Diamonds are forever, unless you’re on Saturn or Jupiter.
Loads of the super-hard stones may be floating among the gas giants’ fluid layers and melted into liquid further into their depths, a pair of planetary scientists proposed this past week.
The research sprang from very humble beginnings - soot in Saturn’s atmosphere, said Kevin Baines, a planetary scientist at the University of Wisconsin-Madison.
Baines was studying thick yellowish ammonia clouds in Saturn’s atmosphere when he noticed other extremely dark clouds cropping up as well.
“It’s almost like clockwork in the southern hemisphere, where we were studying these thunderstorms,” said Baines, who is part of the science team for NASA’s Cassini mission and also works at Jet Propulsion Laboratory in La Canada Flintridge in California. “Whenever you have a thunderstorm, you get both these types of clouds.”
The dark stuff turned out to be soot, bits of pure carbon with no internal structure trapped in frozen ammonia, he said. But where was this soot coming from? Baines and collaborator Mona Delitsky, a planetary scientist at California Specialty Engineering in Pasadena came up with an idea.
Saturn’s atmosphere is mostly hydrogen, Baines said, but about 0.5 percent of it is methane, a molecule made up of carbon and hydrogen. During a thunderstorm, lightning can fry that methane to a crisp, releasing the hydrogen and reducing the carbon to little black bits. The researchers think those bits of soot are blown up into the ammonia clouds during the thunderstorms.
“So we have this reservoir of carbon dust,” Baines said. “The natural question is, what happens to the carbon dust eventually? Eventually it’s going to drift on down.”
The researchers think that as the soot particles fall, they start to find one another and glom together. These bits of pure carbon may also act as seeds that pull the carbon atoms out of methane molecules they meet, further fueling its growth.
By the time the soot particles have floated several hundred miles within the planet’s atmosphere, the heat and pressure crush the carbon into graphite, with atoms arranged into two-dimensional structures layered on top of one another. While it has some crystalline order, graphite’s still pretty soft. Those two-dimensional layers rub off easily, which is why it’s so useful as pencil lead.
Then about 3,700 miles down - roughly the distance from Earth’s surface to its core - the pressure rises to 100,000 times that of Earth at sea level. It’s so powerful that it crushes the graphite into carbon’s three-dimensional crystalline form, diamond. These diamonds grow into large pebbles as they bob around in the planet’s fluid layers, Baines said.
The diamonds precipitate down through Saturn’s layers for another 18,600 miles or so, Baines said. (Keep in mind, Earth’s diameter is a mere 7,918 miles.) But at that point, the temperature is so high that even the diamonds can’t take it anymore, and they melt.
This entire process probably takes a long time, Baines estimated, perhaps on the order of a thousand years.
What would a melted diamond look like? It’s unclear, given that diamonds are defined by their crystalline structure, Delitsky said, though scientists have some ideas.
“When diamond melts, the liquid kind of retains some diamond-like geometry,” said Delitsky, who presented the work at an American Astronomical Society conference in Denver.
The idea that diamonds may reside in the bowels of other planets is not a new one, Baines said. The ice giants Neptune and Uranus have high enough pressures near their cores to forge the precious stone, but not enough heat to melt it.
But Saturn - and Jupiter, which is even more massive than its ringed neighbor - may showcase a novel way that a planet could potentially be making the diamonds, he added.
As for whether Earthlings could one day send sturdy robots to mine the alien diamonds, it’s somewhere in the realm of possibility, Baines said, maybe in 500 years’ time. However, it might not be the most practical endeavor.
“It’s conceivable that that could happen, but it would be very expensive,” he said.
But if someone did, he added, a single shipment could flood Earth’s diamond market.
Diamonds are forever, unless you’re on Saturn or Jupiter.
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