqueously altered) show distinct features of hydrated silicates (hydrosilicates) compared to the olivine and pyroxene rich ordinary chondrites (dry and equilibrated meteorites). Also the iron concentration of the olivine in carbonaceous chondrites differs from ordinary chondrites, which can be probed by the wavelength peak position of the olivine spectral features. The transmission spectra of chondrites (not differentiated) are also strongly different from the achondrite HED meteorites (meteorites from differentiated bodies like 4 Vesta), where the latter show much stronger pyroxene signatures.The two observables that spectroscopically separate the different meteorites groups (and thus the different types of parent bodies) are the pyroxene-olivine feature strength ratio and the peak shift of the olivine spectral features due to an increase in the iron concentration of the olivine.

The Northwest Africa 8159 martian meteorite: Expanding the martian sample suite to the early Amazonian

Northwest Africa (NWA) 8159 is an augite-rich shergottite, with a mineralogy dominated by Ca-, Fe-rich pyroxene, plagioclase, olivine, and magnetite. NWA 8159 crystallized from an evolved melt of basaltic composition under relatively rapid conditions of cooling, likely in a surface lava flow or shallow sill. Redox conditions experienced by the melt shifted from relatively oxidizing (with respect to known Martian lithologies, ∼QFM) on the liquidus to higher oxygen fugacity (∼QFM+2) during crystallization of the groundmass, and under subsolidus conditions. This shift resulted in the production of orthopyroxene and magnetite replacing olivine phenocryst rims. NWA 8159 contains both crystalline and shock-amorphized plagioclase (An50–62), often observed within a single grain; based on known calibrations we bracket the peak shock pressure experienced by NWA 8159 to between 15 and 23GPa. The bulk composition of NWA 8159 is depleted in LREE, as observed for Tissint and other depleted shergottites; however, NWA 8159 is distinct from all other martian lithologies in its bulk composition and oxygen fugacity. We obtain a Sm-Nd formation age of 2.37±0.25Ga for NWA 8159, which represents an interval in Mars geologic time which, until recently, was not represented in the other martian meteorite types. The bulk rock 147Sm/144Nd value of 0.37±0.02 is consistent with it being derived directly from its source and the high initial ε143Nd value indicates this source was geochemically highly depleted. Cr, Nd, and W isotopic compositions further support a unique mantle source. While the rock shares similarities with the 2.4-Ga NWA 7635 meteorite, there are notable distinctions between the two meteorites that suggest differences in mantle source compositions and conditions of crystallization. Nevertheless, the two samples may be launch-paired. NWA 8159 expands the known basalt types, ages and mantle sources within the Mars sample suite to include a second igneous unit from the early Amazonian.

The stardust abundance in the local interstellar cloud at the birth of the Solar System

Primitive Solar System materials, such as certain types of meteorites, interplanetary dust particles and cometary matter, contain small quantities of refractory dust grains that are older than our Solar System. These ‘presolar grains’ condensed in the winds of evolved stars and in the ejecta of stellar explosions, and they were part of the interstellar gas and dust cloud from which our Solar System formed 4.57 billion years ago 1 . Interstellar dust is not only stardust but forms in the interstellar medium as well, predominantly as silicates, and, to a lesser extent, as carbonaceous dust and iron particles 2 . Presolar grains represent a sample of stardust, and their abundances in primitive Solar System materials can be used to constrain the fraction of stardust among interstellar dust. Here we show that the size distribution of presolar silicates follows that observationally derived for interstellar dust, at least in the diameter range 100–500 nm, that current estimates of presolar grain abundances (mass fractions) are at least a factor of 2 too low, and that several per cent of the interstellar dust in the interstellar cloud pre-dating our Solar System was stardust, making it a minor but still important ingredient of the starting material from which our Solar System formed.

Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) with Raman Imaging Applied to Lunar Meteorites.

