The Viking biology experiments indicated that extant martian life is unlikely on the planet's surface, but they did not address the possibility of ancient martian life. Interest in the possibility of extinct martian life resurged in 1996 following the announcement that chemical evidence within the ALH84001 martian meteorite was consistent with ancient bacteria (McKay et al., 1996). ALH84001 (Figure 8.3) is the one martian meteorite with an orthopyroxenite composition and a 4.5 Ga crystallization age (Mittlefehldt, 1994). It contains large globules of carbonates (Figure 4.11) (Romanek et al., 1994) which formed —3.9 Ga ago (Borg et al., 1999). Within
the carbonates, McKay et al. (1996) found polycyclic aromatic hydrocarbons (PAHs), magnetite, and iron sulfides which they argued were formed by martian bacteria. They also noted small features (Figure 8.4) which they suggested were fossilized nanobacteria.
The ALH84001 evidence has been the topic of many heated debates among the scientific community since it was announced. At present, most scientists believe that the PAHs and iron sulfides are either non-biologic in origin or result from terrestrial contamination (Scott, 1999; Zolotov and Shock, 2000). The "nanofossils" have been determined to be either an artifact from the preparation of the sample for scanning electron microscope analysis or a terrestrial weathering effect (Sears and Kral, 1998). The strongest evidence remaining for a biogenic origin of the ALH84001 mineralogies is the magnetite, whose shape is consistent with an origin by bacteria (Thomas-Keprta et al., 2000). However, most scientists believe that the evidence in ALH84001 is too weak to support the assertion that it contains evidence of ancient martian biologic activity.
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