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 farther into their depths, a pair of planetary scientists proposed last week.
The research sprang from very humble beginnings – soot in Saturn’s atmosphere, said Kevin Baines, a planetary scientist at the University of Wisconsin.
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 (planet’s) 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 the Jet Propulsion Laboratory 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, 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 clump 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 is 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, diamonds. These diamonds grow into large pebbles as they bob around in the planet’s fluid layers, Baines said.
This entire process probably takes, Baines estimated, around 1,000 years.