Study: Moon is made up of early Earth's mantle

Scientists say the moon was formed by a high-energy impact which vaporized most of the Earth and created a massive magma atmosphere.

By Brooks Hays
Study: Moon is made up of early Earth's mantle
New research suggest the moon was formed from a high-energy impact that vaporized much of Earth. Photo by Dana Berry/SwRI

ST. LOUIS, Sept. 12 (UPI) -- New isotopic analysis confirms the moon was forged from proto-Earth's mantle several billion years ago.

Scientists say their latest findings, detailed in the journal Nature, finally settle the debate over whether Earth or its ancient impactor formed the moon.


Previously, the most popular explanation for the birth of the moon was the giant impact model, which suggested the moon was formed by a grazing impact between a Mars-sized body and the Earth. The giant impact model explained a variety of the moon's physical characteristics, including its size and rotation.

But the model suggested the moon was mostly formed by the material of the impactor. Isotopic analysis suggested the opposite, with lunar and Earth samples revealing nearly exact concentrations of the three oxygen isotopes.

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Isotopes serve as fingerprint-like chemical signatures of the solar system's many bodies. The chance Earth's impactor featured the same isotopic composition was too small to consider.

Until recently, scientists suggested more precise isotopic measurements might reveal small differences, but 2015 testing quashed the notion.

"These are the most precise measurements we can make, and they're still identical," Washington University in St. Louis geochemist Kun Wang said in a news release.


So scientists went back to the drawing board, attempting to augment the giant impact model in a way that could explain the isotopic similarities of the Earth and its moon.

One hypothesis suggested a silicate vapor atmosphere spread about the impacted Earth and vaporized the impactor, allowing the exchange of materials between the Earth and its impactor.

"They're trying to explain the isotopic similarities by addition of this atmosphere," Wang said, "but they still start from a low-energy impact like the original model."

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Simulations showed such a scenario would take too long to account for the isotopic similarities, as the moon would have formed fairly quickly from the condensed magma in the wake of the impact.

A second hypothesis proposed a high-energy impact which vaporized most of the Earth and created a massive magma atmosphere, thoroughly mixing the materials of Earth and its impactor. It is from this haze of melted rock that the Earth reformed and the moon was born.

New analysis of potassium isotope levels in lunar and terrestrial rocks support the high-energy impact theory.

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"The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar," Wang and his colleagues wrote in a paper, newly published in the journal Nature.


The enrichment of lunar rocks with heavier potassium isotopes can't be explained by a silicon vapor atmosphere, researchers report.

"Our K isotope result is inconsistent with the low-energy disk equilibration model," researchers wrote, "but supports the high-energy, high-angular-momentum giant impact model8 for the origin of the moon."

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