S. Mostefaoui

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

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.

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

60Fe: A Heat Source for Planetary Differentiation from a Nearby Supernova Explosion

From a sample of the Semarkona (LL 3.0) ordinary chondrite we report the in situ discovery of 60Ni isotopic anomalies attributable to the decay of short-lived 60Fe (half-life 1.5 Myr) in the mineral phases troilite (FeS) and magnetite (Fe3O4). The troilite shows a 60Ni excesses of up to ~100 parts per thousand (‰) relative to its solar isotopic abundance. A positive correlation between 60Ni excesses and 56Fe/58Ni ratios provides evidence for live 60Fe in the early solar system. The inferred 60Fe/56Fe ratio of (0.92 ± 0.24) × 10-6 is the highest measured in any meteorite sample so far. This ratio is higher than predictions for production within asymptotic giant branch stars, but falls within the range expected for a Type II supernova source. This result is strongly suggestive of injection of freshly synthesized 60Fe into the nascent solar nebula by a nearby supernova explosion. Such a high abundance of 60Fe will exclude irradiation with solar energetic particles as the sole mechanism responsible for the production of short-lived radionuclides. It further shows that the decay of 60Fe was an important heat source for early planetary melting and differentiation and for keeping asteroids thermally active for much longer than would be possible from the decay of 26Al alone.

Extreme Deuterium Excesses in Ultracarbonaceous Micrometeorites from Central Antarctic Snow

Dust to Dust Interplanetary dust particles are thought to sample the most primitive materials in the solar system. Because of their large deuterium enrichments, they are thought to have formed in interstellar molecular clouds—the birthplaces of stars—and to predate the solar system. Duprat et al. (p. 742; see Perspective by Nittler) describe two large interplanetary dust particles collected from Antarctic snow. The particles contain large zones of organic matter with deuterium excesses 10 to 30 times the terrestrial value. Because the organic matter is associated with crystalline silicates similar to those formed within the solar accretion disk, it is expected that the particles themselves formed in the Suns protoplanetary disk, contradicting the idea that all organics with deuterium excesses are of interstellar origin. Interplanetary dust particles recovered from Antarctic snow may provide a sample of the early solar system. Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.

Timescales of shock processes in chondritic and martian meteorites

The accretion of the terrestrial planets from asteroid collisions and the delivery to the Earth of martian and lunar meteorites has been modelled extensively. Meteorites that have experienced shock waves from such collisions can potentially be used to reveal the accretion process at different stages of evolution within the Solar System. Here we have determined the peak pressure experienced and the duration of impact in a chondrite and a martian meteorite, and have combined the data with impact scaling laws to infer the sizes of the impactors and the associated craters on the meteorite parent bodies. The duration of shock events is inferred from trace element distributions between coexisting high-pressure minerals in the shear melt veins of the meteorites. The shock duration and the associated sizes of the impactor are found to be much greater in the chondrite (∼1 s and 5 km, respectively) than in the martian meteorite (∼10 ms and 100 m). The latter result compares well with numerical modelling studies of cratering on Mars, and we suggest that martian meteorites with similar, recent ejection ages (105 to 107 years ago) may have originated from the same few square kilometres on Mars.

Discovery of Abundant In Situ Silicate and Spinel Grains from Red Giant Stars in a Primtive Meteorite

We report the discovery of 12 in situ presolar silicate and spinel grains, 140-590 nm in size, in the Acfer 094 meteorite. These grains represent a matrix-normalized abundance of presolar O-rich dust of 170 parts per million. Among the 10 silicate grains are three olivines, four pyroxenes, and three grains with glasslike composition. Eleven grains have large excesses in 17O with 17O/16O ratios of up to 2.9 times the solar ratio and slightly lower than or close-to-solar 18O/16O ratios. These grains most likely formed in 1.5-1.65 M☉ red giant branch (RGB) or asymptotic giant branch (AGB) stars with close-to-solar metallicity. One pyroxene grain has close-to-solar 17O/16O and 18O/16O of 3.8 times solar. Silicon- and Fe-isotopic ratios of this grain are suggestive of the formation in an RGB or AGB star, probably with higher-than-solar metallicity. 29Si/28Si and 30Si/28Si ratios of the silicate grains vary by ~16%, are positively correlated with one another, and fall to the 30Si-poor side of the Si mainstream line characteristic for presolar SiC from AGB stars. This gives independent confirmation for the view that the Si mainstream line reflects the Galactic chemical evolution of the Si isotopes, except for a small shift due to dredge-up of matter from the He shell in AGB stars.

