title: Observation of indigenous polycyclic aromatic hydrocarbons in 'giant' carbonaceous antarctic micrometeorites.
abstract: Two-step laser desorption/laser ionization mass spectrometry (microL2 MS) was used to establish the nature and mass distribution of polycyclic aromatic hydrocarbons (PAHs) in fragments of fifteen 'giant' (approximately 200 microns) carbonaceous Antarctic micrometeorites (AMMs). Detectable concentrations of PAHs were observed in all AMMs showing a fine-grained matrix. The range of integrated PAH signal intensities varied between samples by over two orders of magnitude. No evidence of contamination whilst in the Antarctic environment could be found. The dramatic variation of both PAH signal intensities and mass distributions between AMMs along with comprehensive contamination checks demonstrates that particles are not exposed to terrestrial PAHs at or above detection limits, either subsequent, during or prior to collection. Comparison of the observed PAH distributions with those measured in three carbonaceous chondrites [Orgueil (CI1), Murchison (CM2) and Allende (CV3)] under identical conditions demonstrated that marked differences exist in the trace organic composition of these two sources of extraterrestrial matter. In general, AMMs show a far richer distribution of unalkylated 'parent' PAHs with more extended alkylation series (replacement of -H with -(CH2)n-H; n = 1, 2, 3 ...). The degree of alkylation loosely correlates with a metamorphic index that represents the extent of frictional heating incurred during atmospheric entry. A search for possible effects of the chemical composition of the fine-grain matrix of host particles on the observed PAH distributions reveals that high degrees of alkylation are associated with high Na/Si ratios. These results, in addition to other observations by Maurette, indicate that 'giant' micrometeorites survive hypervelocity (> or = 11 km s-1) atmospheric entry unexpectedly well. Because such micrometeorites are believed to represent the dominant mass fraction of extraterrestrial material accreted by the Earth, they may have played a significant role in the prebiotic chemical evolution of the early Earth through the delivery of complex organic matter to the surface of the planet.
authors.affiliation: Department of Chemistry, Stanford University, CA 94305-5080, USA.
authors.forename: S J
authors.initials: SJ
authors.lastname: Clemett
authors.forename: X D
authors.initials: XD
authors.lastname: Chillier
authors.forename: S
authors.initials: S
authors.lastname: Gillette
authors.forename: R N
authors.initials: RN
authors.lastname: Zare
authors.forename: M
authors.initials: M
authors.lastname: Maurette
authors.forename: C
authors.initials: C
authors.lastname: Engrand
authors.forename: G
authors.initials: G
authors.lastname: Kurat
mesh_terms: D000864:Antarctic Regions; D018534:Earth, Planet; D019418:Evolution, Chemical; D005118:Extraterrestrial Environment; D019110:Meteoroids; D008855:Microscopy, Electron, Scanning; D000069497:Origin of Life; D011084:Polycyclic Aromatic Hydrocarbons / Q000032:analysis*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D011084:Polycyclic Aromatic Hydrocarbons