The earth started out as a molten mass and cooled to form a...

The earth started out as a molten mass and cooled to form a solid surface. The oceans were formed by watery comets crashing into the surface. Ordinarily the Earth would be solid to the core by now, but the presence of radioactive elements such as uranium and potassium and the heat given of when they decay has kept the core predominantly molten. The Earths magnetic field is generated by the internal rotation of the core, like a dynamo. Mars has no magnetic field because it has no internally rotating core.