Lunar meteorites provide a more random sampling of the surface of the Moon than do the returned lunar samples, and they provide valuable information to help estimate the chemical composition of the lunar crust, the lunar mantle, and the bulk Moon. As of July 2014, ∼96 lunar meteorites had been documented and ten of these are unbrecciated mare basalts. Using Raman imaging with multivariate curve resolution-alternating least squares (MCR-ALS), we investigated portions of polished thin sections of paired, unbrecciated, mare-basalt lunar meteorites that had been collected from the LaPaz Icefield (LAP) of Antarctica-LAP 02205 and LAP 04841. Polarized light microscopy displays that both meteorites are heterogeneous and consist of polydispersed sized and shaped particles of varying chemical composition. For two distinct probed areas within each meteorite, the individual chemical species and associated chemical maps were elucidated using MCR-ALS applied to Raman hyperspectral images. For LAP 02205, spatially and spectrally resolved clinopyroxene, ilmenite, substrate-adhesive epoxy, and diamond polish were observed within the probed areas. Similarly, for LAP 04841, spatially resolved chemical images with corresponding resolved Raman spectra of clinopyroxene, troilite, a high-temperature polymorph of anorthite, substrate-adhesive epoxy, and diamond polish were generated. In both LAP 02205 and LAP 04841, substrate-adhesive epoxy and diamond polish were more readily observed within fractures/veinlet features. Spectrally diverse clinopyroxenes were resolved in LAP 04841. Factors that allow these resolved clinopyroxenes to be differentiated include crystal orientation, spatially distinct chemical zoning of pyroxene crystals, and/or chemical and molecular composition. The minerals identified using this analytical methodology-clinopyroxene, anorthite, ilmenite, and troilite-are consistent with the results of previous studies of the two meteorites using electron microprobe analysis. To our knowledge, this is the first report of MCR-ALS with Raman imaging used for the investigation of both lunar and other types of meteorites. We have demonstrated the use of multivariate analysis methods, namely MCR-ALS, with Raman imaging to investigate heterogeneous lunar meteorites. Our analytical methodology can be used to elucidate the chemical, molecular, and structural characteristics of phases in a host of complex, heterogeneous geological, geochemical, and extraterrestrial materials.

3D shape of asteroid (6) Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites

Context. The high-angular-resolution capability of the new-generation ground-based adaptive-optics camera SPHERE at ESO VLT allows us to assess, for the very first time, the cratering record of medium-sized (D~100-200 km) asteroids from the ground, opening the prospect of a new era of investigation of the asteroid belt's collisional history. Aims. We investigate here the collisional history of asteroid (6) Hebe and challenge the idea that Hebe may be the parent body of ordinary H chondrites, the most common type of meteorites found on Earth (~34% of the falls). Methods. We observed Hebe with SPHERE as part of the science verification of the instrument. Combined with earlier adaptive-optics images and optical light curves, we model the spin and three-dimensional (3D) shape of Hebe and check the consistency of the derived model against available stellar occultations and thermal measurements. Results. Our 3D shape model fits the images with sub-pixel residuals and the light curves to 0.02 mag. The rotation period (7.274 47 h), spin (343 deg,+47 deg), and volume-equivalent diameter (193 +/- 6km) are consistent with previous determinations and thermophysical modeling. Hebe's inferred density is 3.48 +/- 0.64 g.cm-3 , in agreement with an intact interior based on its H-chondrite composition. Using the 3D shape model to derive the volume of the largest depression (likely impact crater), it appears that the latter is significantly smaller than the total volume of close-by S-type H-chondrite-like asteroid families. Conclusions. Our results imply that (6) Hebe is not the most likely source of H chondrites. Over the coming years, our team will collect similar high-precision shape measurements with VLT/SPHERE for ~40 asteroids covering the main compositional classes, thus providing an unprecedented dataset to investigate the origin and collisional evolution of the asteroid belt.

Meteorite flux to Earth in the Early Cretaceous as reconstructed from sediment-dispersed extraterrestrial spinels

We show that Earth’s sedimentary strata can provide a record of the collisional evolution of the asteroid belt. From 1652 kg of pelagic Maiolica limestone of Berriasian–Hauterivian age from Italy, we recovered 108 extraterrestrial spinel grains (32–250 μm) representing relict minerals from coarse micrometeorites. Elemental and three oxygen isotope analyses were used to characterize the grains, providing a first-order estimate of the major types of asteroids delivering material at the time. Comparisons were made with meteorite-flux time “windows” in the Ordovician before and after the L-chondrite parent-body breakup. In the Early Cretaceous, ∼80% of the extraterrestrial spinels originated from ordinary chondrites. The ratios between the three groups of ordinary chondrites, H, L, LL, appear similar to the present, ∼1:1:0.2, but differ significantly from Ordovician ratios. We found no signs of a hypothesized Baptistina LL-chondrite breakup event. About 10% of the grains in the Maiolica originate from achondritic meteorite types that are very rare (<1%) on Earth today, but that were even more common in the Ordovician. Because most meteorite groups have lower spinel content than the ordinary chondrites, our data indicate that the latter did not dominate the flux during the Early Cretaceous to the same extent as today. Based on studies of three windows in deep time, we argue that there may have been a gradual long-term (a few hundred million years) turnover in the meteorite flux from dominance of achondrites in the early Phanerozoic to ordinary chondrites in the late Phanerozoic, interrupted by short-term (a few million years) meteorite cascades from single asteroid breakup events. (Less)

