Are there diamonds in Jupiter and Saturn's skies?

On Earth, diamonds bankroll dictatorships and criminal syndicates and furnish a multibillion dollar industry that touts the jewels as promises of love.

On Jupiter and Saturn, diamonds don't do any of this. But their mere theoretical presence in these two planet's skies is as enthralling a surprise to scientists as a diamond ring is to an enthusiastic bride-to-be, or to a crook dreaming of big profits from the little gem.

While diamonds have long been believed to salt the cold planets Uranus and Neptune, there had been little work on if the gems could be formed on the hotter planets with more pressurized cores, let alone exist in solid form. Most of the data had suggested that this was improbable: Jupiter and Saturn are methane poor (methane yields carbon, which is what diamonds are made of) and have cores at such galling pressures that diamond would not seem to exist there.

But, in recent years, scientists have revised their understanding of the processes and conditions on our solar system's fifth and sixth planets. At the same time, scientists are also revising their predictions for diamonds’ behavior under extreme pressure and temperature. This week, these cross-disciplinary revisions in findings culminated with a paper presented at the American Astronomical Society’s conference in Denver, Colo. 

The paper’s authors are Mona L. Delitsky, a researcher at California Specialty Engineering, and Kevin H. Baines, a professor at the University of Wisconsin-Madison.

The idea that diamonds could stud the skies of Saturn and Jupiter began in 2004, with the Cassini probe’s arrival in the Saturn system. As Cassini surveyed Saturn’s skies, scientists reported that there appeared to be lightening storms in in the planet’s atmosphere. This was exciting news – these storms could be brewing up countless chemical reactions. Then, three years ago, Delitsky and Baines reported that the darkest, stormiest regions of these thunderstorms were in fact brimming with methane, a gas that, when broken down, yields carbon.

These two factors combined into an intriguing coincidence: What little methane Saturn had was harbored in just the right spot to fuel an extraterrestrial diamond rush. The lightning, the scientists now propose, provides the requisite energy to disassociate methane into hydrogen and carbon. These storms boil at some 53,540 degrees Fahrenheit, the researchers say; tests have shown that carbon can be broken down at about 3,140 degrees Fahrenheit.

And where there is carbon, there can be diamonds – if the temperature and the pressure are right. The new carbon, denser than methane, begins to fall through the atmosphere, the scientists say. As it reaches denser and denser altitudes, it too becomes denser. At a certain point, it becomes graphite. Then, under more and more pressure, it becomes diamond.

So it could be raining diamonds. But for how long could those diamonds last? How far could those diamonds seep down through the atmosphere before mounting pressure melts them into liquid carbon?

The researchers’ work is a based on a European team's 2011 paper that fine tunes the melting point of diamond to account for the interplay of temperature and pressure: that paper put the isotope’s melting point at 500 gigapascals to be 13,940 degrees Fahrenheit. Based on this number, the team suggests that diamond could remain stable well into both planets’ atmospheres, existing possibly as a layer, the authors said.  

“There may be diamond rain or diamond oceans sitting as a layer” on Saturn and Jupiter, the authors wrote (there is no such thing, though, as a “diamond ocean” – melted diamond is liquid carbon).

As the diamonds near the cores of the planets, though, they do begin to melt. In its interior, Saturn could have some regions capable of entertaining stable diamond reserves, while other regions would be hot and pressurized enough to melt the gem into liquid, the researchers said. Jupiter’s super hot and extremely pressurized interior would contain just liquid carbon, no diamonds, they said.

“It appears that diamonds are forever on Uranus and Neptune but not on Jupiter and Saturn,” the authors of the new paper wrote.

It has been proposed since at least 1981, in Marvin Ross's paper called “The ice layer in Uranus and Neptune—diamonds in the sky?,” that both Uranus and Neptune boast treasure troves of diamonds. Both of the planets, which are thought to have three layers, are believed to keep the jewels in their middle “ice” layers.

Jon Eggert, a researcher at the Lawrence Livermore National Laboratory in Livermore, California who was not involved in the research, said that it is “highly likely” that Jupiter and Saturn’s methane could convert to carbon and that the planets’ could have liquid carbon in their cores. Dr. Eggert is an author on a 2010 paper, published in Nature, proposing that diamond could remain stable even at the cores of the seventh and eight planets, based on work modeling the theoretical phase changes of carbon.

But to comment on if and where carbon could exist in diamond form on Jupiter and Saturn, Eggert said would need to see more specifics on the numbers the team used to draw their conclusion, once the paper is published. He also said that he would need to see more evidence supporting the conclusion that diamond might form a layer on the planets, rather than continuing to fall toward the core.

“It that’s true, that would be a big deal,” says Eggert, noting that such a layer would fundamentally change current understanding of the processes, such as temperature flow, within Saturn.