McKay et al. (1996) found fields of aligned bacteria-shaped objects, and inferred that they were fossil bacteria. However, Bradley et al. (1997) suggested that the bacteria-shaped objects were magnetite crystals that had grown in alignment on the host carbonate mineral. Here, the authors show that at least one grain of magnetite is in good crystallographic alignment with its host carbonate mineral; this relationship is called epitaxy. This alignment seems unlikely if the magnetites were formed by bacteria. But the alignment is expected if, as the authors suggest, the magnetite grew at high temperature from a vapor. "The widely published images of aligned nanofossils in ALH 84001 are probably magnetite whiskers with parallel orientations resulting from epitaxial growth on carbonate substrates."
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The claim that ALH 84001's carbonate globules contains traces of ancient martian life (McKay et al., 1996) requires that the globules formed at a temperature comfortable for life, but recent evidence suggests that some of the carbonates formed at very high temperatures (Scott et al., 1997).
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The furor about possible traces of life in ALH 84001 has highlighted some important issues in the search for life on Mars. First, proof of biological activity must be based on many lines of evidence ("multiple biomarkers"). Second, experimental controls are crucial, as non-biological chemicals and structures can easily be misinterpreted as biological. And third, the mineral and chemical thermometers now available cannot distinguish materials formed at biologically reasonable temperatures (< ~ 120°C) from materials formed at higher temperatures unsuitable for life. New thermometric schemes need to be developed.