Detection of an active magnetic field or remnant magnetization in crustal rocks indicates that the planet contains an interior conductive layer. The conductive layer in terrestrial planets is a metallic iron-rich core, probably mixed with some sulfur. The core must be at least partially liquid for production of an active magnetic field. As long as the core is composed of material which conducts electricity, it can produce a time-varying electric field which induces a magnetic field. The area of physics which studies how motions within a fluid core set up the electric currents which produce planetary magnetic fields is called magnetohydrodynamics (MHD) or magnetic dynamo theory (Stevenson, 2003).
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