Researchers suspect that tiny diamonds could pepper the lower cloud decks of Jupiter and Saturn.
Arthur C. Clarke could only be so right. In the sequels to his famous 2001: A Space Odyssey, the renowned sci-fi author described Jupiter’s core as a gigantic diamond — one that, if my childhood memory serves, is later used to build space elevators linking the surface with a giant space-station ring around Earth.
In reality, Jupiter’s core is too hot for solid diamond to exist. But new work based on lab experiments and theoretical calculations suggests that both Jupiter and Saturn do have solid bits of diamond floating deep inside them.
Astronomers have suspected since 1981 that solid diamond exists inside Uranus and Neptune. Experiments published in Nature that year showed that shock waves could fracture methane (CH4) molecules, liberating the carbon atoms. These atoms would then clump together and, as they descended into the increasing temperatures and pressures inside the planets, be transformed into diamond.
But experiments couldn’t recreate the more extreme conditions inside Jupiter and Saturn until about a decade ago, leaving scientists in the dark about which forms carbon took there.
Building on their own observations with Cassini, as well as on investigations by Nadine Nettelmann (University of California, Santa Cruz) and others, Mona Delitsky (California Specialty Engineering) and Kevin Baines (University of Wisconsin–Madison) have determined that diamond should be the most stable form of carbon for a sizeable fraction of the gas giants’ interiors. On Saturn, the solid diamond should exist from about 6,000 to 36,000 kilometers (4,000 to 23,000 miles) below Saturn’s cloud tops, reaching about halfway down into the planet (Saturn’s radius is about 60,000 km). On Jupiter the conditions are slightly different, but there should be a thinner diamond layer in that planet, too.
In Delitsky and Baines’s scenario, carbon is yanked from methane by lightning. Their team’s Cassini observations of lightning on Saturn showed that a bolt’s shock wave and heat (we’re talking up to 30,000 kelvin, or 54,000° F) tears methane’s hydrogen and carbon atoms apart, and these carbon atoms clump together to form carbon soot — basically, the black fluffy stuff coming from a fireplace, Delitsky said in a press conference October 9th at the American Astronomical Society’s Division for Planetary Sciences meeting in Denver.
As these carbon particles fall deeper into the atmosphere, the temperature and pressure increase, affecting which form the carbon takes. In the top 12% of Saturn’s atmosphere, the most stable form of carbon is graphite, the stuff in pencils. Deeper down, these particles become diamond. The diamond layer extends from a temperature of 3,000 K and an atmospheric pressure 70,000 times that at Earth’s sea level to 8,200 K and 5 million times sea-level pressure.
This layer is actually pretty sparse. Lightning on Saturn should produce about 1,000 metric tons of carbon per year — which sounds like a lot, until you consider the gas giant’s total volume. The diamond layer takes up about 600 trillion cubic kilometers inside Saturn, and even assuming the diamond lasts there for 1,000 years, there should only be one millimeter-size diamond per cubic kilometer of atmosphere. Below the latitude bands where lightning regularly appears (around 35° N and S), that concentration could be ten times higher. But Baines stresses that these numbers are all rough estimates; the real values could easily be 10 times higher or lower.
Still, if we could pan the entire 600 trillion cubic kilometers of this layer in Saturn, we’d gather about 1 million tons of diamond. Due to the higher gravity and lower availability of methane on Jupiter, the king of the planets should have roughly a factor of 20 less diamond than Saturn, Baines says.
Deeper inside Jupiter and Saturn, the temperature and pressure are too high for solid diamond to survive: at about 8,000 K (for both planets), the diamond melts. Whether the resulting substance would actually be liquid diamond instead of liquid graphite is unclear: it depends on the atomic geometry.
In contrast, Uranus and Neptune never reach high enough core temperatures to melt their diamond. Pity that Neptune didn’t suit Clarke’s purposes: if he’d postulated a diamond-encrusted core for the blue ice giant, he might have come closer to the truth.