麻豆传媒

The strange magnetic field of Mars, which is concentrated in the planet鈥檚 southern hemisphere, could have been caused by a giant impact.

The finding clears up one of the biggest remaining mysteries about the planet.

The study, led by of the University of Toronto, has shown that the asymmetric field could be linked to the planet鈥檚 strange surface features.

The relatively smooth, flat surface of Mars鈥� northern hemisphere lies around 6 kilometres lower than the more mountainous surface of the southern hemisphere. Earlier this year, researchers proposed that this 鈥淢ars dichotomy鈥� can be explained if a huge object, almost as big as Earth鈥檚 moon, hit the northern hemisphere of Mars at a shallow angle.

It has long been known that the planet鈥檚 oddly shaped magnetic field 鈥� first observed by the Mars Global Surveyor in 1985 鈥� originated about the same time as the Mars dichotomy. So Stanley and her colleagues decided to investigate whether they had a common cause.

Recorded in the rock

The researchers used computer models to work out how heat would move around the interior of the planet after the impact.

They found that the strike would have warmed the mantle 鈥� the semi-molten layer beneath the crust 鈥� in the northern hemisphere, reducing the temperature difference between the planet鈥檚 core and the mantle.

鈥淭his warmer material results in less heat flowing across the core-mantle boundary in the northern hemisphere,鈥� Stanley says.

In the southern hemisphere, by contrast, the strong heat flow churns up mantle rock, which contains magnetic minerals. The models show that this can create a self-sustaining magnetic dynamo as the molten rock rises and falls in convection currents.

鈥淪ince the generated magnetic field is only strong in the southern hemisphere, only the rocks in the southern hemisphere become strongly magnetised,鈥� Stanley says.

The study provides the most plausible explanation so far for the 鈥榮 1985 observation of a strong southern field, according to of the University of Nantes, France.

鈥楥ompelling solution鈥�

鈥淭his is a great paper,鈥� he says. The solution is particularly elegant, Langlois reckons, because it fits with current hypotheses about the history of the planet, and has the potential to sit well with the unfolding details of the planet鈥檚 geology. 鈥淲e will know more once the internal structure of Mars is more accurately known,鈥� he says.

of the Colorado School of Mines, who proposed the oblique-impact model, is similarly impressed.

The details of the theory will require further testing against observations, but the study 鈥渙ffers a very interesting and compelling solution鈥� to the puzzle of Mars鈥� magnetic field, he says.

Andrews-Hanna says he is excited by the rapid progress of research: 鈥淢ars presents us with many mysteries, but as time goes on more of the pieces of the puzzle seem to be falling into place.鈥�

Journal reference: (DOI: 10.1126/science.1161119)