Patrick D. Swan

Identification of Complex Aromatic Molecules in Individual Interplanetary Dust Particles

Seventeen stratospherically collected particles—eight of which are classified as interplanetary dust particles (IDPs), seven of which are classified as probable terrestrial contaminants, and two of which have uncertain origins—were studied with a microprobe two-step laser mass spectrometer. Many polycyclic aromatic hydrocarbons(PAHs) and their alkylated derivatives were identified in two of the eight IDPs. The PAHs observed include a high-mass envelope not found in meteorites or terrestrial contaminants and prominent odd-mass peaks suggestive of nitrogen-containing functional groups attached to aromatic chromophores. In addition, the complexity of the IDP mass spectra has no precedence in previous studies of meteorite samples or their acid residues. Extensive checks were performed to demonstrate that the PAH signals are not caused by terrestrial contaminants.

Pristine presolar silicon carbide

We report the results of a study of 81 micrometer-sized presolar SiC grains in the size range 0.5-2.6 m from the Murchison (CM2) carbonaceous chondrite. We describe a simple, nondestructive physical disaggregation technique used to isolate the grains while preserving them in their pristine state, as well as the scanning electron microscopy energy-dispersive X-ray mapping procedure used to locate them. Nine-tenths of the pristine SiCs are bounded by one or more planar surfaces consistent with cubic (3C polytype) crystal faces based on manifest symmetry elements. In addition, multiple polygonal depressions (generally 100 nm deep) are observed in more than half of these crystal faces, and these possess symmetries consistent with the structure of the 3C polytype of SiC. By comparison of these features with the surface features present on heavily etched presolar SiC grains from Murchison separate KJG, we show that the polygonal depressions on pristine grains are likely primary growth features. The etched SiCs have high densities of surface pits, in addition to polygonal depressions. If these pits are etched linear defects in the SiC, then defect densities are quite high (as much as 10 8 -10 9 /cm 2 ), about 10 3 -10 4 times higher than in typical synthetic SiCs. The polygonal depressions on crystal faces of pristine grains, as well as the high defect densities, indicate rapid formation of presolar SiC. No other primary minerals are observed to be intergrown with or overgrown on the pristine SiCs, so the presence of overgrowths of other minerals cannot be invoked to account for the survival of presolar SiC in the solar nebula. We take the absence of other primary condensates to indicate that further growth or back-reaction with the gas became kinetically inhibited as the gas-phase densities in the expanding asymptotic giant branch (AGB) stellar atmospheres (in which most of the grains condensed) became too low. However, we did observe an oxygen peak in the X-ray spectra of most pristine grains, implying silica coatings of as much as several tens of nm thickness, perhaps due to oxidation of the SiC in the solar nebula. We see little or no evidence on the pristine grains of the surface sputtering or cratering that are predicted theoretically to occur in the interstellar medium (ISM) due to supernova shocks. A possible implication is that the grains may have been protected during their residence in the ISM by surface coatings, including simple ices. Residues of such coatings may indeed be present on some pristine SiCs, because many (60%) are coated with an apparently amorphous, possibly organic phase. However, at present we do not have sufficient data on the coatings to draw secure inferences as to their nature or origin. A few irregular pristine SiCs, either fragments produced by regolith gardening on the Murchison parent body or by grain- grain collisions in the ISM, were also observed. Copyright

Micro-FTIR measurements on individual interplanetary dust particles

The unique characteristics of micro-FTIR spectroscopy for the study of individual interplanetary dust particles (IDPs) are discussed: (1) quick and nondestructive identification of dominant silicate phases (pyroxenes, olivines, layer lattice silicates) and selection of unique IDPs for subsequent intensive study by complementary analytical techniques; (2) identification of potential carbonaceous carrier phases of isotopic anomalies; (3) measurement of IR absorption features in IDPs that can be compared with IR emission features obtained by telescopic observations from comets and protostars.