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Moon Formed When Mars-Size Rock Collided With Young Earth Still Covered With Magma Ocean, Theory Suggests

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A new theory suggests that the moon may have formed after a Mars-size rock collided with the newborn Earth when it was still covered in molten rock.

Giant Impact Hypothesis

A popular theory known as the "Giant Impact Hypothesis" suggests that the moon formed as a result of a collision of two protoplanets.

One of the two objects was the young Earth, and the other was the giant rock called Theia. The moon is said to have formed from the debris of the impact.

New evidences now challenge the idea. Computer models of the scenario show that more than 60 percent of the moon should be composed of material from Theia. Researchers, however, have found no sign of Theia's chemical fingerprints on the moon. Lunar rocks brought back by astronauts of the Apollo missions also show that the moon is consist mainly of material from Earth.

In a new theory published in the Nature Geoscience, Natsuki Hosono, from Japan's Agency for Marine-Earth Science and Technology, and colleagues offer a new idea that can help explain why the chemical makeup of Earth and the moon is alike.

Molten Earth Theory To Explain Formation Of The Moon

Hosono and colleagues used computer models to simulate what happens when a young Earth covered with an ocean of magma hits a Mars-size rock about a tenth of its mass. Most models of the Earth's formation suggest it had a magma ocean soon after its birth.

The researchers found that if Earth was in a molten state when it collided with Theia, much more Earth material could be ejected into space, leaving the moon mostly made out of our planet's material instead of Theia.

The new model also shows that the amount of debris from the impact was comparable to the current mass of the moon.

Future research, however, may have to consider larger masses for Theia since earlier studies have suggested that to build the moon, the impact needs to generate an amount of debris equivalent to about three to four times the mass of the moon.

"These calculations demonstrate that, because of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean, even in a highly oblique collision," the researchers wrote in their study.

"We show that this model reconciles the compositional similarities and differences between the Moon and Earth while satisfying the angular momentum constraint."

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