ALBERT

Description: TEM and IR spectroscopy investigations of the interplanetary dust particles (IDPs) collected in the stratosphere have shown the majority of IDPs in the layer-lattice silicate and pyroxene classes to not have been heated to temperatures above 600 C during atmospheric entry. This implies that they arrive at the upper atmosphere with low geocentric encounter velocities, and limits the possible encounter trajectories for these particles to relatively circular prograde orbits. On this basis, it is judged unlikely that these IDPs are from earth-crossing comets or asteroids; collected IDPs dominated by olivine include a larger portion of above-600 C-heated particles, suggesting their capture from more eccentric orbits.

Description: The nature of the carbon-bearing phases in IDP's provides information regarding the chemical and physical processes involved in the formation and evolution of the early solar system. Several carbon-bearing materials have been observed in IDP's, but details of their nature, abundance, and distribution are still poorly known. A knowledge of the abundance and nature of carbon in IDP's is useful in constraining the sources of IDP's and for comparisons with other chondritic materials. Estimates of carbon abundance in anhydrous and hydrated IDP's indicate that most of these particles have significantly higher carbon than the carbonaceous chondrites. Mineralogical analyses show that carbonates are only a minor component of most hydrated IDP's, and so the high carbon abundances in this group of IDP's indicates that other carbon-bearing phases are present in significant concentrations. Using the technique of electron energy-loss spectroscopy (EELS), we have identified two forms of carbon in a hydrated IDP, oxidized carbon (carbonates), and amorphous elemental carbon.

Description: The ways of establishing the extraterrestrial nature of different subsets of interplanetary dust collected in the stratosphere by high-altitude aircraft are discussed. Consideration is given to microanalytic techniques which make it possible to obtain detailed experimental information on the mineralogical and petrographic characteristics, the mid-IR absorption spectra, the Raman spectra, and the isotopic properties of individual particles. The implications of data obtained by these techniques for the origin of interplanetary dust are examined, showing that the particles are less altered than those solar-system material samples found in meteorites. It is suggested that many of the particles come from comets, although an unknown fraction originate from asteroids. Small regions of isotopically distinct material suggest that part of the dust consists of interstellar-cloud material that predates the solar system.

Description: Comet dust is primitive and shows significant diversity. Our knowledge of the properties of primitive particles has expanded significantly through microscale investigations of cosmic dust samples ( IDP's(Interplanetary Dust Particles) and AMM's (Antarctic Micrometeorites)) and of comet dust samples (Stardust and Rosetta's COSIMA), as well as through remote sensing (spectroscopy and imaging) via Spitzer and via spacecraft encounters with 103P/Hartley 2 and 67P/Churyumov-Gerasimenko. Microscale investigations show that comet dust and cosmic dust are particles of unequilibrated materials, including aggregates of materials unequilibrated at submicron scales. We call unequilibrated materials "primitive" and we deduce they were incorporated into ice-rich (H2O-, CO2-, and CO-ice) parent bodies that remained cold, i.e., into comets, because of the lack of aqueous or thermal alteration since particle aggregation; yet some Stardust olivines suggest mild thermal metamorphism. Primitive particles exhibit a diverse range of: structure and typology; size and size distribution of constituents; concentration and form of carbonaceous and organic matter; D-, N-, and O- isotopic enhancements over solar; Mg-, Fe-contents of the silicate minerals; the compositions and concentrations of sulfides, and of less abundant mineral species such as chondrules, CAIs and carbonates. The uniformity within a group of samples points to: aerodynamic sorting of particles and/or particle constituents; the inclusion of a limited range of oxygen fugacities; the inclusion or exclusion of chondrules; a selection of organics. The properties of primitive particles imply there were disk processes that resulted in different comets having particular selections of primitive materials. The diversity of primitive particles has implications for the diversity of materials in the protoplanetary disk present at the time and in the region where the comets formed.

Description: We report Transmission Electron Microscopy (TEM) investigations of micro-craters that originated from hypervelocity impacts of comet 81P/Wild 2 dust particles on the aluminium foil of the Stardust collector. The craters were selected by Scanning Electron Microscopy (SEM) and then prepared by Focused Ion Beam (FIB) milling techniques in order to provide electron transparent cross-sections for TEM studies. The crater residues contain both amorphous and crystalline materials in varying proportions and compositions. The amorphous component is interpreted as resulting from shock melting during the impact and the crystalline phases as relict minerals. The latter show evidence for shock metamorphism. Based on the residue morphology and the compositional variation, the impacting particles are inferred to have been dominated by mixtures of submicron olivine, pyroxene and Fe-sulfide grains, in agreement with prior results of relatively coarse-grained mineral assemblages in the aerogel collector.

Description: The overall theme of the work was the identification of the sources and formation/aggregation mechanisms of the various classes of interplanetary dust particles (IDP's) and to clarify the relationship between IDP's and conventional meteorites. IDP's are believed to be derived from a much broader range of parent bodies than conventional meteorites. Some of these parent bodies (e.g., comets) have escaped that post accretional processing that has affected the parent bodies of meteorites. Therefore, IDP's are likely to preserve a record of early solar system and possibly presolar grain forming reactions. Using analytical electron microscopy (AEM) and more recently micro-infrared (IR) microspectroscopy to examine ultramicrotomed thin sections, we have addressed the questions of IDP formation mechanisms, sources, and their relationship to conventional meteorites. The following sections describe specific findings resulting from these studies.

Description: Chondritic interplanetary dust particles (IDPs) are heterogeneous aggregates of predominantly submicron mineral grains and carbonaceous material, whose bulk compositions agree within a factor of two with type CI/CM carbonaceous chondrites. The mineralogy and petrography of 25 such particles were studied by analytical electron microscopic examination of ultramicrotomed thin sections (500-1000 A thick). Four classes of chondritic IDPs were recognized, referred to as pyroxene, olivine, smectite, and serpentine, and their relative abundances were 9:4:10:2, respectively. Quantitative thin-film analyses indicate that pyroxene particles most closely resemble material emitted from comet Halley. Smectite particles may have formed from pyroxene particles by aqueous alteration of glass and enstatite crystals. Serpentine particles are the only class that are similar to the matrices of carbonaceous chondrites, but these are the least abundant chondritic IDPs. Collectively, chondritic particles are a mineralogically diverse group of extraterrestrial materials.

Description: GEMS (Glass Embedded with Metals and Sulfides) are highly enigmatic yet common components of anhydrous IDPs. We have recently proposed a model of GEMS formation from shock-accelerated crystalline dust in superbubbles[1] which explains the three most perplexing properties of GEMS: pseudomorphism[2], their chemistry[3], and their size range. In this Abstract, we briefly review the main points of the model, and suggest tests that will either prove or rule out this hypothesis.