You are free to share this article under the Attribution 4.0 International license. “Diamond rain,” an exotic type of precipitation long hypothesized to occur on giant ice planets, could be more common than previously thought, according to a study. In a previous experiment, researchers mimicked the extreme temperatures and pressures found deep inside the ice giants Neptune and Uranus and, for the first time, observed the diamond rain as it formed. By investigating this process in a new material that more closely resembles the chemical composition of Neptune and Uranus, scientists from the Department of Energy’s SLAC National Accelerator Laboratory and their colleagues discovered that the presence of oxygen makes diamond formation more likely, allowing them to form and develop. in a wider range of conditions and on more planets. The new study provides a more complete picture of how diamond rain forms on other planets and, here on Earth, could lead to a new way of making nanodiamonds, which have a very wide range of applications in drug delivery, medical sensors, non- invasive surgeries. sustainable manufacturing and quantum electronics. “The previous paper was the first time we saw direct diamond formation from any mixtures,” says Siegfried Glenzer, director of SLAC’s High Energy Density Division. “Since then, several experiments have been done with different pure materials. But inside the planets, it’s much more complicated. there are a lot more chemicals in the mix. And so what we wanted to understand here was what kind of effect these additional chemicals have.” The team, led by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the University of Rostock in Germany, as well as France’s École Polytechnique in collaboration with SLAC, report the results in Science Advances.
From plastic to “diamond rain”
In the previous experiment, the researchers studied a plastic material made from a mixture of hydrogen and carbon, key components of the overall chemical composition of Neptune and Uranus. But in addition to carbon and hydrogen, ice giants also contain other elements, such as large amounts of oxygen. In the latest experiment, the researchers used PET plastic – often used in food packaging, plastic bottles and containers – to more accurately replicate the composition of these planets. “PET has a good balance between carbon, hydrogen and oxygen for simulating activity on ice planets,” says Dominik Kraus, a physicist at the HZDR and professor at the University of Rostock. The researchers used a high-powered optical laser in the Matter in Extreme Conditions (MEC) instrument at SLAC’s Linac Coherent Light Source (LCLS) to generate shock waves in PET. They then investigated what happened to the plastic with X-ray pulses from the LCLS. Using a method called X-ray diffraction, they watched the material’s atoms rearrange themselves into small areas of diamond. At the same time they used another method called small-angle scattering, which had not been used in the first work, to measure how quickly and how large these regions grew. Using this additional method, they were able to determine that these diamond regions grew by as much as a few nanometers. They found that, with oxygen present in the material, the nanodiamonds were able to grow at lower pressures and temperatures than previously observed. “The effect of oxygen was to accelerate the breakdown of carbon and hydrogen and thus encourage the formation of nanodiamonds,” says Kraus. “It meant that carbon atoms could more easily combine to form diamonds.”
Neptune and Uranus
The researchers predict that the diamonds on Neptune and Uranus will grow to be much larger than the nanodiamonds produced in these experiments—perhaps millions of carats in weight. Over the course of thousands of years, the diamonds could slowly sink through the planets’ ice layers and accumulate in a thick layer of bling around the solid planetary core. The team also found evidence that, combined with diamonds, hyperionic water can also form. This newly discovered phase of water, often described as “hot, black ice,” exists at extremely high temperatures and pressures. In these extreme conditions, the water molecules break apart and the oxygen atoms form a crystalline lattice in which the hydrogen nuclei float freely. Because these free-floating cores are electrically charged, superionic water can conduct electricity and could explain the unusual magnetic fields on Uranus and Neptune. The findings could also affect our understanding of planets in distant galaxies, as scientists now believe that ice giants are the most common form of planet outside our solar system. “We know that Earth’s core is mostly iron, but many experiments are still investigating how the presence of lighter elements can change the melting and phase transition conditions,” says SLAC scientist and fellow Silvia Pandolfi. “Our experiment shows how these elements can change the conditions under which diamonds form in ice giants. If we want to accurately model the planets, then we need to get as close as we can to the true composition of the planetary interior.”
Fabrication of nanodiamonds
The research also points to a possible way forward for the production of nanodiamonds by laser impact compression of inexpensive PET plastics. Although they are already included in abrasives and polishes, in the future, these tiny gems could potentially be used for quantum sensors, medical contrast agents and reaction accelerators for renewable energy sources. “The way nanodiamonds are made right now is by taking a bunch of carbon or diamond and blowing it up with explosives,” says SLAC scientist and fellow Benjamin Ofori-Okai. “This creates nanodiamonds of various sizes and shapes and is difficult to control. What we see in this experiment is a different reactivity of the same species under high temperature and pressure. “In some cases, diamonds appear to form faster than others, suggesting that the presence of these other chemicals may speed up this process. Laser production could provide a cleaner and more easily controlled method of producing nanodiamonds. If we can design ways to change some things about the reactivity, we can change how fast they form and therefore how big they grow.” Next, the researchers are planning similar experiments using liquid samples containing ethanol, water and ammonia — the stuff Uranus and Neptune are mostly made up of — that will bring them even closer to understanding exactly how diamond rain forms on other planets. “The fact that we can recreate these extreme conditions to see how these processes play out on very fast, very small scales is exciting,” says SLAC scientist and collaborator Nicholas Hartley. “The addition of oxygen brings us closer than ever to seeing the full picture of these planetary processes, but there is still more work to be done. It’s a step on the way to getting the most realistic mix and seeing how these materials actually behave on other planets.” The research was supported by the DOE Office of Science and the National Nuclear Security Administration. LCLS is a user facility of the DOE Office of Science. Source: Ali Sundermier for Stanford University via Helmholtz-Zentrum Dresden-Rossendorf