A chondrite strewn field was found in east of Lop Nor, Xinjiang

The meteorites are the most important extraterrestrial samples for understanding the formation and the evolution history of the solar system, although ~382 kg lunar samples and ~1500 asteroid mineral grains have been returned by human space exploration activities. The Northwest region of China, especially the Xinjiang, has a vast area of sandy deserts and Gobi Deserts. The lower annual rainfall and the dry climate of this region provide a good condition for meteorite preservation. In recent years, the number of recovered meteorites from the Gobi Deserts in Xinjiang increased significantly. The Sanfengshan area, located about 65 km east of Lop Nor, is one of the research regions. We first collected three ordinary chondrites in Sanfengshan area on October 9, 2013. Then, we carried out 11 meteorite search campaigns in this area. More than 130 meteorite fragments with a total mass over 160 kg have been recovered. The map of all collected Sanfengshan meteorites depicts a clearly 3 km×11 km elliptic strewn field. The sizes are sorted in the SE-NW-oriented ellipsoid of the strewn field, therefore indicating that the meteor entered the atmosphere from SE at a low angle relative to the ground. In this study, six meteorites (Tuya 002–007) from the Sanfengshan field have been measured using a Scios-FIB field emission scanning electron microscope (SEM), a JXA 8230 electron microprobe analysis (EMA), and a self-made noble gas mass spectrometer. Among all of the collected meteorites, Tuya 002 is the only one unusual sample, which doesn’t show any fusion crust on its surface. We inferred that Tuya 002 is not paired with the other collected meteorites. The well-defined chondrule boundaries and heavy weathering grade, i.e. W3 (the occurrence of large amounts of calcium sulfate in Tuya 002 indicates that it suffered a heavy terrestrial weathering) also support the conclusion that Tuya 002 is obviously different from the other meteorites collected in the Sanfengshan area. In addition to Tuya 002, all the meteorites collected in this region most likely originate from one single meteoric shower. These meteorites all belong to the L5 ordinary chondrite group, with a moderate shock stage of S2 and a heavy weathering grade of W3. The cosmic ray exposure age of Tuya 003 (the representative meteorite of the Sanfenghshan meteoric shower) is 45.6±4.4 Ma. This indicates that the meteoroid that produced the Sanfengshan meteoric shower could be produced in the 45 Ma break-up event on L chondrite parent body. On the other hand, petrographic observations and noble gas measurements shown that Tuya 002 belongs to the L4 ordinary chondrite group. A rather lower CRE age relative to the Sanfengshan L5 chondrites of 17.0±4.1 Ma has been calculated. This also indicates that Tuya 002 is not paired with other L5 meteorites collected in the same strewn field. Our studies shown that the meteorites collected in Sanfengshan area consist of more than one meteorite fall. It includes the main L5 meteorite fall (produced the Sanfengshan L5 strewn field) and the L4 meteorite fall (represented by Tuya 002). This indicates that the Sanfengshan area is not a meteorite dense area. In recent years, several meteorite dense areas (e.g., Loulan Yizhi, Lop Nur, Argan, and Xingdi) have been gradually reported. The limited meteorite types found in such reported meteorite dense areas indicates that these regions are maybe not the real meteorite dense areas, but probably represents the meteorites strewn field. Therefore, it is necessary, in the future, to carry out detailed petrology, mineralogy, cosmic ray exposure age as well as terrestrial age measurements to identify the meteorite dense area in/around the Xinjiang.