NITROGEN AND CARBON ISOTOPIC COMPOSITION OF THE SUN INFERRED FROM A HIGH-TEMPERATURE SOLAR NEBULAR CONDENSATE

We report high-precision measurements of nitrogen and carbon isotopic compositions of a carbon-bearing titanium-nitride (osbornite) in a calcium-aluminum-rich inclusion (CAI) from the CH/CB-like carbonaceous chondrite Isheyevo. The mineralogy and petrography of the CAI and thermodynamic calculations indicate that the osbornite formed by gas-solid condensation in a high-temperature (similar to 2000 K) region of the solar nebula. Because isotopic fractionation at high temperature is small, the measured nitrogen [N-15/N-14 = (2.356 +/- 0.018) x 10(-3)] and carbon [C-13/C-12 = 0.01125 +/- 0.00008; 1 sigma] isotopic compositions of the Isheyevo osbornite are representative of the solar nebula and, hence, of the Sun. This conclusion is supported by the observations that ( 1) the measured C-13/C-12 ratio is indistinguishable from the spectroscopic determination of the C-13/C-12 ratio of the solar photosphere and ( 2) the measured N-15/N-14 ratio of osbornite is in excellent agreement with the Galileo spacecraft measurement of the nitrogen isotopic composition of the Jovian atmosphere, the second largest reservoir of nitrogen in the solar system. The inferred N-15/N-14 ratio of the solar nebula is also similar to the nitrogen isotopic composition of the vast majority of chondritic nanodiamonds, suggesting their solar nebula origin.

Biological forcing controls the chemistry of reef‐building coral skeleton

[1] We present analyses of major elements C and Ca and trace elements N, S, Mg and Sr in a Porites sp. exoskeleton with a spatial resolution better than similar to 150 nm. Trace element variations are evaluated directly against the ultrastructure of the skeleton and are ascribed to dynamic biological forcing. Individual growth layers in the bulk fibrous aragonite skeleton form on sub-daily timescales. Magnesium concentration variations are dramatically correlated with the growth layers, but are uncorrelated with Sr concentration variations. Observed (sub) seasonal relationships between water temperature and skeletal trace-element chemistry are secondary, mediated by sensitive biological processes to which classical thermodynamic formalism does not apply.

Proto-Planetary Disk Chemistry Recorded by D-Rich Organic Radicals in Carbonaceous Chondrites

Insoluble organic matter (IOM) in primitive carbonaceous meteorites has preserved its chemical composition and isotopic heterogeneity since the solar system formed ∼4.567 billion years ago. We have identified the carrier moieties of isotopically anomalous hydrogen in IOM isolated from the Orgueil carbonaceous chondrite. Data from high spatial resolution, quantitative isotopic NanoSIMS mapping of Orgueil IOM combined with data from electron paramagnetic resonance spectroscopy reveals that organic radicals hold all the deuterium excess (relative to the bulk IOM) in distinct, micrometer-sized, D-rich hotspots. Taken together with previous work, the results indicate that an isotopic exchange reaction took place between pre-existing organic compounds characterized by low D/H ratios and D-rich gaseous molecules, such as H2D + or HD2 + . This exchange reaction most likely took place in the diffuse outer regions of the proto-planetary disk around the young Sun, offering a model that reconciles meteoritic and cometary isotopic compositions of organic molecules.