Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001

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μL²MS was used to analyze fresh fractured samples of ALH84001 for the presence of PAHs. The μL²MS instrument is capable of the simultaneous measurement of all PAHs present on a sample surface to a spatial resolution of 40 μm, and detection limits are in the sub-attomole (>10⁷ molecules) range (1 amol = 10⁻¹⁸ mol).

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PAH concentration is estimated from comparison of the averaged spectra of interior fracture surfaces of ALH84001 with known terrestrial standards and the Murchison (CM2) meteorite matrix. In the case of Murchison (CM2), the total concentration of PAHs has been independently measured to be in the range of 18 to 28 parts per million;[;

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At a single laser ionization wavelength, μL²MS is unable to distinguish between structural isomers; however, because different isomers have different photoionization cross sections, mass assignments are based on the most probable isomer. In the case of masses 178 and 202, the possible isomer combinations are phenanthrene or anthracene and pyrene or fluoranthene. At the photoionization wavelength used in this study (266 nm), the μL²MS instrument is ∼19 times as sensitive to phenanthrene as to anthracene and ∼23 times as sensitive to pyrene as to fluoranthene [R. Zenobi and R. N. Zare, in Advances in Multiphoton Processes and Spectroscopy, S. H. Lin, Ed. (World Scientific, Singapore, 1991), vol. 7, pp. 1-167]. Hence, masses 178 and 202 are assigned to phenanthrene and pyrene. In the case of higher masses, more structural isomers exist, and assignments are based on those PAHs known to have high cross sections from comparisons with standards.

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Sources of laboratory contamination fall into three categories: sample handling, laboratory air, and virtual leaks (that is, sources of PAHs inside the μL²MS vacuum chamber). Possible contamination during sample preparation was minimized by performing nearly all sample preparation at the NASA-JSC meteorite curation facility. In cases where subsequent sample handling was required at Stanford, all manipulation was performed in less than 15 minutes, using only stainless steel tools previously rinsed and ultrasonicated in methanol and acetone. Dust-free gloves were worn at all times and work was performed on a clean aluminum foil surface. To quantify airborne contamination from exposure to laboratory air, two clean quartz discs were exposed to ambient laboratory environments both at NASA-JSC and Stanford. Each disc received an exposure typical of that experienced subsequently by samples of ALH84001 during sample preparation. No PAHs were observed on either quartz disk at or above detection limits. Because contamination can depend on the physical characteristics of the individual sample (for example, a porous material will likely give a larger contamination signal than a nonporous one), additional contamination studies have been previously conducted at Stanford [see (29)]. Briefly, samples of the meteoritic acid residues of Barwell (L6) and Bishunpur (L3.1) were exposed to laboratory air for 1 and 4 days, respectively. Barwell (L6) is known to contain no indigenous PAHs and none were observed on the exposed sample. Bishunpur (L3.1), in contrast, contains a rich suite of PAHs, but no discernible differences in signal intensities were observed between exposed and unexposed samples. To test for contamination from virtual leaks, the μL²MS instrument was periodically checked using samples of Murchison (CM2) meteorite matrix whose PAH distribution has been previously well characterized. No variations in either signal intensity or distribution of PAHs were observed for μL²MS instrument exposure times in excess of 3 days. No sample of ALH84001 was in the instrument for longer than 6 hours. The μL²MS vacuum chamber is pumped by an oil-free system: two turbomolecular pumps and a liquid nitrogen-cooled cyropump.

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We handpicked small chips in a clean bench from our curatorial allocation of ALH84001. Most chips were from a region near the central part of the meteorite and away from the fusion crust, although for comparison we also looked at chips containing some fusion crust. For high-resolution work we used an Au-Pd coating estimated to be ∼2 nm thick in most cases. On some samples we used a thin (<1 nm) coating, about 10 s with our sputter coater; these samples usually showed charging effects and could not be used for highest resolution imaging. We more typically used 20 to 30 s. For backscatter and chemical mapping we used a carbon coat of about 5 to 10 nm thick. We monitored the possible artifacts from the coating using other reference samples or by looking at fresh mineral surfaces on ALH84001. We could also estimate relative coating thicknesses by the size of the Au and Pd peaks in the energy dispersive x-ray spectrum. For our heaviest coating, a slight crazing texture from the coating is barely visible at a magnification of 50,000× on the cleanest fresh grain surfaces. The complex textures shown in most of the SEM photographs are not artifacts of the coating process but are the real texture of the sample. We used a JEOL 35 CF and a Philips SEM with a field emission gun (FEG) at the JSC. The JEOL and Philips SEMs are equipped, respectively, with a PGT and Link EDS system. We achieved about 2.0 nm resolution at 30kV. Some images were also taken at lower kV, ranging from 1 to 10 kV. In most cases, the chips were coated after handpicking with no further treatment. For comparison, several chips were ultrasonically cleaned for a few seconds in liquid Freon before coating to remove adhering dust. Several samples were examined uncoated at low kV. We carbon-coated and examined all of the surfaces analyzed for PAHs only after the PAH analyses had been completed.

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We thank L. P. Keller, V. Yang, C. Le, R. A. Socki, M. F. McKay, M. S. Gibson, and S. R. Keprta for assistance; and R. Score and M. Lindstrom for their help in sample selection and preparation. The guidance of J. W. Schopf at an early stage of this study is appreciated. We also thank G. Horuchi, L. Hulse, L. L. Darrow, D. Pierson, and personnel from Building 37. The work was supported in part by NASA's Planetary Materials Program (D.S.M. and R.N.Z.) and the Exobiology Program (D.S.M.). Additional support from NASA-JSC Center Director's Discretionary Program (E.K.G.) is recognized. C.S.R. and H.V. acknowledge support from the National Research Council. X.D.F.C. acknowledges support from the Swiss National Science Foundation.