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Project Progress

1. Organic Carbon in Meteorites and IDPs

   Interplanetary dust particles (IDPs) are typically several microns in diameter and contain carbon and other materials with structure on a scale of tens- to hundreds-of-nanometers across. A heterogeneous distribution of carbon crystallinity and chemical composition has been inferred from other studies using standard micro-Raman techniques, but until now IDP structure had not been directly imaged. A group led by Postdoctoral Fellow Marc Fries conducted the first confocal Raman imaging of interplanetary dust particles in order to better characterize the organic matter they contain. Three particles were shown to have chemical variability, especially in terms of the degree of disorder in their organic matter, on a ~200-nm spatial scale. Unfortunately, the particle with the highest degree of heterogeneity was lost before isotopic measurements could be performed. Future work will attempt to correlate Raman spectra with D/H and ¹⁵N/¹⁴N ratios in IDPs, as well as with synchrotron XANES data.

   Co-Investigators Nittler and Alexander also used isotopic imaging to characterize the D/H spatial heterogeneity of fragments of the Tagish Lake (unique carbonaceous chondrite) and Al Rais (CR2) meteorites. They found that D/H is highly variable on a 1-2 µm scale in both meteorites, with D/H ratios in some C-rich hotspots approaching the highest values previously observed in IDPs. Transmission-electron-microscopy (TEM) analysis of several D-rich fragments by Co-I Stroud is underway.

   Using the focused ion beam workstation at the Naval Research Laboratory, Co-I Stroud has developed techniques for preparing ultra-thin sections of isotopically characterized interplanetary dust particles (Figure 1). These techniques will be applied to analyzing the structure and chemical composition of the IDPs showing large isotopic variations in D/H, in order to determine any association with particular organic carriers.

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   On the basis of stable isotope and solid-state nuclear magnetic resonance measurements by Co-Is Cody, Alexander, and Fogel, the D/H ratio of organic matter in carbonaceous chondrites correlates strongly with the extent of parent body processing. In particular, the insoluble macromolecular material in Tagish Lake appears much more hydrothermally processed than CR2 chondrites and apparently has lost much of its aliphatic component that was originally present. The lower bulk D/H of Tagish Lake compared to Al Rais supports this conclusion in general, but the presence of organic-rich hotspots with very high D/H ratios indicates that some very primitive organic matter has survived on small scales.

   Ion microprobe analysis of organic residues has shown that at least some of the heterogeneities seen in the analyses of the bulk meteorites survive the process of making the residues. This work also suggests that ion microprobe measurements may underestimate the D anomalies in the organic material by up to a factor of two.

   One of the aims of our work on meteorite organics is to provide standards for the development of microanalytical techniques that will be applied to IDPs and the Stardust mission samples. We have recently determined the bulk elemental and isotopic compositions of the insoluble organics from a number of primitive meteorites. We have also acquired high-resolution XANES spectra of insoluble organics at the Advanced Light Source (ALS) at the Lawrence Berkeley Laboratory. Transmission electron microscopy is generally the technique of choice for studying IDPs and, probably, will also be for studying Stardust samples. We have begun a quantitative comparison of the XANES spectra acquired at ALS with electron energy-loss spectra obtained by TEM (Figure 2). The features associated with C, N, and O are clearly resolved. The C (1s) near-edge fine structure is similar to amorphous carbon. We are currently analyzing several other CM chondrites to determine the degree of heterogeneity in composition and bonding character of C.

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2. The Fate of Carbon during Planetary Differentiation

   In the first year of funding for this subtask, Co-Is McCoy, Nittler, and Stroud continued their collaboration on the lodranite GRA 95209, in which carbon isotopic anomalies in a partially-differentiated meteorite were first discovered. Combining the petrographic observations of McCoy, the isotopic measurements of Nittler, and TEM observations of graphite morphology by Stroud, it is now clear that isotopic anomalies were preserved in graphite during melting of enveloping metal in GRA 95209, but once melt began to migrate and graphite agglomerated, these isotopic anomalies were destroyed.

   A major focus of this research effort is experimental melting of primitive achondrites to examine the temperature to which isotopic anomalies in graphite persist. To this end, Co-I McCoy and collaborators G. Benedix (research scientist, Washington University), T. Rushmer (faculty member, University of Vermont), R. Ford (master’s degree student, University of Vermont), and C. Corrigan (postdoctoral fellow, Smithsonian Institution) spent much of the past year developing the techniques for melting primitive achondrites at appropriate temperatures and fO₂.

   In the past year, the team also secured additional samples of GRA 95209 from the Meteorite Working Group, which will be used in the planned experiments.

3. The Martian Hydrosphere: Clues from Meteorites

   Co-I Vicenzi has continued his study of the microchemistry of preterrestrial alteration products in Martian meteorites. He is developing a new technique, using hyperspectral X-ray microanalysis, to investigate the possibility of a chemical linkage between complex low-temperature aqueous assemblages formed in the Martian subsurface and global surface dust. Elli Pauli (George Washington University), who has a pre-doctoral appointment at the Smithsonian Institution, is using a variety of microscale spectroscopic methods to characterize sulfate-bearing alteration in Martian meteorites. The disposition of Fe, S, Cl and other astrobiologically important elements, and the suitability of these once watery microenvironments to sustain microbial life, are clearly important areas of exploration with regard to possible extant life on Mars.

   With the addition in January 2004 of Detlef Rost as a postdoctoral fellow in the Smithsonian Institution group, work has initiated on measuring relative sensitivity factors for time-of-flight secondary ion mass spectrometry (ToF-SIMS) on a set of deep-sea volcanic glasses. These data are required to quantify the submicrometer-resolution, secondary ion images collected from Martian secondary mineral-filled veinlets. Data analysis is underway to evaluate the mechanism of formation of the low-temperature alteration materials, e.g., hydrothermal versus evaporitic precipitation.