Dynamo theory makes certain predictions. One is that a dynamo-induced magnetic field can exist only where a molten core exists within a fast-spinning planet. Small celestial bodies, such as Mars and the asteroids, long ago lost their heat, which shuts down the necessary convection near their cores. Venus is large enough to have retained its heat, but it spins very slowly, reducing the motion of its core and weakening the field. Mercury, which is too small to retain its primordial heat, does have a magnetic field as a result of its extreme inclination, which is allowed for in dynamo theory.
