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Diamonds are forever, unless you’re on Saturn or Jupiter.

Loads of the superhard stones may be floating among the gas giants’ fluid layers and melted into liquid further into their depths, two planetary scientists proposed last week.

The research sprang from 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.

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“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 works at the Jet Propulsion Laboratory in La Cañada Flintridge in California.

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, which is why it’s so useful as pencil lead.

About 3,700 miles down — roughly the distance from Earth’s surface to its core — the pressure is 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. But at that point, the temperature is so high that even the diamonds can’t take it anymore, and they melt.

This process probably takes a long time, Baines estimated, perhaps on the order of a thousand years.

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