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Exploring Mars ... from Grenoble
March 18, 2004A neutron diffraction experiment carried out recently at the Institut Laue-Langevin in Grenoble by the physicists Bachir Ouladdiaf (ILL), Gérard Fillion and Rafik Ballou (Laboratoire Lois Néel, CNRS, GRENOBLE), in partnership with the geophysicists Pierre Rochette (CNRS and Université d'Aix-Marseille) and Lon Hood (University of Arizona, USA), has brought us a step closer to understanding the magnetisation of the Martian crust.
We know from the measurements taken by the American probe Mars Global Surveyor (MGS) in 1999 that the magnetic fields on Mars are very localised and very contrasted. The southern Martian crust, for example, is highly magnetised, while in the same hemisphere a non-magnetised zone surrounds the Argyre and Hellas basins, two giant impact craters. How can these differences be explained?
The North-South magnetic dichotomy could be explained by the shutdown of the Martian dynamo before the formation of the younger northern crust. On the other hand, in the older crust on the heavily cratered terrain of the Noachian in the south, there is no apparent difference between the crust which is magnetised and the crust which is not magnetised. Is this because of the influence of the proximity of the giant impact basins of Hellas and Argyre, assuming these impacts occurred after the dynamo shutdown? In which case, what mechanism is responsible for demagnetising the crust at distances of several crater radii? The mechanism in question cannot be thermal, since the warming induced by the impact is too limited at such distances. It must therefore be related to pressure.
This is where the neutrons of the ILL step in. Neutron diffraction is in fact a unique tool which allows scientists to probe not only crystalline orders, as with X-ray diffraction, but also magnetic orders (by using complex environments, such as pressure, magnetic field, etc.).
Pyrrhotite is known to undergo a transition to a non-magnetic state at moderately high pressure (between 1 GPa (gigapascal) and 5 GPa). In order to quantitatively test the hypothesis of demagnetisation by impact around Hellas and Argyre, it was therefore necessary to determine precisely the pressure at which the magnetism disappears. It was also necessary to obtain data on the remanence in relation to pressure and, finally, to calculate the peak pressure in relation to the distance around the two giant impact basins.
The neutron diffraction experiment conducted at the ILL demonstrated that pyrrhotite undergoes a total demagnetisation at between 2.8 and 3 GPa. And it just so happens that calculations by researchers predict that the isobaric curve of 3 GPa, describing the maximum pressure of the impact shock wave, lies approximately at the limit between the magnetic and non-magnetic zones of the Noachian crust!
These preliminary results strengthen the hypothesis that pyrrhotite is the mineral responsible for Martian remanence. Moreover, they explain the contrast between the magnetic and non-magnetic zones of the Martian crust. Finally, they have an impact on what we know about the paleomagnetic signals of Martian meteorites.
This work is therefore a major step forward in our understanding of the global magnetic map of Mars.
Institut Laue-Langevin (ILL)
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