- A new study challenges the idea that the moon was formed by a rogue planetary body slamming into the Earth and knocking off a disc.
- Instead, the study’s authors think the moon may have emerged from an even more violent collision that would have created a rapidly spinning doughnut of molten, vaporized rock and liquid called a synestia.
- As the synestia cooled, the moon may have emerged from its outer regions before the inner part cooled and became Earth.
- This theory helps answer some lingering questions about how the moon formed.
Our understanding of the moon’s formation is violent enough – but the story of how Earth acquired its satellite may be a tale of even greater destruction, according to a new study published Wednesday in the Journal of Geophysical Research: Planets.
In the traditional moon-formation story, known as the Giant Impact Model, a Mars-sized object called Theia came hurtling through space and glanced off our planet, and the molten rock and material thrown off in the collision became our moon.
But there are problems with this explanation. Though the moon lacks certain volatile elements found on Earth, its chemical and isotopic composition is very close to our planet’s, a fact that doesn’t fit with the idea that it’s partially or largely from a rogue object. Plus, getting the right-sized disc into the exact orbit of the moon would have required a precise and unlikely collision.
So the researchers behind the study – from Harvard and the University of California at Davis – came up with a different explanation that relies on a new type of planetary object, called a synestia, which two of the authors 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 vaporise the structures. It’d be too hot for them to exist as planetary bodies, so they’d become rapidly spinning doughnuts of molten, vaporized rock and liquid. The terms mantle, atmosphere, and disk would no longer apply.
The researchers 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.
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 earlier 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, most likely for only hundreds of years, the study says.
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 pressurised rock that rained down on this moon seed could have collected, according to the study.
The new theory posits that 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 who led the study, said in a news release. “Over time, the whole structure shrinks, and the moon emerges from the vapour.”
This helps explain the similarities and differences between the Earth and the moon, which are chemically alike, except for certain elements. If it had been part of a synestia, the moon could have collected the less-easily-vaporized and less volatile elements that also make up the Earth.
“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 who helped write the study, said in the release.
Such giant impacts creating 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 vapour and orbiting what would become the Earth.
It may have taken only 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 similar composition of the Earth and the moon, this theory is plausible because several types of impacts could have created the synestia, the authors say, making it more likely than the model that depends on a specific glancing blow to knock off a moon-disc.
“Basically, this is the first model 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 know what exactly 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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