This figure shows a model of the Earth's magnetic field being generated within the Earth.
Click on image for full size
Gary A Glatzmaier (Los Alamos National Laboratory) and Paul H Roberts (University of California, Los Angeles).
Generating a Magnetic Field
Scientists think, although not certain, there are two essential ingredients for generating a magnetic field. Those two ingredients are
A bar of iron can be made into a magnet by wrapping it with wires and running a current through the wires.
It is believed a planet, or a star, can generate a magnetic field if it has both of the two ingredients above. It must have enough magnetic material, and it must have currents moving inside the magnetic material. If a planet does not have enough of either of these two ingredients, it will not have a magnetic field. Planets which do not have magnetic fields include Venus (moves very slowly), and Mars (most the iron is on the surface, and not molten).
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