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THEIA: AN ANCIENT PROTO-PLANET THAT MAY HAVE MERGED WITH THE YOUNG EARTH TO FORM THE MOOON

About 4.45 billion years ago, 150 million years after the solar system formed, Earth was hit by a Mars-size object called Theia.

The collision created the moon, but debate has raged exactly what happened during this event - and a mystery has persisted on why the moon and Earth are so similar in their composition.

The impact of Theia with Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon.

This cloud would have been composed of some Earth material, explaining the similarity between Earth and the moon, and other material.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

But one mystery has persisted, revealed by rocks the Apollo astronauts brought back from the moon - why are the moon and Earth so similar in their composition?

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon.

Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon.

Or Theia could have, coincidentally, been isotopically similar to young Earth.

A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact.

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