Sarah Stewart/UC Davis based on NASA rendering
A shapeshifting Earth might have formed the moon. Roughly 4.5 billion years ago, a collision caused our planet to mushroom outward into a seething, spinning cloud of vaporised rock that resembled a squished jelly doughnut. And there, within its puffy edges, the moon formed. That’s what a new model suggests.
Scientists have long suspected that our planetary companion was built when a Mars-sized body – commonly known as Theia – struck the young Earth, throwing molten rock into orbit that coalesced into the Moon. But that model forms a moon that is mostly composed of rock from Theia, whereas Apollo moon rocks suggest that the moon’s make-up is almost identical to Earth’s.
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A few years ago, Simon Lock, a graduate student at Harvard University, stumbled upon a potential solution. When he and his advisor simulated the early impact, they did not see a young Earth surrounded by a disk of debris. Instead, it looked like the collision vaporised the planet – heating it up, spinning it up and causing it to shape-shift into that cosmic jelly doughnut.
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The previously unrecognised planetary structure was baffling. “We bashed our head against the wall for like two years,” Lock says. “And slowly, piece-by-piece – like small eureka moment by small eureka moment – we pieced together what actually was happening.”
They called it a synestia, and argued that most planets and even some stars might form these oddities at some point in their lives. Now, they argue that such a structure can even explain our moon.
Moonstruck
Conditions within the synestia are torrid. At roughly 3000°C, it lacks a surface but has an outer edge marked by clouds of molten rock that form silica raindrops. There are also chunks of debris that soar throughout the structure.
Should some of those chunks of debris slam together, they could easily form a proto-moon. Then the silica rain – which falls at a rate 10 times higher than rainfall in a hurricane on Earth today – would collect onto the moon, helping it grow.
Meanwhile, the synestia is continuously cooling and shrinking until it’s smaller than the young moon’s orbit. That causes our planetary companion to pop out from the synestia, leaving it in orbit around the body that will keep cooling until it resembles Earth.
The model helps explain why the moon is almost Earth’s chemical clone yet lacks those elements that are easily vaporized, such as potassium and sodium.
Josh Eisner at the University of Arizona in Tucson, who was not involved in the study, was impressed by the details included within the new model. “They tried to be pretty careful about making this a real model and not just a diagram or a sketch,” he says.
Common worlds
The previous line of thought argued that Theia was either fairly hefty or quite puny but could not be anything in between, but Lock and his colleagues found that Theia can run the gamut and still form a synestia.
That means synestias form easily and just might pop up all over the place. And if that’s the case, large moons might also. It’s an exciting result given that some scientists argue that large moons and habitability go hand in hand, Eisner says.
Our moon, for example, helps keep our spin axis relatively stable so that the climate doesn’t swing wildly between hot and icy. So, if large moons are abundant across the galaxy, habitable planets might be as well.
Journal reference: Journal of Geophysical Research Planets, DOI: 10.1002/2017JE005333
Read more: Lunar volcanoes and lava lakes gave the early moon an atmosphere
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