Diamonds gain their value from how rare they are on the surface of Earth. Just last week we learned there may be a "diamond factory" beneath our feet produced where Earth’s core meets the mantle, while diamonds in the sky turn out to be very real. From a more universal perspective, however, it looks increasingly like they are as common as muck.
Years ago planetary scientists proposed the extreme pressures inside the so-called “ice giants” Uranus and Neptune produce diamond rain, and even managed to make nanodiamonds by replicating these conditions in the lab. There’s evidence the way the diamonds sink through the gaseous material, generating heat from friction as they fall, is a common enough phenomenon to influence the planets’ heat balance.
Our Solar System has two ice giants to four rocky planets, but on a galactic scale, these may actually be the most common type of planet. Modeling of ice giants with somewhat more complex atmospheric compositions in Science Advances suggests diamond formation may be at least as common elsewhere.
Uranus and Neptune are both very low in oxygen in their gaseous outer layers, but their ammonia-water oceans require oxygen for the H2O. Their counterparts elsewhere may be richer in oxygen. The team that produced nano-diamonds under Neptune-like temperatures and pressures in a pure methane atmosphere sought to investigate whether the same thing would occur in an otherwise-similar oxygen-rich planet.
They turned to an unexpected material – PET, a clear, strong plastic often used in typical soft drink bottles. “PET has a good balance between carbon, hydrogen, and oxygen to simulate the activity in ice planets,” said Professor Dominik Kraus of the University of Rostock in a statement. The authors bombarded PET samples with X-ray lasers.
Rather than preventing diamond formation, Kraus and co-authors found oxygen actually makes it more likely. With oxygen present, diamonds form at lower temperatures and pressures than without.
“The effect of the oxygen was to accelerate the splitting of the carbon and hydrogen and thus encourage the formation of nanodiamonds,” Kraus said. “It meant the carbon atoms could combine more easily and form diamonds.”
The diamonds formed in this study are just a few nanometers wide, but the authors think they would grow under ice giant conditions to weigh millions of carats (at least 200 kilograms). Although the dense gases on worlds such as this would slow their fall, eventually the diamonds would sink to form a layer around the solid core.
Even the most imaginative schemes for planetary mining are unlikely to find a path to retrieving precious stones from beneath tens of thousands of kilometers of gas dense enough to produce pressures that form them. However, the discovery could prove practical in another way, with the researchers believing their technique for making nano-diamonds by shining lasers on PET could be commercially viable.
These diamonds would be unlikely to be large enough to threaten the gemstone industry, but could be used for abrasives, quantum sensors, and medical contrast agents.
“The way nanodiamonds are currently made is by taking a bunch of carbon or diamond and blowing it up with explosives,” said Stanford University’s Dr Benjamin Ofori-Okai. “This creates nanodiamonds of various sizes and shapes and is hard to control. What we're seeing in this experiment is a different reactivity of the same species under high temperature and pressure.” This might produce more consistent outcomes.