Moon is proto-Earth’s mantle, relocated

Data confirm model in which impact pulverizes Earth like a sledgehammer hitting a watermelon

Planetary smackdown: An artist’s conception of the giant impact that created Earth’s moon. New research suggests the impact was even more violent than this image suggests. Illustration Credit: Dana Berry/SwRI




The isotopic study, which supports the high-energy model, is published in the advance online edition of Nature Sep.12, 2016. “Our results provide the first hard evidence that the impact really did (largely) vaporize Earth,” said Wang, assistant professor in Earth and Planetary Sciences in Arts & Sciences.

An isotopic crisis



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So the giant impact hypothesis had a major problem. It could match many physical characteristics of Earth-Moon system but not their geochemistry. The isotopic composition studies had created an “isotopic crisis” for the hypothesis.

At first, scientists thought more precise measurements might resolve the crisis. But more accurate measurements of oxygen isotopes published in 2016 only confirmed that the isotopic compositions are not distinguishable. “These are the most precise measurements we can make, and they’re still identical,” Wang said.

A slap, a slug or a wallop?

“So people decided to change the giant impact hypothesis,” Wang said. “The goal was to find a way to make the Moon mostly from Earth rather than mostly from the impactor. There are many new models — everyone is trying to come up with one — but two have been very influential.”



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“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.”

But exchanging material through an atmosphere is really slow, Wang said. You’d never have enough time for the material to mix thoroughly before it started to fall back to Earth.

So another model, proposed in 2015, assumes the impact was extremely violent, so violent that the impactor and Earth’s mantle vaporized and mixed together to form a dense melt/vapor mantle atmosphere that expanded to fill a space more than 500 times bigger than today’s Earth. As this atmosphere cooled, the Moon condensed from it.

The thorough mixing of this atmosphere explains the identical isotope composition of Earth and Moon, Wang said. The mantle atmosphere was a “supercritical fluid,” without distinct liquid and gas phases. Supercritical fluids can flow through solids like a gas and dissolve materials like a liquid.

Why potassium is decisive


Wang and Jacobsen examined seven lunar rock samples from different lunar missions and compared their potassium isotope ratios to those of eight terrestrial rocks representative of Earth’s mantle. They found that the lunar rocks were enriched by about 0.4 parts per thousand in the heavier isotope of potassium, potassium-41.

The only high-temperature process that could separate the potassium isotopes in this way, said Wang, is incomplete condensation of the potassium from the vapor phase during the Moon’s formation. Compared to the lighter isotope, the heavier isotope would preferentially fall out of the vapor and condense.

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Kun Wang, Stein B. Jacobsen. Potassium isotopic evidence for a high-energy giant impact origin of the Moon. Nature, 2016; DOI: 10.1038/nature19341
