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Records of an ancient Martian magnetic ¢eld in ALH84001



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Earth and Planetary Science Letters 201 2002 449 463 www elsevier com locate epsl Records of an ancient Martian magnetic eld in ALH84001 Benjamin P Weiss a Hojatollah Vali b Franz J Baudenbacher c Joseph L Kirschvink a Sarah T Stewart a David L Shuster a a b Division of Geological and Planetary Sciences 170 25 California Institute of Technology Pasadena CA 91125 USA Electron Microscopy Centre Department of Anatomy and Cell Biology and Department of Earth and Planetary Sciences McGill University 3640 University Street Montreal QC Canada H3A 2B2 c Department of Physics and Astronomy Vanderbilt University 6301 Stevenson Center Nashville TN 37235 USA Received 12 February 2002 received in revised form 16 April 2002 accepted 16 May 2002 Abstract Although Mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the Mars Global Surveyor above surfaces formed V3 or more Ga We present magnetic and textural studies of Martian meteorite ALH84001 demonstrating that 4 Ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization Because 40 Ar 39 Ar thermochronology demonstrates that most ALH84001 carbonates have probably been well below the Curie point of magnetite since near the time of their formation Weiss et al Earth Planet Sci Lett 2002 this issue their magnetization originated at 3 9 4 1 Ga on Mars This magnetization is at least 500 million years Myr older than that known in any other planetary rock and its strong intensity suggests that Mars had generated a geodynamo and global magnetic field within 450 650 Myr of its formation The intensity of this field was roughly within an order of magnitude of that at the surface of the present day Earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001 and possibly for the production of the strong crustal anomalies Chromite in ALH84001 may retain even older records of Martian magnetic



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