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 a new experiment. They fired a laser at a thin film of simple PET plastic and examined what was happening with intense laser flashes. One result was that the researchers were able to confirm their earlier claim that diamonds really do rain in the ice giants at the edge of our solar system. And another was that this method could be a new way to produce nanodiamonds, which are needed, for example, for highly sensitive quantum sensors. The group presented its findings in the journal scientific progress.
Conditions inside icy giant planets like Neptune and Uranus are extreme: temperatures reach several thousand degrees Celsius and pressures millions of times greater than in Earth’s atmosphere. Still, such states can be briefly simulated in the lab: Powerful laser flashes hit a film-like sample of material, heating it to 6,000 degrees Celsius in the blink of an eye, and generating a shock wave that compresses the material to a million times the atmospheric pressure for a few nanoseconds. “Until now, we used hydrocarbon films for this kind of experiment,” explains Dominik Kraus, physicist at HZDR and professor at the University of Rostock. “And we found that this extreme pressure produced tiny diamonds, known as nanodiamonds.”
Using these films, however, it was only partially possible to simulate the interiors of planets – because ice giants contain not only carbon and hydrogen, but also enormous amounts of oxygen. While searching for 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 in ice planets,” explains Kraus. The team conducted its experiments at the SLAC National Accelerator Laboratory in California, the site of the Linac Coherent Light Source (LCLS), a powerful accelerator-based X-ray laser. They analyzed what happens when intense laser flashes hit a PET film, using two measurement methods at once: X-ray diffraction to determine whether nanodiamonds were produced and so-called small-angle scattering to see how fast and how large the diamonds grew.
A big help: oxygen
“The effect of the oxygen was to accelerate the splitting of carbon and hydrogen and thus encourage the formation of nanodiamonds,” said Dominik Kraus, who reports on the results. “It meant that the carbon atoms could more easily combine and form diamonds.” This further supports the assumption that it literally rains diamonds into the ice giants. The findings are likely relevant not only to Uranus and Neptune, but also to countless other planets in our galaxy. While such ice giants used to be considered rarities, it now seems clear that they are probably the most common form of planet outside the solar system.
The team also encountered other hints: 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 can move freely.” Because the nuclei are electrically charged, superionic water can conduct electrical current and help create the magnetic field of the ice giants. However, in their experiments, the research group was not yet able to unequivocally prove the existence of superionic water in the mixture with diamonds. This is planned in close collaboration with the University of Rostock at the European XFEL in Hamburg, the world’s most powerful X-ray laser. There, HZDR leads the international user consortium HIBEF, which offers ideal conditions for this type of experiment.
Precision factory for nano diamonds
In addition to this rather fundamental knowledge, the new experiment also opens up prospects for a technical application: the tailor-made production of nanometer-sized diamonds, which are already contained in abrasives and polishes. In the future, they should be used as highly sensitive quantum sensors, medical contrast agents and efficient reaction accelerators, for example for splitting CO2. “Until now, these types of diamonds have mainly been produced by detonating explosives,” explains Kraus. “Using laser flashes, they could be manufactured much cleaner in the future.”
The scientists’ vision: A high-powered laser fires ten flashes per second at a PET film that is illuminated by the beam at intervals of one-tenth of a second. The resulting nanodiamonds shoot out of the film and end up in a container filled with water. There they are slowed down and can then be filtered and harvested effectively. The essential advantage of this method as opposed to production by explosives is that “the nanodiamonds can be cut to size with respect to size or even doped with other atoms,” emphasizes Dominik Kraus. “The X-ray laser means we have a lab tool that can precisely control the growth of the diamonds.”
Materials supplied by Helmholtz Center Dresden-Rossendorf. Note: Content is editable for style and length.