The moon's formation may have been far more violent than we thought
- A new study challenges the traditional idea that the moon was formed after a rogue planetary body slammed into the Earth, knocking off a disc that would become the moon.
- Instead, the study authors think the moon may have emerged from an even more violent collision that would have created a rapidly spinning donut of molten, vaporized rock and liquid called a synestia.
- As the synestia cooled, the moon might have emerged from its outer regions before the inner part cooled and became Earth.
- This helps answer some lingering questions about how the moon formed.
Our traditional understanding of the moon's formation is violent enough. But the true story of how Earth acquired its satellite may be a tale of even greater destruction.
That's according to a study newly published in the Journal of Geophysical Research - Planets.
In the traditional moon-formation story, a Mars-sized object called Theia came hurtling through space and glanced off our planet. The molten rock and material thrown off in the collision became our moon. That line of thought is known as the Giant Impact Model.
But there are problems with this explanation. The moon's chemical and isotopic composition is very close to that of Earth, a fact that doesn't fit with the idea that the moon is a disc partially or largely from a rogue planet. Instead, the moon seems more like Earth, though it lacks certain volatile elements found here. Plus, getting the right-sized disc into the exact orbit of the moon would have required a precise and unlikely collision.
So the team of researchers behind the study - from the University of California, Davis and Harvard - have a different explanation.
It relies on a new type of planetary object, called a synestia, which two of the authors of the study proposed last year.
The idea behind a synestia is that if planet-sized objects were to collide at high enough speeds, there could be enough energy to vaporize the structures. It'd be too hot for them to exist as traditional planetary bodies -they'd become rapidly spinning donuts of molten, vaporized rock and liquid. The terms mantle, atmosphere, and disk would no longer apply.
The researchers behind the new study argue that a synestia-creating impact is responsible for our moon. It's similar to the Giant Impact Model, but cranked up to 11 in intensity.
NASA image adapted by Lock et al
Scientists estimate that Earth took its terrestrial form around when our solar system settled into its current layout 4.5 billion years ago.
According to these new calculations, the pre-Earth form of the planet could have been slammed by another planetary object. Everything would have vaporized, creating a synestia. Spinning in the vastness of space, this structure couldn't last in this superheated state for long, likely only for hundreds of years, according to the study.
But in that cooling and condensing process, it's likely that some molten rock in an outer layer could have condensed into a seed for a proto-moon. Superheated and pressurized rock would have rained down on this moon seed, and it could have collected vaporized rock, according to the study.
According to this new theory, the moon would have formed inside the synestia at high pressures and a temperature between 5,000 and 7,000 degrees Fahrenheit before it cooled and condensed.
"The rate of rain fall is about ten times that of a hurricane on Earth," Simon Lock, a graduate student in Harvard's Department of Earth and Planetary Sciences and lead author of the new study, said in a news release. "Over time, the whole [synestia] structure shrinks, and the Moon emerges from the vapor."
This helps explain the similarities and differences between the moon and Earth. The moon itself is chemically similar to Earth minus certain elements. If it was part of a synestia, it could have collected the less easily vaporized and less volatile elements that also make up Earth, but fewer elements that would have been easily vaporized.
"This is the first model that can match the pattern of the moon's composition," Sarah Stewart, a professor of Earth and Planetary Sciences at UC Davis and an author of the study, said in a statement to Business Insider.
Such giant impacts that created moons in a synestia could once have been common in the universe, the authors wrote.
As the synestia continued to cool and condense, it would have shrunk, with the moon emerging from the cloud of vapor, orbiting what would become the Earth.
It might have only taken a few decades for the moon to emerge from the cooling cloud, which could have condensed within another 1,000 years or so.
In addition to helping explain the chemical composition of Earth and the moon, the authors say this theory is plausible because a number of types of impacts could have created the synestia. That makes it more likely than the model that depends on a specific glancing blow to knock a moon-disc off a rogue planet.
"Basically, this is the first model that that has been able to explain the complications, and that has been able to do it quantitatively," Lock said. "This is a dramatically different way of forming the Moon. You just don't think of a satellite forming inside another body, but this is what appears to happen."
It's hard to be certain of exactly what happened before the Earth as we know it existed, but this model presents a new plausible explanation and answers lingering questions.
"We've done calculations of each of the processes that go into forming the Moon and shown that the model could work," Lock said, "but there are various aspects of our theory that will need more interrogation."
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