Searching for traces of life associated with carbonates in martian meteorites

Martian meteorites provide an obvious starting point for the search for evidence of life on Mars. Peculiar structures shown by electron microscopy of fragments from the Antarctic meteorite ALH84001 have been reported to demonstrate shapes and a size distribution similar to those of cultured terrestrial nanobacteria. However, the association of these putative fossil forms with bacteria is only morphological, and no traces of undisputed biological organic matter have yet been demonstrated in close association with these structures. Similar, and larger, apparent microbial fossils have been found within samples of the Martian basaltic shergottite meteorite Los Angeles 001 (LA 001). Here they are associated with calcium carbonate deposition, known from isotopic studies to be terrestrial in origin, and also with silica-bearing globular encrustations, reminiscent of biofilm. This association is typical of bacterially-mediated mineral precipitation, which may occur directly on bacterial cells or on their surroundings. We have used analytical electron microscopy to locate and document possible bacterial shapes in LA 001, prior to Focussed Ion Beam cross-sectioning and Secondary Ion Mass Spectrometry of the internal sections revealed. We intend to compare the morphology and composition of internal structures to those of bacteria preserved in terrestrial carbonate stromatolites, siliceous sinters and abiotic precipitates, in order to determine whether major element compositional variation, isotopic and trace element partitioning can be used to reliably fingerprint bacterial activity as responsible for the meteoritic structures.