• Research in anthropology including biological and physical, as well as cultural anthropology.
  • Science Sessions: The PNAS Podcast Program

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

  1. Michael Winklhofer‖
  1. *Department of Biological Science, Florida State University, Tallahassee, FL 32306-1100; †Space Science Division 245-3, National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035; §Servei de Microscòpia Electrònica, Universitat de Lleida, 25196 Lleida, Spain; Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain; and ‖Institut für Geophysik, Universität München, D-80333 Munich, Germany

  1. Edited by Rita R. Colwell, National Science Foundation, Arlington, VA, and approved January 10, 2001 (received for review October 27, 2000)

Abstract

The presence of magnetite crystal chains, considered missing evidence for the biological origin of magnetite in ALH84001 [Thomas-Keprta, K. L., Bazylinski, 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. Cosmochim. 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 of elongated crystals along the chain axis, flexibility of chains, and a halo that is a possible remnant of a membrane around chains), observed or inferred to be present in magnetotactic bacteria but incompatible with a nonbiological origin, are shown to be present. Although it is unlikely that magnetotactic bacteria were ever alive in ALH84001, decomposed remains of such organisms could have been deposited in cracks in the rock while it was still on the surface on Mars.

Footnotes

    • ↵‡ To whom reprint requests should be sent at the † address, the present address: E-mail: friedmann{at}bio.fsu.edu or friedmann{at}gal.arc.nasa.gov.

    • This paper was submitted directly (Track II) to the PNAS office.

  • Abbreviations

    SEM-BSE,
    scanning electron microscopy in backscattered electron mode;
    SEM-SE,
    electron microscopy in secondary electron mode;
    LEDS,
    low-electron-density substance
    • Received October 27, 2000.

    Online Impact