IN 1974, the spacecraft Mariner 10 measured a magnetic field around the planet Mercury. Although it is 100 times weaker than Earth鈥檚 field, the puzzle has always been how such a small planet can be generating any magnetic field at all. Now it seems the answer may lie within the planet鈥檚 crust.
Earth鈥檚 magnetic field is thought to be produced by the flow of material in its molten core. But Mercury is so small, at just 4879 kilometres across, that its core should have cooled and solidified long ago. Most scientists trying to explain its magnetic field have been searching for reasons why the core might still be molten, but a new calculation supports an alternative explanation for the magnetism: its source could be in the crust rather than the core.
It is a radical idea. A symmetrical crust can鈥檛 hold a magnetic field because it would cancel itself out. Lumps and bumps in the crust, like craters and mountains, are too small to destroy this symmetry. But Ohed Aharonson from the California Institute of Technology in Pasadena and his colleagues point out that Mercury is actually lopsided, because of its elliptical orbit and the fact that it spins three times for every two revolutions around the sun. This uneven pattern leaves the crust with areas that are permanently hot or cold.
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Rocks that form from magma can acquire a magnetic field as they cool, which is 鈥渇rozen鈥� into the rock below a certain temperature. The variations in Mercury鈥檚 surface temperature mean that the depth at which this happens differs across the planet, and the thickness of the crust available to be magnetised varies accordingly.
Aharonson and his co-authors have now shown that these differences are enough to stop the field from cancelling out, meaning that the crust could still be clinging on to remnants of the magnetic field once generated at the planet鈥檚 core (Earth and Planetary Science Letters, vol 218, p 261). For this to happen, either the rocks in the crust must be composed of minerals that are easy to magnetise, perhaps iron sulphide, or the core once had a particularly vigorous magnetic dynamo.
It will soon be possible to put this theory to the test. NASA鈥檚 Messenger spacecraft is due to be launched in May this year, and should be orbiting Mercury by 2009. Magnetometers aboard the craft will make detailed readings of Mercury鈥檚 magnetic field to determine exactly where it sits. 鈥淚f the field is generated in the crust then small features like craters will perturb the field, but if it is generated in the core these surface features will have a negligible effect,鈥� explains Aharonson.