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More than a billion miles away from Earth, on the ice giants of Neptune and Uranus, diamonds are forever. This is not cosmic poetry, but a reasonable scientific conclusion: we know that under extreme pressures and high temperatures miles below a planet’s surface, hydrocarbons are beaten into a crystalline bling coveted by hobbyists. But in distant Neptune and Uranus, the universe’s diamond-making process is a little more curious. Since the 1970s, scientists have believed that diamonds could rain down into the rocky interiors of mostly muddy planets—a diamond shower, if you will.
In 2017, researchers in Germany and California found a way to replicate these planetary conditions, making tiny diamonds called nanodiamonds in the lab out of polystyrene (aka Styrofoam). Five years later and they’re at it again, this time using some polyethylene terephthalate (PET), according to a study published Friday in Science Advances. The research has implications not only for our understanding of space, but opens a path to creating nanodiamonds that are used in a variety of contexts from plastic waste.
So why are we making diamonds out of the same plastic that things like food containers and water bottles are made of? There’s a good reason for that, Dominik Kraus, a scientist at Germany’s Helmholtz-Zentrum Dresden-Rossendorf research laboratory and lead author of the study, said in an email.
When Kraus and his colleagues first tried to make nanodiamonds out of polystyrene, which contains the same elements of carbon and hydrogen found in Neptune and Uranus, they did so by bombarding the material with the Linac coherent light source, a high power X-ray laser. the SLAC National Acceleration Laboratory in California. This process quickly heated the polystyrene to 5,000 Kelvin (about 8,540 degrees Fahrenheit) and compressed it to 150 gigapascals, similar to conditions found about 6,000 miles inside the icy planets.
Although the researchers were able to make the microscopic bling with two quick hits of the laser, they later realized that a vital chemical ingredient was missing: oxygen. So they turned to PET, which has a good balance of not only carbon and hydrogen but also oxygen, making it a closer chemical substitute for the ice giants than polystyrene.
“The chemistry under these conditions is very complex and modeling extremely difficult. ‘Anything can happen’ is a typical phrase when discussing these scenarios with theorists,” Kraus said. “In fact, there were some predictions showing that the presence of oxygen is helping [carbon separate from hydrogen] and diamond formation, but also ideas that it can be the other way around.
To put the theoretical pedal to the metal, Kraus and his colleagues took a piece of PET, put it through the same experimental motions as in 2017, but also included something called small-angle X-ray diffraction to see how fast and how big diamonds grow.
“We found that the presence of oxygen enhances rather than prevents diamond formation, making ‘diamond rain’ within these planets a more likely scenario,” Kraus said. “We [also] see that diamonds grow larger at higher pressures and with progressive time in experiments.”
They were also able to extract many tiny diamonds from a single X-ray shot, on the order of a few billion crystallites (or a few micrograms if you’re talking total weight). But Kraus said that’s not enough, at least right now, for application purposes like diamond quantum sensors, which are used to detect magnetic flux, or chemical catalysts, which need a couple of milligrams at least. However, it could eventually be expanded to serve these purposes and be the first step towards a more elegant way to recycle plastic.
“If the industrial scale of the formation process really works as discussed above, and nanodiamonds will be needed in very large quantities for certain processes, for example catalysis for light-induced CO2 reduction reactions that help to reduce global warming, this can become a potential way to recycle large amounts of PET,” said Kraus.
While making shiny micro-bags is all well and good, it’s important not to lose sight of the scientific intent: to better understand how the extreme environmental conditions of our icy planetary neighbors result in literal showers of diamonds. To that end, Kraus and his team believe they also found more evidence of a strange type of water that was first theorized but then definitively discovered in 2019.
Called superionic water, it acts like a strange cross between solid and liquid, the New York Times reported in 2018, and is believed to fill the mantles of Neptune, Uranus and potentially countless other ice giants and other planets. It may have no application to us on Earth, but its presence may explain why some celestial bodies have peculiar magnetic fields. Kraus said the finding that nanodiamonds actually form inside ice giants makes it more likely that the conditions for superionic water will occur.
“In our experiments, we still did not see direct evidence of superionic water forming along with diamonds,” Kraus said. “[But] Our experiments show that carbon is separating from hydrogen and oxygen, allowing regions of pure water to form inside the planets. Thus, making diamond precipitation a more realistic scenario within these planets, the formation of superionic water is also more likely.”
Aside from superionic water, Kraus and his colleagues need to do a little more research on the nanodiamond front. They are looking for ways to make large quantities of small jewelry in minutes and with more accessible, but still high-energy laser systems. They could even try introducing elements like nitrogen to see how that would affect the shape of the nanodiamond. (Nitrogen is quite common in diamonds: about 98 percent of natural diamonds contain tens to several hundred parts per million nitrogen atoms.)
If anyone reading this is thinking of making a Breaking Bad but with diamonds and old water bottles, maybe just leave the complicated physics to the experts.
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