A gamma-ray spectroscopy survey of Omani meteorites

The gamma-ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani-Swiss meteorite search campaigns 2001-2008 were nondestructively measured using an ultra-low background gamma-ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean 26Al activities in groups of ordinary chondrites with a) weathering degrees W0-1 and low 14C terrestrial age and b) weathering degree W3-4 and high 14C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in 26Al. Among the least weathered samples one meteorite (SaU 424) was found to contain detectable 22Na identifying it as recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is ~120 events/106 km2*a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass ~4.5 t) show significant variations of 26Al activities, including the highest values measured, consistent with a meteoroid radius of ~115 cm. Activities of 238U daughter elements demonstrate terrestrial contamination with 226Ra and possible loss of 222Rn. Recent contamination with small amounts of 137Cs is ubiquitous. We conclude that gamma-ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.

A Preliminary Study On the Electrical Signatures of Some Iron and Stony Meteorites and Their Dependence On Nickel Content

The present study is considered as an exploratory study of electrical properties of meteorites at variable current frequencies, called the electrical signature. The electric resistivity has been measured at different frequencies, varying between 1 and 100 KHz for some iron meteorites (Mundrabilla and Gibeon meteorites), stony meteorite samples (NWA 869, NWA 7629 and Ghubara) and Fe-Ni alloys, of known Ni concentration, which have been prepared and used as standards to be compared with the studied meteorites. In addition, SEM studies supported by EDX technique have been applied. The SEM and EDX displays enabled us to obtain the chemical composition and internal structural fabrics of the studied samples. Porosity and density (bulk and grain densities) have also been measured for both types of meteorites. Porosity values of the studied meteorites are very low (∅ ≤ 3%) and correspond to micro vugs and cracks. The grain density of non-weathered samples varies from 3.48 g/cm3 for the stony meteorites to 7.91 g/cm3 for the iron meteorites. The obtained electrical signatures are diagnostic for each type and can be used to detect quantitatively the concentration of Ni. The electrical signature of stony meteorites is much simpler than that of iron meteorites, and simpler signatures have been obtained at higher Ni concentrations.

CLOUDS IN SUPER-EARTH ATMOSPHERES: CHEMICAL EQUILIBRIUM CALCULATIONS

ABSTRACT Recent studies have unequivocally proven the existence of clouds in super-Earth atmospheres. Here we provide a theoretical context for the formation of super-Earth clouds by determining which condensates are likely to form under the assumption of chemical equilibrium. We study super-Earth atmospheres of diverse bulk composition, which are assumed to form by outgassing from a solid core of chondritic material, following Schaefer &amp; Fegley. The super-Earth atmospheres that we study arise from planetary cores made up of individual types of chondritic meteorites. They range from highly reducing to oxidizing and have carbon to oxygen (C:O) ratios that are both sub-solar and super-solar, thereby spanning a range of atmospheric composition that is appropriate for low-mass exoplanets. Given the atomic makeup of these atmospheres, we minimize the global Gibbs free energy of formation for over 550 gases and condensates to obtain the molecular composition of the atmospheres over a temperature range of 350–3000 K. Clouds should form along the temperature–pressure boundaries where the condensed species appear in our calculation. We find that the composition of condensate clouds depends strongly on both the H:O and C:O ratios. For the super-Earth archetype GJ 1214b, KCl and ZnS are the primary cloud-forming condensates at solar composition, in agreement with previous work. However, for oxidizing atmospheres, K2SO4 and ZnO condensates are favored instead, and for carbon-rich atmospheres with super-solar C:O ratios, graphite clouds appear. For even hotter planets, clouds form from a wide variety of rock-forming and metallic species.

A new type of solar-system material recovered from Ordovician marine limestone.

From mid-Ordovician ∼470 Myr-old limestone >100 fossil L-chondritic meteorites have been recovered, representing the markedly enhanced flux of meteorites to Earth following the breakup of the L-chondrite parent body. Recently one anomalous meteorite, Österplana 065 (Öst 65), was found in the same beds that yield L chondrites. The cosmic-ray exposure age of Öst 65 shows that it may be a fragment of the impactor that broke up the L-chondrite parent body. Here we show that in a chromium versus oxygen-isotope plot Öst 65 falls outside all fields encompassing the known meteorite types. This may be the first documented example of an ‘extinct' meteorite, that is, a meteorite type that does not fall on Earth today because its parent body has been consumed by collisions. The meteorites found on Earth today apparently do not give a full representation of the kind of bodies in the asteroid belt ∼500 Myr ago.