Conel M. Od. Alexander

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

The Provenances of Asteroids, and Their Contributions to the Volatile Inventories of the Terrestrial Planets

Constraining the Birthplace of Asteroids Many primitive meteorites originating from the asteroid belt once contained abundant water that is now stored as OH in hydrated minerals. Alexander et al. (p. 721, published online 12 July) estimated the hydrogen isotopic compositions in 86 samples of primitive meteorites that fell in Antarctica and compared the results to those of comets and Saturns moon, Enceladus. Water in primitive meteorites was less deuteriumrich than that in comets and Enceladus, implying that, in contradiction to recent models of the dynamical evolution of the solar system, the parent bodies of primitive meteorites cannot have formed in the same region as comets. The results also suggest that comets were not the principal source of Earths water. Hydrogen isotopic analysis of primitive meteorites implicates asteroids as early sources of Earth’s water. Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn’s moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth’s volatiles.

Silicon Nitride from Supernovae

Seven presolar silicon nitride (Si3N4) dust grains have been identified (five unambiguously and two probably) in separates of the Tieschitz (H3.6) and Murchison (CM2) meteorites, confirming previous tentative identifications of this mineral as a presolar component. These rare (2 ppb in Murchison) grains have isotopic compositions similar to those of the uncommon class of meteoritic SiC known as grains X (~60 ppb in Murchison), namely 28Si and 15N excesses relative to solar, both 13C excesses and deficits, and extremely high inferred 26Al/27Al ratios. These isotopic compositions coupled with Ca and Ti isotopic anomalies seen in some SiC grains X point to an origin in Type II supernova ejecta for SiC grains X, and by analogy for the Si3N4 grains as well. However, substantial discrepancies exist between the isotopic characteristics of the grains and the compositions predicted by supernova models.

Origin and Evolution of Prebiotic Organic Matter As Inferred from the Tagish Lake Meteorite

The study of organic matter in a well-preserved meteorite provides insight into processes that affected its parent asteroids. The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites’ asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration.

Aluminum-, calcium- and titanium-rich oxide stardust in ordinary chondrite meteorites

We report O-, Al-Mg-, K-, Ca-, and Ti-isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3 ,M gAl2O4, hibonite (CaAl12O19), and Ti oxide. This work greatly increases the presolar hibonite database, and is the first report of presolarTioxide.O-isotopiccompositionsof thegrainsspanpreviouslyobservedrangesandindicateanoriginin red giant and asymptotic giant branch (AGB) stars of low mass (<2.5 M� ) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in hibonite grains are attributable to in situ decay of now-extinct 41 Ca. Inferred initial 41 Ca/ 40 Ca ratios are in good agreement with model predictionsfor low-mass AGB star envelopes,provided that ionizationsuppresses 41 Cadecay. Stable Mg and Ca isotopic ratios of most of the hibonite grains reflect primarily the initial compositions of the parent starsandaregenerallyconsistentwithexpectationsforGalacticchemicalevolution,butrequiresomelocalinterstellar chemical inhomogeneity. Very high 17 O/ 16 Oo r 25 Mg/ 24 Mg ratios suggest an origin for some grains in binary star systemswheremasstransferfromanevolvedcompanionhasalteredtheparentstarcompositions.Asupernovaorigin for the hitherto enigmatic 18 O-rich Group 4 grains is strongly supported by multielement isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep 16 O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — supernovae: general

Characterization of Presolar Silicate and Oxide Grains in Primitive Carbonaceous Chondrites

Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abundancesof 320and200ppm,respectively.Theseabundances,whicharematrix-normalizedandcorrectedforinstrumental detectionefficiencies,aremuchhigherthanthoseofotherpresolarphases,withtheexceptionof nanodiamonds,although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar silicates provides further constraints on the effects of Galactic chemical evolution. Subject headingg circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolution

High precision iron isotope measurements of meteoritic material by cold plasma ICP-MS

The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ56Fe and ± 0.25 ‰ for δ58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ56Fechondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.

Establishing a molecular relationship between chondritic and cometary organic solids

Multidimensional solid-state NMR spectroscopy is used to refine the identification and abundance determination of functional groups in insoluble organic matter (IOM) isolated from a carbonaceous chondrite (Murchison, CM2). It is shown that IOM is composed primarily of highly substituted single ring aromatics, substituted furan/pyran moieties, highly branched oxygenated aliphatics, and carbonyl groups. A pathway for producing an IOM-like molecular structure through formaldehyde polymerization is proposed and tested experimentally. Solid-state 13C NMR analysis of aqueously altered formaldehyde polymer reveals considerable similarity with chondritic IOM. Carbon X-ray absorption near edge structure spectroscopy of formaldehyde polymer reveals the presence of similar functional groups across certain Comet 81P/Wild 2 organic solids, interplanetary dust particles, and primitive IOM. Variation in functional group concentration amongst these extraterrestrial materials is understood to be a result of various degrees of processing in the parent bodies, in space, during atmospheric entry, etc. These results support the hypothesis that chondritic IOM and cometary refractory organic solids are related chemically and likely were derived from formaldehyde polymer. The fine-scale morphology of formaldehyde polymer produced in the experiment reveals abundant nanospherules that are similar in size and shape to organic nanoglobules that are ubiquitous in primitive chondrites.

Chondrule formation in particle-rich nebular regions at least hundreds of kilometres across

Chondrules are millimetre-sized spherules (mostly silicate) that dominate the texture of primitive meteorites. Their formation mechanism is debated, but their sheer abundance suggests that the mechanism was both energetic and ubiquitous in the early inner Solar System. The processes suggested—such as shock waves, solar flares or nebula lightning—operate on different length scales that have been hard to relate directly to chondrule properties. Chondrules are depleted in volatile elements, but surprisingly they show little evidence for the associated loss of lighter isotopes one would expect. Here we report a model in which molten chondrules come to equilibrium with the gas that was evaporated from other chondrules, and which explains the observations in a natural way. The regions within which the chondrules formed must have been larger than 150–6,000 km in radius, and must have had a precursor number density of at least 10 m-3. These constraints probably exclude nebula lightning, and also make formation far from the nebula midplane problematic. The wide range of chondrule compositions may be the result of different combinations of the local concentrations of precursors and the local abundance of water ice or vapour.

Automated isotopic measurements of micron-sized dust: Application to meteoritic presolar silicon carbide

identified SiC AB, X, Y, and Z subgroups were identified, as was a highly unusual grain with an extreme 30 Si enrichment, a modest 29 Si enrichment, and isotopically light C. The stellar source responsible for this grain is likely to have been a supernova. Minor differences in isotopic distributions between the present work and prior data can be partially explained by terrestrial contamination and grain aggregation on sample mounts, though some of the differences are probably intrinsic to the samples. We use the large new SiC database to explore the relationships between three previously identified isotopic subgroups—mainstream, Y, and Z grains—all believed to originate in asymptotic giant branch stars. The isotopic data for Z grains suggest that their parent stars experienced strong CNO-cycle nucleosynthesis during the early asymptotic giant branch phase, consistent with either cool bottom processing in low-mass (M 2.3MJ) parent stars or hot-bottom burning in intermediate-mass stars (M 4MJ). The data provide evidence for a sharp threshold in metallicity, above which SiC grains form with much higher 12 C/ 13 C ratios than below. Above this threshold, the fraction of grains with relatively high 12 C/ 13 C decreases exponentially with increasing 29 Si/ 28 Si ratio. This result indicates a sharp increase in the maximum mass of SiC parent stars with decreasing metallicity, in contrast to expectations from Galactic chemical evolution theory. Copyright