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.
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.
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.
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)