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

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Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian ME RESEARCHGARTICLE am dicates that they are indigenous to the earch for Past Life on Mars: meteorite and were not formed during its 13,000-yearresidence in the Antarctic en- Possible Relic Biogenic Activity vironment (13). The 813C values of the carbonate in ALH84001 range up to 42 per mil for the in Martian Meteorite ALH84001 large carbonate spheroids (12) and are higher than values for carbonatesin other David S. McKay,Everett K. Gibson Jr., SNC meteorites.The sourceof the carbon Kathie L. Thomas-Keprta,Hojatollah Vali, is the martian atmospheric C02, which has ChristopherS. Romanek, Simon J. Clemett, been recycledthrough water into the car- bonate (12). The carbonisotopic composi- Xavier D. F. Chillier, Claude R. Maechling, Richard N. Zare tions of ALH84001 are similar to those measuredin CM2 carbonaceouschondrites Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic (14). Consequently, the carbonates in aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate ALH84001 and the CM2 meteorites are globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. believed to have been formedby aqueous High-resolution scanning and transmission electron microscopy study of surface tex- processeson parent bodies. The b83C in tures and internal structures of selected carbonate globules show that the globules martianmeteorite carbonatesranges from contain fine-grained, secondary phases of single-domain magnetite and Fe-sulfides. The -17 to +42 per mil (12, 15, 16). This carbonate globules are similar in texture and size to some terrestrial bacterially induced range of 13Cvalues exceeds the range of 13C carbonate precipitates. Although inorganic formation is possible, formation of the glob- generatedby most terrestrialinorganic pro- ules by biogenic processes could explain many of the observed features, including the cesses(17, 18). Alternatively,biogenic pro- PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phas- cessesare known to producewide rangesin es and textures could thus be fossil remains of a past martian biota. 813C on Earth(19, 20). ALH84001 arrived on Earth 13,000 yearsago (15, 21) and appearsto be essen- tially free of terrestrialweathering (8). A long-standingdebate over the possibility biomarkerson the basis of what we know ALH84001 does not have the carbon iso- of present-daylife on Marswas addressed by about life on Earth.Therefore, if there is a topic compositionstypically associated with the Viking landerexperiments in 1976. Al- martianbiomarker, we may not be able to weatheredmeteorites (12, 15), and detailed though the resultswere generallyinterpret- recognizeit, unlessit is similarto an earthly mineralogical studies (8) show that ed to be negative for life in the tested biomarker.Additionally, no informationis ALH84001 has not been significantlyaf- surfacesoils, the possibilityof life at other available on the geologic context of this fected by terrestrialweathering processes. locations on Mars could not be ruled out rock on Mars. ALH84001 is somewhat friable and (1). The Viking lander'smass spectrometry ALH84001 is an igneousorthopyroxen- breaks relatively easily along preexisting experimentsfailed to confirmthe existence ite consistingof coarse-grainedorthopyrox- fractures.It is these fracturesurfaces that of organicsfor the martiansurface samples ene [(Mg,Fe)SiO3]and minor maskelynite displaythe carbonateglobules. We analyzed analyzed.Furthermore, the Viking results (NaAISi308), olivine [(Mg,Fe)SiO4], chro- freshly broken fracturesurfaces on small contained no informationon possiblefos- mite (FeCr204),pyrite (FeS2), and apatite chips of ALH84001for polycyclicaromatic sils. Another source of informationabout [Ca3(PO4)2j (4-6). It crystallized 4.5 bil- hydrocarbons(PAHs) using a microprobe possible ancient martian life is the Sher- lion yearsago (Ga) (6). It recordsat least two-step laser mass spectrometer (piL2MS) gotty-Nakhla-Chassigny(SNC) class of two shock events separatedby a period of (22, 23). meteorites,which appearto have come to annealing.The age of the first shock event Polycyclic aromatic hydrocarbons. Spa- the Earthby impactevents on Mars(2, 3). has been estimatedto be 4.0 Ga (7). Unlike tial distribution maps of individual PAHs We have examined ALH84001, collected the other SNC meteorites,which contain on interior fracture surfaces of ALH84001 in Antarcticaand recently recognizedas a only trace carbonate phases, ALH84001 demonstrate that both total PAH abun- meteoritefrom Mars (4). Ourobjective was containssecondary carbonate minerals that dance and the relative intensities of indi- to look for signs of past (fossil) life within formglobules from 1 to -250 ,umacross (4, vidual species have a heterogeneous distri- the pore spaceor secondaryminerals of this 6, 8, 9). These carbonate globules have bution at the 50-p,m scale. This distribution martianmeteorite. Our task is difficultbe- been estimatedto have formed3.6 Ga (10). appears to be consistent with partial geo- cause we only have a small piece of rock Petrographicand electron microprobere- chromatographic mobilization of the PAHs fromMars and we are searchingfor martian sults (4, 11) indicate that the carbonates (24). The average PAH concentration in formed at relatively high temperatures the interior fracture surfaces is estimated to D. S. McKay, Mail Code SN, NASA Lyndon B. Johnson (-700?C); however,the stableoxygen iso- be in excess of 1 part per million (25). The Space Center (JSC), Houston, TX 77058, USA. tope data indicate that the carbonates PAHs were found in highest concentration E. K. Gibson Jr., Mail Code SN4, NASA-JSC, Houston, 00 in rich in carbonates. TX 77058, USA. formed between and 80?C (12). The regions K. L. Thomas-Keprta, Lockheed Martin,Mail Code C23, carbonateglobules are found along fractures From averaged spectra we identified two 2400 NASA Road 1, Houston, TX 77058, USA. and in pore spaces.Some of the carbonate groupings of PAHs by mass (Fig. IA). A H. Vali, Department of Earth and Planetary Sciences, globules were shock-faulted(4, 5). This middle-mass envelope of 178 to 276 atomic McGillUniversity, 3450 UniversitySt., Montreal, Quebec, H3A 2A7 Canada. shock event occurredon Marsor in space, mass units (amu) dominates and is com- C. S. Romanek, Savannah River Ecology Laboratory, and thus rules out a terrestrialorigin for posed mostly of simple 3- to 6-ring PAH Drawer E, Universityof Georgia, Aiken, SC 29802, USA. the globules (3, 8, 13). The isotopic com- skeletons with alkylated homologs account- S. J. Clemett, X. D. F. Chillier,C. R. Maechling, R. N. Zare, Department of Chemistry, Stanford University,Stanford, position of the carbon and oxygen associ- ing for less than 10% of the total integrated CA 94305-5080, USA. ated with the carbonate globules also in- signal intensity. Principal peaks at 178, 202, 924 SCIENCE * VOL. 273 * 16 AUGUST 1996 228, 252, and 278 amu are assigned to by gL2MS did not show the presence of (31). We also conducted experimentsin phenanthrene (CI4Ho), pyrene (C16H1o), terrestrial PAHs within detection limits, which chips of ALH84001were cultured in chrysene (C18H12), perylene or benzopy- which suggests an upper limit for terrestrial nutrientmedium under aerobic and anero- rene (C20H12), and anthanthracene contamination of ALH84001 of 1%. Mea- bic conditions;we found the chips to be (C22H12) (26). A second weak, diffuse surements of four interior fragments of two sterile. high-mass envelope extends from about 300 Antarctic ordinary chondrites (ALH83013 With the use of the piL2MStechnique, to beyond 450 amu. The peak density is and ALH83101) of petrologic classes H6 PAHs have been found in a wide rangeof high and shows a periodicity at 14 and 2 and L6 showed no evidence of indigenous extraterrestialmaterials, including carbona- amu. This distribution implies that there is PAHs. These represent equivalent desorp- ceous and ordinarychondrites (29), inter- a complex mixture of PAHs whose parent tion matrix blanks; previous studies have planetarydust particles(23, 32), and inter- skeletons have alkylated side chains with shown that no indigenous organic material stellargraphite grains (33). Eachmaterial is varying degrees of dehydrogenation; specif- is present in meteorites of petrologic class characterizedby differing PAH distribu- ic assignments are ambiguous. six (29). tions reflectingthe differentenvironments Contamination checks and control ex- Studies of exterior fragments of in which the PAHs formedand their sub- periments indicate that the observed organ- ALH84001 with intact fusion crust show that sequentevolution (for example,as a result ic material is indigenous to ALH84001. no PAHs are present within the fusion crust of aqueousalteration and thermalmetamor- The accumulation of PAHs on the Green- or a zone extending into the interior of the phism). Comparisonof the mass distribu- land ice sheet over the past 400 years has meteorite to a depth of -500 gm (Fig. 1, B tion of PAHs observedin ALH84001 with been studied in ice cores (27). The total through E). The PAH signal increases with that of PAHs in other extraterrestrialma- concentration of PAHs in the cores varies increasing depth, leveling off at -1200 gm terials indicates that the closest match is from 10 parts per trillion for preindustrial within the interior, well away from the fusion with the CM2 carbonaceouschondrites times to 1 part per billion for recent snow crust. This concentration profile is consistent
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