Diamonds are forever, unless you're on Saturn or Jupiter. Loads of the super-hard precious stones may be floating among the gas giants' fluid layers and melted into liquid further into their depths, say a pair of planetary scientists.
The research, being presented at the Division for Planetary Sciences conference this week in Denver, sprang from very humble beginnings — soot in Saturn's atmosphere, said Kevin Baines, a planetary scientist at the University of Wisconsin-Madison and one of the work's coauthors.
Baines, who works on the Cassini mission's Saturn-orbiting spacecraft, 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," Baines said. "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, Baines said. But where was this soot coming from? He and planetary scientist Mona Delitsky of California Specialty Engineering in Pasadena came up with an idea.
Saturn's atmosphere is mostly hydrogen, Baines said, but there's about a half percent of 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 and so the natural question is, what happens to the carbon dust eventually?" Baines said. "Eventually it's going to drift on down."
The researchers think that as the soot particles fall slowly through Saturn, they start to find one another and glom together. These bits of pure carbon may also act as seeds that pull the carbons out of the methane molecules they meet, growing over time.
By the time they've floated several hundred miles into the planet, the growing heat and pressure crush the carbon into graphite, where the atoms are 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 into the atmosphere, 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 22,400 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," she said.
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 go and mine the diamonds, sending sturdy robots to scoop the gems out of Jupiter and Saturn's layers, it's somewhere in the realm of possibility, Baines said, maybe in 500 years' time. And it might not be the most financially 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.