In its infancy, beginning about 4.5 billion years ago, the earth glowed like a faint star. Incandescent yellow-orange oceans of magma roiled the surface following repeated collisions with immense boulders, some the size of small planets, orbiting the newly formed sun. Averaging 75 times the speed of sound, each impactor scorched the surface--shattering, melting and even vaporizing on contact.
Early on, dense iron sank out of the magma oceans to form the metallic core, liberating enough gravitational energy to melt the entire planet. Massive meteorite strikes continued for hundreds of millions of years, some blasting craters more than 1,000 kilometers in diameter. At the same time, deep underground, the decay of radioactive elements produced heat at rates more than six times greater than they do today.