About thirty million years into its existence, the early Earth was beginning to take
its present shape. Some of the lighter elements that had floated to Earth's
surface were already beginning to cool and form a crust. Even so, the inner solar
system remained a very crowded place, with many asteroids still wandering about
and perhaps as many as twenty planet-sized objects still vying for permanent status.
Inevitably, there were collisions, which gradually reduced the number of
competitors to the four inner planets we have today.One such collision involved Earth and a planet roughly the size of Mars, known as Theia. The force of the collision destroyed Theia, but because the blow was glancing, the impact caused a significant portion of Earth's mantle to be ejected into space, along with most of Theia's mantle. Earth's massive gravity held this matter in orbit, however, and about half of it eventually fell back down. The rest coalesced to form the Moon.
Evidence strongly supporting this hypothesis was gathered on the Moon by the Apollo 11 astronauts. The rocks that they brought back turned out to be nearly identical in chemical composition to Earth rocks, except for two key differences: The Moon rocks had no iron, and they had no water.
The lack of iron was perfectly consistent with the Theia hypothesis (also known as the big splash) because neither Earth's mantle nor Theia's would have had much iron in it at the time of the collision. (Upon impact, Theia's iron core simply sank into Earth's core.) The lack of water was similarly predictable. Rocks on Earth have large amounts of water locked away in their crystalline structures, but the cinderlike rocks from the Moon had none. This is because the collision between Earth and Theia would have generated a great deal of heat, enough to reliquefy Earth's crust and therefore certainly enough to have evaporated any water that might have existed in the ejected matter. (Adapted from ‘The Bedside Baccalaureate’, edited by David Rubel)
