Polycyclic Aromatic Hydrocarbons in the Martian (SNC) Meteorite ALH 84001: Hydrocarbons from Mars, Terrestrial Contaminants, or Both?

Details Polycyclic Aromatic Hydrocarbons in the Martian (SNC) Meteorite ALH 84001: Hydrocarbons from Mars, Terrestrial Contaminants, or Both? Polycyclic Aromatic Hydrocarbons in the Martian (SNC) Meteorite ALH 84001: Hydrocarbons from Mars, Terrestrial Contaminants, or Both?

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.587t&link_type=abstract

Meteoritics, vol. 30, no. 5, page 587

Mathematics

Logic

6

Meteorites, Allan Hills 84001, Altross, Elephant Moraine 79001, Lewis Cliff 85320, Nakhla, Snc, Zagami, Organic Compounds, Polycyclic Aromatic Hydrocarbons, Scanning Electron Microscope

Scientific paper

Previous work has shown that pre-terrestrial polycyclic aromatic hydrocarbons (PAHs) exist in interplanetary dust particles (IDPs) and certain meteorites [1-3]. We previously reported the first observation of PAHs in the newest member of the SNC group, Allan Hills 84001 [4] and determined that particular types of organic compounds are indigenous to ALH 84001 because they are associated with certain mineralogical features [4]. We also analyzed two diogenites from Antarctica: one showed no evidence for aromatic hydrocarbons while the other contained PAHs with the same major peaks as those in ALH 84001[4]. PAHs in the diogenite meteorite are not associated with mineral features on the analyzed surface and the most abundant PAHs in the diogenite are lower by a factor of 3 than those in ALH 84001. Furthermore, ALH 84001 contains a number of minor PAHs not found in the diogenite or typical terrestrial soils [4]. In this study we are analyzing a more complete group of Antarctic and non-Antarctic meteorites, including SNCs, to determine: (1) PAHs abundance and diversity in Antarctic meteorites and (2) the contribution of PAHs in SNCs from martian and, possibly, terrestrial sources. ALH 84001 is an unusual orthopyroxenite which contains abundant carbonate spheroids which are ~100-200 micrometers in diameter and range in composition from magnesite to ferroan magnesite [5-7]. These spheroids are not the result of terrestrial contamination: oxygen isotopic compositions indicate that the carbonates probably precipitated from a low-temperature fluid within the martian crust [5] and carbon isotopic abundances are consistent with martian atmospheric CO2 as the carbon source [5]. PAHs may coexist with other low-temperature carbon-bearing phases in a subsurface martian environment. Samples: We are analyzing freshly-fractured meteorite samples, or chips, which have been extracted from the internal regions of the following meteorites: ALH 84001 (crush and uncrush zones), EETA79001 (Antarctic SNC), four diogenites and two eucrites from Antarctica, and three non-Antarctic meteorites, Altrouss (diogenite), Nakhla (SNC), and Zagami (SNC). Some chips, removed from exterior meteorite surfaces, will also be analyzed: fusion crust from ALH 84001 and Antarctic evaporite deposits on LEW 85320 (H5 chondrite). Methods: The samples are processed in the meteorite clean lab at NASA/JSC to minimize laboratory contamination. Previous work has shown that the contribution of contaminant PAHs from the laboratory handling under these conditions is insignificant [4]. After initial processing, the chips are analyzed for the presence of PAHs using the Stanford microprobe two-step laser mass spectrometer (micrometers L(^2)MS) [1,3,4]. This instrument is capable of selectively detecting PAHs at the sub-attomole (<10^-18 mole) level with a spatial resolution of 40 micrometers. Contour maps of all major PAHs on the chip surface at this resolution are obtained using the micrometers L^(2)MS instrument. The chip surface is subsequently analyzed using a scanning electron microscope and energy dispersive spectroscopy to determine if the spatial concentration of PAHs corresponds to textural or mineralogical features on the sample surface. A more complete discussion of the technique is found in [4]. Summary: Close comparison of PAHs concentration maps and mineralogical features has revealed correlations between PAHs "hot spots" and carbonate and glass phases in the interior of ALH 84001. These correlations, along with isotopic data indicating that the carbonates were formed on Mars, are consistent with the supposition that at least some of the PAHs may be indigenous to Mars. References: [1] Clemett S. J. et al. (1993) Science, 262, 721. [2] Thomas K. L. et al. (1994) LPS XXV, 1391. [3] Clemett S. J. et al. (1992) LPS XXIII, 233. [4] Thomas K. L. et al. (1995) LPS XXVI, 1409. [5] Romanek C. S. et al. (1994) Nature, 372, 655. [6] Romanek C. S. et al. (1995) LPS XXVI, 1183.

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