about the same time evolved as separate celestial bodies in the opinion of the scientists of the US National Aeronautics and Space Administration NASA the Moon evolved normally for its first 500 million years then they said the most cataclysmic period came four billion years ago when celestial bodies the size of large cities and small countries came crashing into the moon and formed its huge basins and towering mountains the huge amounts of radioactive materials left by the collisions began heating rock beneath the surface melting massive