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In the initial report by McKay et al. on ALH84001 several lines of evidence were given to suggest the
presence of biogenic activity on Mars: (i) the presence of the carbonate globules within fractures and pores
of a 4.5 Gy old igneous rock after the primary crystallization event; (ii) formation age of the carbonates is
younger than the age of the host igneous rock but older than the carbonate's age; (iii) SEM and TEM
images of carbonate globules and assorted features resemble terrestrial biogenic structures and fossilized
nanobacteria; (iv) the occurrence of magnetite and iron sulfide particles could have resulted from oxidation
and reduction reactions known to be important in terrestrial microbial systems; and (v) presence ofPAHs
associated carbonate globules indicating potential indigenous organic molecules. As noted in McKay et
al., none of these observations is in itself conclusive proof for the existence of past life on Mars. Although
there are alternative explanations for each ofthese phenomena taken individually, when they are considered
collectively, particularly in view of their close spatial association, it was concluded that they may represent
the first direct evidence for primitive life on early Mars. Since the initial report, additional supporting
evidence and contradictory evidence, including alternative inorganic explanations have been
presented. The time and temperatures of carbonate formation in ALH84001 continues to be hotly debated.
Knott et al. suggests the carbonates were formed at 3.6 Gy, whereas Wadhwa and Lugmair noted the
formation may be as late as 1.3 Gy. Turner et al. argue that the 3.6 Gy date is not well defined and
additional studies are needed to define the carbonate formation date.
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