Evidence for possible past life in a Martian meteorite: a current assessment

摘要

In the initial report by McKay et al.l 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 of PAHs associated carbonate globules indicating potential indigenous organic molecules. As noted in McKay et al.l, none of these observations is in itself conclusive proof for the existence of past life on Mars. Although there are alternative explanations for each of these 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^-S and contradictory evidence9-14; including alternative inorganic explanations have been presented. The time and temperatures of carbonate formation in ALH84001 continues to be hotly debated. Knott et al.l5 suggests the carbonates were formed at 3.6 Gy, whereas Wadhwa and Lugmair1^ noted the formation may be as late as 1.3 Gy. Turner et aU? argue that the 3.6 Gy date is not well defined and additional studies are needed to define the carbonate formation date.