Biology
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.496c&link_type=abstract
Meteoritics, vol. 30, no. 5, page 496
Biology
1
Carbon, Interplanetary Dust Particlesy, L200E4, X-Ray Microscope, Xanes Spectra
Scientific paper
The nature and distribution of carbon in interplanetary dust particles (IDPs) are important because IDP carbon has been proposed as a major source of pre-biotic organic material on the early Earth [1] and may have been equally important on the surface of early Mars [2], thus both the abundance and the types of carbon in IDPs have exobiology implications. An ~12 micrometers chondritic-porous IDP (L2008F4) was embedded in sulfur and thin sections (~ 50 - 80 micrometers thick) were prepared by ultramicrotomy. The thin sections were supported on SiO membranes and analyzed using the scanning transmission x-ray microscope (STXM) at the National Synchrotron Light Source, Brookhaven National Laboratory [3]. This microscope operates by focusing monochromatic x-rays to a 50 nm diameter spot, through which the specimen is scanned, and detecting the flux transmitted through the specimen. The focusing lens is a zone plate, which operates efficiently over the range from 2.0 to 5.0 nm (250 to 600 eV). At the carbon edge, where this analysis was done, the flux on the sample is typically 1 x 10^6 photons/second for a 0.2 eV energy resolution. High resolution images (200 by 300 pixels over an area of 10 x 15 micrometers) of the IDP section were taken at energies of 281.8 eV and 302.4 eV, which are respectively below and above the carbon K-edge. Above the edge carbon absorbs strongly, while it is virtually transparent at energies below the edge. By taking differences in the logarithm of the intensity at each pixel, maps proportional to the carbon density were obtained. The C-mapping indicated that the thin section contains an estimated 34 vol.% carbon. The bonding state of carbon can be inferred from the x-ray absorption near-edge structure (XANES) [4]. Carbon-XANES spectra, shown in Figure 1, were obtained at 4 locations on this IDP section, identified as having high carbon concentrations from the carbon map. In most areas the spectra showed three pre-edge resonances, though one region (patch 2-4) showed no peaks. The three peaks are pi* resonances due to carbon double or triple bonds. The lowest energy peak can be identified as the C=C double bond. At least two distinct phases of carbon were identified. The section was thinner than optimal for this technique, so the change in contrast of the carbon rich areas across the carbon edge was only 10%. Even so, individual carbon-XANES spectra were obtained in approximately 2 minutes. The STXM can be used to map the carbon density distribution, determine the carbon abundance, and identify the particular carbon bond. References: [1] Anders E. (1989) Nature, 342, 255-257. [2] Flynn G. J. (1993) LPS XXIV, 493-494. [3] Jacobsen C. et al. (1991) Opt. Comm., 86, 351-364. [4] Ade H. et al. (1994) Science, 258, 972-975. Fig. 1. Carbon XANES spectra of 4 regions on a single ultramicrotome section of L2008F4, found by taking the negative of the logarithm of transmission spectra. The spectra have been vertically displaced for clarity. _
Bajt Saa
Chapman Henry N.
Flynn George James
Keller Lindsay P.
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