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/ Source: Space.com
By Charles Q. Choi, Space.com
A new study reveals that the moon may have formed after a giant rock the size of Mars struck a newborn earth covered with magma.
The Earth met about 4.5 billion years ago and previous research had suggested that the moon rose soon after. Over the past three decades, the main explanation for the origin of the moon was that it resulted from the collision of two protoplanets, or embryonic worlds. One of them was the Newborn Earth, and the other was a rock the size of Mars called Theia, named after the mother of the moon in the Greek myth. The moon then fused debris.
This "giant impact hypothesis" seemed to explain many details about the Earth and the Moon, such as the large size of the Moon compared to that of the Earth and the rotational rates of the two bodies. However, over the past 15 years or so, evidence has been found to challenge it and suggest a multitude of alternatives.
Related: How the moon formed: 5 wild lunar theories
Computer models of the giant-impact scenario often say that more than 60% of the moon should be made of Theia material. The problem is that most solar system bodies have a unique chemical composition and that the Earth, Theia and therefore the Moon should also be. However, rock samples from the moon show that its composition is strangely more similar to that of the Earth than what such models could predict with respect to the versions of elements called isotopes. (The isotopes of an element each have a different number of neutrons.)
To solve this mystery, a recent model of lunar formation suggested that the moon formed from an impact so violent that it vaporized much of the early Earth. Another suggested that the collision involved a rapidly rotating proto-Earth. However, one of the drawbacks of these models is that these scenarios require improbable impact conditions, said the lead author of the study, Natsuki Hosono, scientist in planetary sciences at the Japan Science and Technology Agency. Earth and Earth technology in Yokohama.
Related: How the Moon Was Made: Explaining Lunar Evolution (Infographic)
To solve this problem, the Hosono team now suggests that a large part of Earth's materials could have reached orbit to help form the Moon if the Earth was partially melted at the time of Giant impact.
Researchers have developed computer models simulating Earth covered with an ocean of magma, which most of the planet's formation models suggest the planet had shortly after its birth. They then analyzed what had happened when a rock of the size of the planet Mars representing about one tenth of the mass of the Earth had touched this molten protoplanet.
Scientists have discovered that even a blow from Theia could remove more than 70% of the moon-forming debris in the ocean's magma ocean, since molten rock was easier to spray than solid materials. These findings could help explain the similarities in composition between the Moon and Earth while explaining details such as their rotation rates, they said.
"Previous work on lunar formation fundamentally ignored the effects of the ocean magma," Hosono told Space.com. "Our research concluded that the ocean magma is one of the most important things for the giant impact forming the moon.
The new model suggests that the amount of debris resulting from an impact on a molten Earth was comparable to the current mass of the moon. However, previous work suggested that to build the moon, the giant impact was to first generate an amount of debris equal to about three to four times the mass of the moon, Hosono said. Future research might consider larger masses for Theia and take into account the rotation of the proto-Earth to see if this could generate enough debris to form a moon of the right size, Hosono added.
Scientists detailed their findings online today (April 29) in the journal Nature Geoscience.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us on twitter @Spacedotcom and on Facebook.
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