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Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars

Abstract

Magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters Hellas and Argyre than over surrounding regions1. The reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact2,3, in the absence of a significant ambient magnetic field. Lower than average magnetic field intensities are also observed above the Vredefort meteorite crater in South Africa, yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on Earth. We find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented, with vector directions changing over centimetre length scales. Moreover, the magnetite grains contributing to the magnetic remanence crystallized during impact, which directly relates the randomization and intensification to the impact event. The strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater. Seen from high altitudes, as for martian craters, the magnetic field appears much lower than that of neighbouring terranes, implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet's internally generated magnetic field at the time of impact.

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Figure 1
Figure 2: Representative demagnetization diagrams of Vredefort rocks.
Figure 3: Equal area stereonet plots of the magnetic measurements.
Figure 4: Comparison of the angular distance between minimum anisotropy of magnetic susceptibility (AMS) axes and magnetization component directions between two samples.

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Acknowledgements

We thank A. Galdeano, G. Hulot, M. Cloete, M. Le Goff, Y. Gallet and G. Plenier for discussions and comments. This work was supported by the French Ministries of Foreign Affairs and Education and Research, INSU and the South African National Research Foundation.

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Carporzen, L., Gilder, S. & Hart, R. Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars. Nature 435, 198–201 (2005). https://doi.org/10.1038/nature03560

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