Complex Organic Compounds
Polycyclic aromatic hydrocarbons (PAHs) were detected in the interior of ALH84001 at concentration in the parts per million range (McKay et al., 1996). PAHs are complex organic compounds with two or more fused aromatic rings (Fig. 1). On Earth, PAHs form as a result of the incomplete combustion of organic materials during industrial and other human activities, and also through diagenesis of organic compounds. McKay et al. (1996) only found a few different types of PAHs in the meteorite, in close association to the carbonate globules. This was taken as indicative of diagenetic alteration of microorganisms accumulated within the ALH84001. The controversy around this finding centered on whether these organic compounds formed on Mars, or represent terrestrial contamination from the Antarctic ice where ALH84001 was found. McKay et al. (1996) showed that the content of PAHs was minimum on the surface of the meteorite and increased towards its center, with a tendency to accumulate around the carbonate globules. Furthermore, ALH84001 is depleted in PAHs near its fusion crust, the edge of the meteorite that was melted during its passage through the Earth's atmosphere (Clemett et al., 1998). This suggests that the PAHs were already in the meteorite when it landed on Earth. On the other hand, positive Carbon-14 analyses indicates that a large portion of the organic carbon present in the meteorite is terrestrial contamination, despite a significant percentage
(~8%) has no Carbon-14 signature and is too old to be terrestrial. This carbon component could be an inorganic carbonate phase or a high molecular weight organic component, but it still remains uncharacterized (Jull et al., 1998; Becker et al., 1999). Steele et al. (2007) conducted spectroscopy and microscopy analyses of carbonate globules in ALH84001 and the Bockfjorden volcanic complex (BVC), Svalbard. The authors identified macromolecular carbon in association with the carbonate globules in both samples, which they linked to the uncharac-terized organic phase. Based on thermodynamic calculations, the authors postulated a non-biogenic origin associated to the primary development of the globules. Small amounts of amino acids, which are nearly identical to amino acids found in Antarctic ice, are also associated to the carbonate globules (Bada et al., 1998), indicating that the rock has in fact been contaminated to a certain degree with specific terrestrial organic compounds. Yet, other meteorites with no indigenous PAHs also found in Allan Hills, are clean of terrestrial PAHs contamination (Clemett et al., 1998), and the heterogeneous distribution of PAHs and their relatively large concentrations within the rock compared to the Allan Hills ice, are also hard to reconcile with contamination processes.
2.3. COEXISTENCE OF IRON-OXIDES, IRON-SULFIDES AND CARBONATES
ALH84001 also contains nanophase magnetite (Fe3O4) and pyrrhotite (Fe1-xS, x = 0 to 0.2) crystals in rims surrounding the carbonate globules (McKay et al., 1996). Although both types of minerals are known to form individually through both biotic and abiotic processes, McKay et al. argued that simple inorganic precipitation models could not explained their coexistence. Life, however, operates under disequilibrium conditions and often favors the co-precipitation of minerals that could not otherwise occur simultaneously. Their close association to the carbonate globules was taken as further evidence for the intervention of life in the formation of these minerals. On Earth, some bacteria are known to co-precipitate intracellular iron-oxide and iron-sulfide particles, a biologically controlled mineralization process that can take place under anaerobic conditions (Blakemore, 1982; Petersen et al., 1986; Bazylinski and Frankel, 2003). Yet, Anders (1996) quickly pointed out that some C-chondrites, a different type of meteorites, also have a similar suit of mineralogies, which were clearly formed under abiotic conditions. The same author proposed and alternative, non-biological processes that could explain these seemingly incompatible mineralogies.