Published in Lunar and Planetary Science XXVI, pp. 1409-1410, LPI, Houston.
Previous work has shown that preterrestrial organic compounds exist in interplanetary dust particles (IDPs) and certain meteorites [1-3]. Polycyclic aromatic hydrocarbons (PAHs) have been found in several IDPs [1,2] and numerous ordinary and carbonaceous chondrites . We document, for the first time, the occurrence of PAHs in the newest member of the SNC meteorite clan, Alan Hills 84001.
This martian meteorite is an unusual orthopyroxenite which contains ~0.5  - 0.1 vol% [4,5] carbonate spheroids, ~100-200 µm in diameter. These spheroids, which range in composition from magnesite to ferroan magnesite [5-7], are not the result of terrestrial contamination; rather carbon isotopic abundances are consistent with martian atmospheric CO2 as the carbon source . Oxygen isotopic composition of carbonates indicates that they probably precipitated from a low-temperature fluid within the martian crust . We selected ALH 84001 to search for organic signatures because it is possible that PAHs may coexist with other low-temperature carbon-bearing phases in a subsurface martian environment. Indeed, a recent report suggests that Mars may have been a wet and warm planet [5,6].
Methods: We fractured 3 fragments of ALH 84001 which range in size from ~2-8 mm in length. The fragments were processed in the meteorite clean lab at NASA Johnson Space Center to minimize laboratory contamination. These freshly-fractured samples were sent to Stanford where their surfaces were analyzed for PAHs using a microprobe two-step laser mass spectrometer (µL2MS). Our µL2MS has a spatial resolution of 40 µm and selectively ionizes aromatic species from the plume of desorbed organic molecules. This instrument has been described previously ; however, the detection sensitivity was recently improved by replacing the 1-inch microchannel plate detector (MCP) with a 2-inch MCP. For PAH mapping studies of a freshly-cut surface, a single-shot mass spectrum was taken on a sample spot followed by translation of the sample by 50 µm. After the sample was analyzed, the infrared laser power was increased and position indicators were burned into the surface producing laser-fused beads of the SNC meteorite matrix. The samples were then resumed to NASA/JSC for analysis with a turbo-pumped JEOL 35 CF scanning electron microscope (SEM) equipped with a PGT energy dispersive spectrometer (EDS). SEM micrographs and EDS analyses were taken of the sample to determine if the spatial concentration of PAHs corresponds to textural or mineralogical features on the surface. To date, we have analyzed one surface using both techniques.
Results: We have determined that certain types of organic compounds are indigenous to ALH 84001 because their presence correlates with mineralogical features in the chips. A spatial map of the PAHs from a 1 mm × 0.5 mm region of one surface was prepared by taking mass spectra over a 20 × 12 array of spots. There was great variability in the signal strengths from the 240 spots. To illustrate this point, we prepared a contour plot (Fig l) of the most intense PAHs. For each spot, the signal intensity of phrenanthrene (C14H10 ; 178 amu), pyrene (Cl6H10; 202 amu), chrysene (C18H12; 228 amu) and benzopyrene (C20H12; 252 amu) were summed and recorded. The data for the 20 × 12 array were spline fit to make the contours less jagged. In Fig l there are three regions of the sample that are richer in PAHs than others. These are referred to as hot spots. SEM observations show that a correlation exists between one surface feature and a region with the highest PAHs concentration (Figs l, 2). The surface feature is a magnesite core, ~30 µm in diameter, located within a carbonate spheroid. The diameter of the PAHs hot spot and the spheroid core are comparable suggesting that the core may be the hydrocarbon carrier. Two other regions of slightly enhanced concentrations of PAHs did not correlate with surface features. Not all spheroid cores in the analyzed chip contain PAHs excesses; in fact, some cores do not have PAHs concentrations above background.
Discussion: PAHs have been observed in ALH 84001 and have been correlated with specific surface features. The intensities for these signatures are much weaker than those observed in carbonaceous meteorites, however PAHs were not concentrated in acid residues as with some meteorites . Although there may not be a correlation between bulk carbon and the presence of PAHs, the total carbon in ALH 84001 (~600 ppm ) is low compared to carbonaceous chondrites. PAHs have been previously measured in IDPs with bulk carbon ranging from 3-14 wt% [1,2] and in chondrites  with bulk carbon abundances ranging from ~<0.l-4.0 wt% [e.g., 8]. Hydrocarbons were reported from another martian meteorite, EETA 79001  and have been observed in bulk samples of ALH 84001 . However, the presence of organic compounds in ALH 84001 has been interpreted as possible terrestrial contamination . A portion of the bulk carbon in ALH 84001, which has been reported to combust at temperatures up to 450°C , may be represented by the PAHs component in our study. The carbonates in this meteorite may have been formed from relatively low temperature circulating fluids [5,6]. If so, these fluids may also have contained dissolved hydrocarbons.
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