What happens inside planets like Neptune and Uranus? To find out, an international team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the University of Rostock and the French École polytechnique conducted an unprecedented experiment. They fired a laser at a thin film of plain PET plastic and investigated what happened using intensive laser flashes. One of the results was that the researchers were able to confirm their earlier thesis that it really is raining diamonds inside the ice giants on the outskirts of our solar system. And another was that this method could establish a new way to produce nanodiamonds, which are needed, for example, for highly sensitive quantum sensors. The group presented their findings in the journal Scientists progress.
Conditions inside icy giant planets like Neptune and Uranus are extreme: temperatures reach several thousand degrees Celsius and pressure is millions of times greater than that of Earth’s atmosphere. Nevertheless, states like this can be briefly simulated in the laboratory: powerful laser flashes strike a sample of film-like material, heat it up to 6,000 degrees Celsius in the blink of an eye and generate a shock wave which compresses the material for a few nanoseconds. a million times atmospheric pressure. “Until now, we used hydrocarbon films for this kind of experiments,” explains Dominik Kraus, physicist at HZDR and professor at the University of Rostock. “And we found that this extreme pressure produced tiny diamonds, called nanodiamonds.”
Using these films, however, it was only partially possible to simulate the interiors of planets, since the ice giants not only contain carbon and hydrogen, but also large amounts of oxygen. While searching for a suitable film material, the group came across an everyday substance: PET, the resin from which ordinary plastic bottles are made. “PET has a good balance of carbon, hydrogen and oxygen to simulate the activity of ice planets,” says Kraus. The team conducted their experiments at the SLAC National Accelerator Laboratory in California, home to the Linac Coherent Light Source (LCLS), a powerful accelerator-based X-ray laser. They used it to analyze what happens when intense laser flashes hit PET film, using two measurement methods at the same time: X-ray diffraction to determine if nanodiamonds have been produced, and so-called small scattering. angles to see how fast and how big the diamonds have grown.
A big help: oxygen
“The effect of the oxygen was to accelerate the separation of carbon and hydrogen and thus to encourage the formation of nanodiamonds”, explains Dominik Kraus, reporting the results. “This meant that carbon atoms could combine more easily and form diamonds.” This further confirms the hypothesis that it is literally raining diamonds inside the ice giants. The findings are likely not only relevant to Uranus and Neptune, but also to countless other planets in our galaxy. While these ice giants were once considered rarities, it now seems clear that they are probably the most common form of planet outside the solar system.
The team also encountered clues of another type: in combination with the diamonds, water should be produced, but in an unusual variant. “So-called superionic water may have formed,” Kraus said. “The oxygen atoms form a crystal lattice in which the hydrogen nuclei move freely.” Because the nuclei are electrically charged, superionic water can conduct electrical current and thus help create the ice giants’ magnetic field. In their experiments, however, the research group has not yet been able to unequivocally prove the existence of superionic water in the mixture with diamonds. This should happen in close cooperation with the University of Rostock at the European XFEL in Hamburg, the most powerful X-ray laser in the world. The HZDR leads the international user consortium HIBEF there, which provides ideal conditions for such experiments.
Precision plant for nanodiamonds
In addition to this rather fundamental knowledge, the new experience also opens up prospects for a technical application: the tailor-made production of nano-sized diamonds, which are already included in abrasives and polishing agents. In the future, they are expected to be used as highly sensitive quantum sensors, medical contrast agents and efficient reaction accelerators, for example to separate CO2. “Until now, diamonds of this type have mainly been produced by detonating explosives,” says Kraus. “With the help of laser flashes, they could be made much cleaner in the future.”
The vision of the scientists: A high performance laser fires ten flashes per second on a PET film which is illuminated by the beam at intervals of a tenth of a second. The nanodiamonds thus created shoot out of the film and land in a collection tank filled with water. There they are slowed down and can then be filtered and harvested efficiently. The essential advantage of this method compared to the production by explosives is that “the nanodiamonds could be cut to measure in terms of size or even doped with other atoms”, underlines Dominik Kraus. “The X-ray laser means we have a lab tool that can precisely control the growth of diamonds.”
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