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Interplanetary dust particles obtained from the earth's stratosphere are investigated to determine the density of these particles. The size of the interplanetary dust particles varies from 5 to 15 micrometers and the density ranges from 0.3 to 6.2 grams per cubic centimeter. The low density value reveals that the particles exhibit porosity and originate from uncompacted bodies. Most of these particles have sulphide grains and chondritic spherules.

Hypervelocity impacts of soda lime glass on porous sintered glass targets is investigated to study the mechanism involved in impacts of small astronomical objects such as meteorites on larger ones such as planets. Strength and density of projectiles and targets are varied. Deep craters are caused when porosity of the target is increased. At lower velocities, the impact damage has a greater localization. Porous targets also have a very low rear-surface spallation. The investigation indicates that threshold of the specific energy required to damage targets of different porosity is propotional to (1 - the porosity value)(super minus 3.6). Melt-bordered agglutinate crater pits are produced during the impacts.

Magnesium stable isotope ratios and minor element abundances of five olivine particles from comet 81P/Wild 2 were examined by secondary ion mass spectrometry (SIMS). Wild 2 olivine particles exhibit only small variations in δ25Mg values from –1.0 +0.4/ –0.5 ‰ to 0.6 +0.5/ –0.6 ‰ (2σ). This variation can be simply explained by mass-dependent fractionation from Mg isotopic compositions of the Earth and bulk meteorites, suggesting that Wild 2 olivine particles formed in the chondritic reservoir with respect to Mg isotope compositions. We also determined minor element abundances, and O and Mg isotope ratios of olivine grains in amoeboid olivine aggregates (AOAs) from Kaba (CV3.1) and DOM 08006 (CO3.01) carbonaceous chondrites. Our new SIMS minor element data reveal uniform, low FeO contents of ∼0.05 wt% among AOA olivines from DOM 08006, suggesting that AOAs formed at more reducing environments in the solar nebula than previously thought. Furthermore, the SIMS-derived FeO contents of the AOA olivines are consistently lower than those obtained by electron microprobe analyses (∼1 wt% FeO), indicating possible fluorescence from surrounding matrix materials and/or Fe,Ni-metals in AOAs during electron microprobe analyses. For Mg isotopes, AOA olivines show more negative mass-dependent fractionation (–3.8 ± 0.5‰ ≤ δ25Mg ≤ –0.2 ± 0.3‰; 2σ) relative to Wild 2 olivines. Further, these Mg isotope variations are correlated with their host AOA textures. Large negative Mg isotope fractionations in olivine are often observed in pore-rich AOAs, while those in compact AOAs tend to have near-chondritic Mg isotopic compositions. These observations indicate that pore-rich AOAs preserved their gas–solid condensation histories, while compact AOAs experienced thermal processing in the solar nebula after their condensation and aggregation. Importantly, one 16O-rich Wild 2 LIME olivine particle (T77/F50) shows negative Mg isotope fractionation (δ25Mg = –0.8 ± 0.4‰, δ26Mg = –1.4 ± 0.9‰; 2σ) relative to bulk chondrites. Minor element abundances of T77/F50 are in excellent agreement with those of olivines from pore-rich AOAs in DOM 08006. The observed similarity in O and Mg isotopes, and minor element abundances suggest that T77/F50 formed in an environment similar to AOAs, probably near the proto-Sun, and then was transported to the Kuiper belt, where comet 81P/Wild 2 likely accreted. [ABSTRACT FROM AUTHOR]

We measured oxygen isotope ratios of 16 silicate fragments from seven aerogel tracks (turnip-like type B tracks 77, 149, 172, 191, and 220; carrot-like type A tracks 22 and 175) of the comet 81P/Wild 2 collector from NASA's Stardust mission using secondary ion mass spectrometry. Thirteen were prepared by ultramicrotomy; three from track 220 were prepared by sputtering resin blocks using a SIMS Kohler beam, a new procedure aiming to mine as many cometary particles encased in aerogel/resin as possible. Combining new and literature results, we recognized that most silicate fragments of individual type B tracks have diverse mineralogy but consistent mass-independent fractionation of oxygen isotopes (Δ17O = δ17O − 0.52 × δ18O). These observations suggest that their impactors are loosely bound aggregates of unequilibrated materials originating mainly from similar protoplanetary disk regions, resembling the cluster IDP U2-20-GCA. Furthermore, silicate fragments from type A track 22 have almost identical mineralogy and Δ17O values, confirming that its impactor is a single chondrule-like fragment. The terminal particle of type A track 175 is pure forsterite with Δ17O of ∼ –23 ‰. Six iron-rich fragments of this study have positive oxygen isotope ratios (Δ17O∼ +2 ‰) and ordinary chondrite chondrule-like olivine compositions. Together with five similar fragments in the literature, a unique population (Mg# ≤ 86) of Wild 2 fragments that resemble chondrules from the inner solar system (O-E-R) chondrites or the outer solar system CH-CB chondrites was identified. The remaining 16O-poor Wild 2 fragments are Mg# ≥ 79 silicates with Δ17O ∼ –2 ‰ and a small amount of Mg# ≤ 79 silicates with Δ17O ∼ 0 ‰, which are most consistent with CR chondrite chondrules. Thus, we conclude that in addition to the possible major source of CR chondrite chondrule-like materials, the inner solar system or CH-CB chondrule-like materials are a minor component of comet Wild 2, like the cluster IDP U2-20-GCA. [ABSTRACT FROM AUTHOR]

