Mathematics – Logic
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..gp41a07k&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #GP41A-07
Mathematics
Logic
1540 Rock And Mineral Magnetism, 6225 Mars
Scientific paper
The controversy surrounding the presence of ancient bacterial life on Mars is at present focused on the existence of bacterial magnetofossils in the ALH84001 meteorite. Thomas-Keprta et al. (GCA 64:4049-4081, 2000) and Friedmann et al. (PNAS 98:2176-2181, 2001) have used six fingerprints of natural selection on the morphology and organization of bacterial magnetites as a guide for evaluating a biological origin of the fine-grained magnetite in the rims of the ALH84001 carbonate blebs. These six factors include unusual crystal morphology, elongation along the magnetically-easy \{111\} axis, high chemical purity, defect-free crystal lattice, a narrow size and shape distribution corresponding to the single-domain stability field, and the alignment of similarly-sized and shaped particles in linear chains. All six of these features can be viewed as maximizing in the net magnetic moment and magnetic stability of the bacterial cells, increasing the biological efficiency of magnetotaxis. All six of these are present in a well-defined sub-population of the Martian magnetites, whereas the closest inorganic analog found to date meets at most 3. Thomas-Keprta et al. (PNAS 98: 2164-2169, 2001) report yet another distinctive feature common to both Martian magnetites and bacterial strain MV-1, which is a hexa-octahedral morphology truncated at the long ends by a series of [100] and [111] faces. The net effect of these truncations is to remove all right-angle edges from the ends of these elongate magnetosomes, rounding them out. This general feature is in fact very common among the magnetosomes present in populations of living magnetotactic bacteria, and leads to the question of why this type of rounding would be supported by natural selection. I suspect that the answer might lie in the detailed magnetization pattern of elongate magnetic particiles. Recent numerical calculations of the magnetization patterns of rectangular magnetite crystals reveals the tendency for them to break down into magnetic 'flower' structures as they approach the single-domain to two-domain transition (Fabian et al., GJI 124: 89-104; Newell & Merrill JGR 105: 19377-19391), with the greatest warping around the sharp corners. In this situation, the bacteria would gain a higher net magnetic moment per Fe atom by building a new crystal along the chain rather than fleshing out the corners in the existing particles. If confirmed by suitable numerical calculations on elongate hexagonal magnetite prisms, this morphology could provide a seventh criterion for the Identification of bacterial magnetofossils in the ALH84001 meteorite.
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