Scientists studying the interior of the Earth and computer simulations of the early solar system conclude that Earth's moon could have formed less than 100 million years after the birth of the solar system.
About 4.6 billion years ago, a nebula collapsed into a star (the sun) surrounded by a dense disk of gas. Driven by electrostatic and gravitational forces, the material began to accrete into larger bodies called planetesimals.[How the Moon Was Born: A Photo Timeline]
For several million years, planetesimals swept up the gas and dust in the nebula surrounding the new sun. Eventually, solar wind blew the remaining gas out of the solar system, and the planets’ growth stopped.
Initially, the solar system formed with many more planets than exist today. Many of these bodies collided and merged into larger planets. About 30 million to 50 million years after the solar system’s birth, a Mars-size body called Theia struck the young Earth at an oblique angle. Both bodies were disrupted before gravity caused them to reassemble into a single planet (Earth) with debris orbiting around it.
The debris blasted into space by the impact of Theia reassembled into two bodies orbiting Earth. The smaller moon was about 600 miles (1,000 kilometers) in diameter. Within about 100 million years after the birth of the solar system, Earth’s two moons crashed. The smaller moon became splattered across one hemisphere of the other moon.
Around 4.1 billion to 3.8 billion years ago, the moon was violently struck by a huge number of asteroids. This Late Heavy Bombardment created large lunar plains (seas) and impact craters.
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