Filamentary enstatite crystals, formed by gas–solid condensation in the solar nebula, are found in chondritic porous interplanetary dust particles of probable cometary origin. We measured the oxygen isotopic composition of four filamentary enstatite grains, two whiskers (1.8 μm and 2.3 μm in length) and two ribbons (3.4 μm and 6.1 μm in length), from the giant cluster interplanetary dust particle U2-20 GCP using NanoSIMS ion imaging. These grains represent both the 16O-rich solar (δ 17,18O ≈ - 70 ‰) and 16O-poor planetary (δ 17,18O ≈ 0 ‰) isotope reservoirs. Our measurements provide evidence for very early vaporization of dust-poor and dust-rich regions of the solar nebula, followed by condensation and outward transport of crystalline dust to the comet-forming region very far from the Sun. Similar processes are likely responsible for the crystalline silicates observed in the outer regions of protoplanetary disks elsewhere in the Galaxy. [ABSTRACT FROM AUTHOR]

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35–65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous "witness plates" to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15N/14N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion–molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates. [ABSTRACT FROM AUTHOR]

Abstract: Large, correlated, mass-dependent enrichments in the heavier isotopes of O, Cr, Fe, and Ni are observed in type-I (metal/metal oxide) cosmic spherules collected from the deep sea. Limited intraparticle variability of oxygen isotope abundances, typically <5‰ in δ18O, indicates good mixing of the melts and supports the application of the Rayleigh equation for the calculation of fractional evaporative losses during atmospheric entry. Fractional losses for oxygen evaporation from wüstite, assuming a starting isotopic composition equal to that of air (δ18O = 23.5‰; δ17O = 11.8‰), are in the range 55%–77%, and are systematically smaller than evaporative losses calculated for Fe (69%–85%), Cr (81%–95%), and especially Ni (45%–99%). However, as δ18O values increase, fractional losses for oxygen approach those of Fe, Cr, and Ni indicating a shift in the evaporating species from metallic to oxidized forms as the spherules are progressively oxidized during entry heating. The observed unequal fractional losses of O and Fe can be reconciled by allowing for a kinetic isotope mass-dependent fractionation of atmospheric oxygen during the oxidation process and/or that some metallic Fe may have undergone Rayleigh evaporation before oxidation began. In situ measurements of oxygen isotopic abundances were also performed in 14 type-S (silicate) cosmic spherules, 13 from the Antarctic ice and one from the deep sea. Additional bulk Fe and Cr isotopic abundances were determined for two type-S deep-sea spherules. The isotopic fractionation of Cr isotopes suggest appreciable evaporative loss of Cr, perhaps as a sulfide. The oxygen isotopic compositions for the type-S spherules range from δ18O = −2‰ to + 27‰. The intraspherule isotopic variations are typically small, ∼5% relative, except for the less-heated porphyritic spherules which have preserved large isotopic heterogeneities in at least one case. A plot of δ17O vs. δ18O values for these spherules defines a broad parallelogram bounded at higher values of δ17O by the terrestrial fractionation line, and at lower values of δ17O by a line parallel to it and anchored near the isotopic composition of δ18O = −2.5‰ and δ17O = −5‰. Lack of independent evidence for substantial evaporative losses suggests that much of this variation reflects the starting isotopic composition of the precursor materials, which likely resembled CO, CM, or CI chondrites. However, the enrichments in heavy isotopes indicate that some mixing with atmospheric oxygen was probably involved during atmospheric entry for some of the spherules. Isotopic fractionation due to evaporation of incoming grain is not required to explain most of the oxygen isotopic data for type-S spherules. However spherules with barred olivine textures that are thought to have experienced a more intense heating than the porphyritic ones might have undergone some distillation. Two cosmic spherules, one classified as a radial pyroxene type and the other showing a glassy texture, show unfractionated oxygen isotopic abundances. They are probably chondrule fragments that survived atmospheric entry unmelted. Possible reasons type-I spherules show larger degrees of isotopic fractionation than type-S spherules include: a) the short duration of the heating pulse associated with the high volatile content of the type-S spherule precursors compared to type-I spherules; b) higher evaporation temperatures for at least a refractory portion of the silicates compared to that of iron metal or oxide; c) lower duration of heating of type-S spherules compared to type-I spherules as a consequence of their lower densities. [Copyright &y& Elsevier]