OBSERVATION OF INDIGENOUS POLYCYCLIC AROMATIC-HYDROCARBONS IN GIANT CARBONACEOUS ANTARCTIC MICROMETEORITES

Two-step laser desorption/laser ionization mass spectrometry (mu (LMS) -M-2) was used to establish the nature and mass distribution of polycy clic aromatic hydrocarbons (PAHs) in fragments of fifteen 'giant' (sim ilar to 200 mu m) carbonaceous Antarctic micrometeorites (AMMs). Detec table 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 contam ination whilst in the Antarctic environment could be found. The dramat ic variation of both PAH signal intensities and mass distributions bet ween AMMs along with comprehensive contamination checks demonstrates t hat particles are not exposed to terrestrial PAHs at or above detectio n limits, either subsequent, during or prior to collection. Comparison of the observed PAH distributions with those measured in three carbon aceous chondrites [Orgueil (CI1), Murchison (CM2) and Allende (CV3)] u nder identical conditions demonstrated that marked differences exist i n the trace organic composition of these two sources of extraterrestri al matter. In general, AMMs show a far richer distribution of unalkyla ted 'parent' PAHs with more extended alkylation series (replacement of -H with -(CH2)(n)-H; n = 1, 2, 3 ...). The degree of alkylation loose ly correlates with a metamorphic index that represents the extent of f rictional heating incurred during atmospheric entry. A search for poss ible effects of the chemical composition of the fine-grain matrix of h ost particles on the observed PAH distributions reveals that high degr ees of alkylation are associated with high Na/Si ratios. These results , in addition to other observations by Maurette, indicate that 'giant' micrometeorites survive hypervelocity (greater than or equal to 11 km s(-1)) atmospheric entry unexpectedly well. Because such micrometeori tes are believed to represent the dominant mass fraction of extraterre strial material accreted by the Earth, they may have played a signific ant role in the prebiotic chemical evolution of the early Earth throug h the delivery of complex organic matter to the surface of the planet.