The original accretion of the Earth's material into a spherical mass is thought to have resulted in a relatively uniform composition.
As material became molten enough to allow movement, the denser iron and nickel, previously evenly distributed throughout the mass, began to migrate to the center of the planet to form the core.
The gravitational potential energy released by the sinking of the dense NiFe globules, along with any cooler, denser solid material, is thought to have been a runaway process, increasing the temperature of the protoplanet above the melting point of most components, resulting in the rapid formation of a molten iron core covered by a deep global silicate magma.
This event, an important process of planetary differentiation, occurred at about 500 million years into the formation of the planet.
By this theory Mars has simply cooled faster than the Earth, gradually solidifying its dynamic iron center, hence shutting down its magnetosphere.