What hypothesis explains why Earth and the moon are about the same age?

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The theory that explains why Earth and the moon are the same age has many names, most notably "The Giant Impact Theory" or "The Theia Hypothesis." This theory states that Earth's moon was created as the result of a large planet-sized impact approximately 4.5 billion years ago (similar to the...

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The theory that explains why Earth and the moon are the same age has many names, most notably "The Giant Impact Theory" or "The Theia Hypothesis." This theory states that Earth's moon was created as the result of a large planet-sized impact approximately 4.5 billion years ago (similar to the approximate age of Earth and the creation of our solar system). During the impact between Earth and the other object (named Theia), a large chunk of Earth is believed to have broken off and begun orbiting around the young planet Earth. Over thousands of years, this chunk coalesced with other debris from the collision under the pull of gravity.

Scientists theorized that this collision occurred after collecting rocks from the moon's surface and comparing their age and composition to igneous rocks on Earth. What they found is that while the geology of the moon and Earth are vastly different (Earth has a lively geology, while the Moon is very much "dead"), their compositions are nearly identical. The rocks were both tested through the measurement of radioactive isotopes and found to be nearly identical in age. The chemistry within the moon rocks is also similar to most Earth rocks containing large amounts of silicon, oxygen, and iron.

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The hypothesis is called The Big Splash or the Theia hypothesis. It is also known as the Giant Impactor Theory.

As per this hypothesis/theory, immediately after the formation of Earth (when our planet was very young, about 50 million years of age), it collided with a planet known as Theia. This collision took place about 4.5 billion years ago. The planet Theia was about the size of the planet Mars and collided with Earth in an oblique impact. Due to this collision, a significant fraction of Theia's mass was combined with Earth's, while the debris of this collision was sent to orbit around Earth. This debris later formed our moon. This also means that the Earth and the moon share similar rocks and hence are of about the same age.

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