Study of ancient Earth–Theia collision sheds new light on geologic history

New Scientist : A new study could alter the long-held belief that the collision that formed the Moon also completely melted Earth’s mantle. It has been proposed that a primordial planet named Theia struck Earth and that debris from the collision congealed into the Moon. Had the energy of the collision totally melted Earth’s mantle, elemental isotopes should have been evenly distributed throughout. However, new observations by Sujoy Mukhopadhyay of Harvard University and colleagues show that the ratio of helium-3 to neon-22 is much higher in the shallow part of Earth’s mantle than in the deeper part. The researchers propose that although the energy from the giant impact would have been sufficient to melt the entire planet, it was not evenly distributed, with the side where Theia collided bearing the brunt of the melting. The new insight could help further research into the different stages of Earth’s development.