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Published August 15, 2002 | public
Journal Article

Records of an ancient Martian magnetic field in ALH84001


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 ∼3 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 fields, possibly extending back to near the time of planetary formation.

Additional Information

© 2002 Elsevier Science B.V. Received 12 February 2002; received in revised form 16 April 2002; accepted 16 May 2002. We thank V. Courtillot, D.J. Stevenson, M. Purucker, M.T. Zuber, and L. Hedges for encouragement and thoughtful advice, D. Mann of High Mountain Petrographics for producing unheated thin sections, T. Kamino of Hitachi Japan for assistance with the FIB and FEG-TEM, K. Thomas- Keprta for providing data for Fig. 5, and T. Puig for valuable logistical help. Financial support for B.P.W. and J.L.K. was provided by the NASA Exobiology program and the NASA Astrobiology Institute. Financial support to H.V. was provided by National Science and Engineering Research Council of Canada (NSERC).[AC]

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