geodynamo

How magnetism works: electron spin, magnetic domains, ferromagnetism, Earth's geodynamo, animal navigation, and MRI, explained mechanism by mechanism.

Planetary magnetic fields are made by dynamos: electrically conducting fluid, stirred by heat and rotation, that regenerates its own field. Earth's runs in liquid iron, Jupiter's in metallic hydrogen, and Uranus and Neptune's in shallower layers that give them tilted, off-center fields. Mars lost its global field billions of years ago.