Chains of magnetite crystals in the meteorite ALH84001: Evidence of biological origin

The presence of magnetite crystal chains, considered missing evidence for t he biological origin of magnetite in ALH84001 [Thomas-Keprta, K. L., Bazyli nski, D. A., Kirschvink, J. L., Clemett, S. J., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr., & Romanek, C. S. (2000) Geochim. Cosmoch im. Acta 64, 4049-4081], is demonstrated by high-power stereo backscattered scanning electron microscopy. Five characteristics of such chains (uniform crystal size and shape within chains, gaps between crystals, orientation o f elongated crystals along the chain axis, flexibility of chains, and a hal o that is a possible remnant of a membrane around chains), observed or infe rred to be present in magnetotactic bacteria but incompatible with a nonbio logical origin, are shown to be present. Although it is unlikely that magne totactic bacteria were ever alive in ALH84001, decomposed remains of such o rganisms could have been deposited in cracks in the rock while it was still on the surface on Mars.