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The planetary collision that formed the Moon may have been way more violent than we thought

The planetary collision that formed the Moon may have been way more violent than we thought


But not all researchers are convinced

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Dana Berry/SwRI

After studying the chemical makeup of lunar rocks, scientists say they have found new evidence that disproves one of the leading theories of how the Moon formed. The evidence hinges on the presence of just one element: potassium, and it suggests that the planetary collision that formed our satellite was extremely violent — an idea that’s very different from what was previously thought.

It could change our understanding of our planetary system’s history

The new theory — detailed in a study published today in Nature — is a radical new concept that could change our understanding of our planetary system’s history. But not all researchers are convinced just yet. "That is definitely a tall claim," Munir Humayun, a geologist at Florida State University who was not involved in the study, tells The Verge. "It’s a little too early with their data to tell that."

For decades, most astronomers have agreed that the Moon is the result of a giant collision between the Earth and a Mars-sized object, called the impactor. But not everyone can agree on the exact mechanics of that collision. Right now, a popular theory suggests that the impact was relatively low-energy, leaving the Earth mostly intact but causing the impactor to melt into magma. This magma formed a disc out in space — what would eventually turn into the Moon.

But study author Kun Wang says that the potassium signatures they found paint a different scenario. The collision that formed the Moon wasn’t low energy at all, he argues. Instead, the impact was extremely violent, pulverizing most of the Earth and the impactor, and turning them into a vapor. In this scenario, the vaporized Earth and impactor mix together into a giant dense atmosphere. This atmosphere then cools and condenses into our planet and its satellite.

"This model is entirely different," Wang, a geochemist at Washington University in St. Louis, tells The Verge. "The impact is much larger."

The two competing theories for how the Moon formed. The first depicts the silicate atmosphere concept, while the second depicts the concept of a more vaporized Earth. (Kun Wang)

The idea that a giant planetary crash formed the Moon has been around since the 1970s; it’s known as the giant-impact hypothesis. But there have been some problems with the model. Originally, the hypothesis suggested that about 80 percent of the Moon came from the impactor and the rest from Earth. That became an issue as researchers started studying the composition of the Moon more closely. It turns out that the Moon and Earth share a lot of the same chemical makeup, meaning the Moon must have been made from a much more significant portion of our planet’s material.

Astronomers have modified the giant-impact hypothesis a bit

To fix this problem, astronomers have modified the giant-impact hypothesis a bit. A new model from 2007 proposed that a silicate atmosphere surrounded the planetary system after the impactor collided with Earth. This atmosphere would have acted like a conduit, allowing materials to be exchanged between the Earth and the impactor’s magma, which eventually formed the Moon. That would solve the mystery of why Earth and its satellite are so similar.

But Wang says his new potassium measurements don’t fit with this model either. Specifically, the researchers analyzed seven lunar rocks and eight Earth rocks. They measured two different variants — or isotopes — of potassium: potassium-41, the heaviest version of the element, and potassium-39, the lightest version. They found that the lunar samples are richer in the heavier element, potassium-41, than the Earth samples.

A color mosaic of the Moon's north pole. (NASA)

These findings don’t support the giant-impact hypothesis the way it stands now, Wang explains. The Moon should be richer in potassium-39, not the other way around. If the "silicate atmosphere" theory is true, both Earth and the newly forming Moon would have been super heated after the collision, and potassium would have been evaporating from both objects. But since the Earth is so much bigger, the planet would have sent way more potassium over to the Moon, not the other way around. And since the lighter element evaporates faster than the heavier one, that should make the Moon more rich in potassium-39 — not potassium-41.

The only way to explain the higher abundance of potassium-41 on the Moon is the much more violent impact, says Wang. In this scenario, the Earth is almost completely vaporized, so all of the potassium from the planet would be mixed up in the dense vapor leftover from the collision. That vapor eventually condensed to form the Moon. In the condensation process, the heavy potassium would have condensed into the Moon more than the light potassium. Thus, the Moon would have more potassium-41. "Our paper is the first hard, real evidence to support that theory," says Wang.

Humayun says that Wang and his team haven’t disproven anything just yet

But Humayun, who has specialized in potassium isotopes, says that Wang and his team haven’t disproven anything just yet. The lunar samples used for the study don’t accurately represent the Moon’s potassium composition, Humayun says. And that means this study doesn’t truly negate any of the Moon’s origin stories — for now. "I’m very pleased overall with what they have done, I just wish they had used better samples," he says.

Wang and his team analyzed a mixture of lunar breccias and lunar basalts. Humayun argues that breccias — rocks made from small meteorites slamming into the surface of the Moon — tend to be contaminated with soil that has more potassium-41. That could confuse the readings. And the basalts used for the study aren’t reliable either, he says. These rocks, which formed from lava that rapidly cooled long ago, sometimes run the risk of outgassing potassium out into space.

"Even a small amount of potassium loss could give rise to a measurable effect," says Humayun. "I can’t say these concerns kill this theory, but you have to work past these concerns before you can claim you have the potassium composition measured."

Meanwhile, Wang and his team are already bracing themselves for some pushback on their research. They say it’s only normal for people to be resistant to a new theory at first. "It will take time for people to accept a new idea," says Wang. "It took people decades to accept this giant-impact hypothesis. Now we’re saying that giant impact hypothesis is not right, so it may take 10 to 20 years to accept the new model."

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