For centuries, magnetic materials have been familiar objects to mankind: they are everywhere in everyday life. But magnetic systems exist on all scales: elementary particles such as neutrons act like tiny magnets; planets such as our Earth, Jupiter, Saturn and (a long time ago) Mars have powerful magnetic dynamos in their cores.

Scientists believe that once upon a time Mars also had a global magnetic field, like the Earth’s, but that the iron-core dynamo that generated it shut down billions of years ago with patches of magnetic zones located in the southern hemisphere. Neutron experiments have shown that the shock waves induced by huge meteorites have led to disappearance of magnetism in the impacted zones of the Mars southern hemisphere.

B. Ouladdiaf, ILL brochure "Neutrons and magnetism", p.9