THE belief that Mars鈥檚 magnetism died out nearly four billion years ago could be wrong. If a study of a meterorite crater in South Africa is anything to go by, the Red Planet may have been magnetically active long after that.
Measurements taken by the Mars Global Surveyor satellite have shown that the massive Hellas and Argyre craters are only weakly magnetic. Scientists took this to mean that Mars鈥檚 magnetism was already weak when meteorites struck to form the craters four billion years ago. This conclusion was based on the idea that if the magnetic field was weak to being with, shock waves from large meteorite impacts could disrupt the field enough to demagnetise the rocks in the crater.
To verify this idea, Stuart Gilder of the Institute of Physics of the Earth in Paris, France, and his colleagues studied the Vredefort meteorite impact crater in South Africa, which also looks weakly magnetic from above. The team collected rocks from 127 different locations inside the crater and measured the strength and direction of the magnetisation in each of the samples.
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They discovered that far from being demagnetised, all the rocks were highly magnetic, but in a jumble of different directions. 鈥淲e think that the meteorite impact randomised the magnetic directions in the rocks,鈥� says Gilder. 鈥淭he magnetic field is strong when you are standing on the surface, but from a few hundred metres above the different directions cancel each other out, making the crater appear to be demagnetised.鈥�
This means that the lack of a magnetic field above Hellas and Argyre on Mars can no longer be used as proof that Mars鈥檚 magnetic field had shut down four billion years ago. 鈥淚t suggests that a magnetic field could have been present on Mars for a lot longer,鈥� says David Dunlop, a geophysicist at the University of Toronto in Canada. The next step will be to use robot rovers to collect and analyse rock samples from the Martian craters.
