Diamond rain on ice giant planets could be more common than previously thought

Revealing a new path to make nanodiamonds here on Earth.

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Ice giant planets such as Neptune and Uranus are highly abundant in our galaxy. Their interiors are mainly composed of a dense fluid mixture of water, methane, and ammonia. Due to extreme conditions, there rains diamond.

In a previous experiment, scientists simulated the severe temperatures, and pressures found deep inside Neptune and Uranus‘s ice giants. For the first time, they were able to watch diamond rain form.

A new study has found that “diamond rain,” a long-hypothesized exotic type of precipitation on ice giant planets, could be more common than previously thought. The study offers a complete picture of how diamond rain forms on other planets and, here on Earth, could lead to a new way of fabricating nanodiamonds, which have a vast array of applications in drug delivery, medical sensors, noninvasive surgery, sustainable manufacturing, and quantum electronics.

Siegfried Glanzer, director of the High Energy Density Division at SLAC, said, “The earlier paper was the first time that we directly saw diamond formation from any mixtures. Since then, there have been many experiments with different pure materials. But inside planets, it’s much more complicated; many more chemicals are in the mix. And so, what we wanted to figure out here was what sort of effect these additional chemicals have.”

In a prior experiment, scientists looked at a plastic material consisting of hydrogen and carbon, two essential elements of Neptune and Uranus’ overall chemical makeup. But ice giants also include additional elements, such as significant amounts of oxygen and carbon, and hydrogen.

In a recent experiment, scientists used PET plastic to reproduce the composition of these planets more accurately.

Dominik Kraus, a physicist at HZDR and professor at the University of Rostock, said, “PET has a good balance between carbon, hydrogen, and oxygen to simulate the activity in ice planets.”

Scientists created shock waves in the PET using a high-powered optical laser at the Matter in Extreme Conditions (MEC) instrument at SLAC’s Linac Coherent Light Source (LCLS). They then explored what happened in the plastic with X-ray pulses from LCLS. 

Scientists later used X-ray diffraction to watch as the atoms of the material rearranged into small diamond regions. At the same time, they used another method called small-angle scattering to measure how fast and large those regions grew. This method helps them determine that these diamond regions grew up to a few nanometers wide. They discovered that nanodiamonds could develop at lower pressures and temperatures than previously noted when oxygen was present in the substance.

Kraus said, “The effect of the oxygen was to accelerate the splitting of the carbon and hydrogen and thus encourage the formation of nanodiamonds. It meant the carbon atoms could combine more easily and form diamonds.”

The team also discovered proof that superionic water might occur in combination with diamonds. This recently identified water phase frequently referred to as “hot, black ice,” can be found at extraordinarily high pressures and temperatures. 

Water molecules break under these severe conditions, and oxygen atoms organize into a crystal lattice where hydrogen nuclei are free to move about. Superionic water can conduct electric current due to the electrical charge on these free-floating nuclei, which may help to explain why Uranus and Neptune have peculiar magnetic fields.

The findings could also impact our understanding of planets in distant galaxies since scientists now believe ice giants are the most common form of a planet outside our solar system.

SLAC scientist and collaborator Silvia Pandolfi said, “We know that Earth’s core is predominantly made of iron, but many experiments are still investigating how the presence of lighter elements can change the conditions of melting and phase transitions. Our experiment demonstrates how these elements can change the conditions diamonds form on ice giants. If we want to accurately model planets, we need to get as close as we can to the actual composition of the planetary interior.”

The study also points to a potential route for manufacturing nanodiamonds from inexpensive PET plastics using laser-driven shock compression. These tiny gems are presently used in abrasives and polishing agents. Still, they may also be utilized in quantum sensors, medicinal contrast agents, and renewable energy reaction accelerators in the future.

SLAC scientist and collaborator Benjamin Ofori-Okai said, “The way nanodiamonds are currently made is by taking a bunch of carbon or diamond and blowing it up with explosives. 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. In some cases, the diamonds seem to be forming faster than in others, which suggests that the presence of these other chemicals can speed up this process. Laser production could offer a cleaner and more easily controlled method to produce nanodiamonds. If we can design ways to change some things about the reactivity, we can change how quickly they form and therefore how big they get.”

Scientists are planning similar experiments using liquid samples containing ethanol, water, and ammonia – what Uranus and Neptune are mostly made of – which will bring them closer to understanding exactly how diamond rain forms on other planets.

SLAC scientist and collaborator Nicholas Hartley said“The fact that we can recreate these extreme conditions to see how these processes play out on very fast, very small scales is exciting. Adding oxygen brings us closer than ever to see the full picture of these planetary processes, but there’s still more work to be done. It’s a step towards getting the most realistic mixture and seeing how these materials truly behave on other planets.”

Journal Reference:

  1. Zhiyu He et al. Diamond formation kinetics in shock-compressed C─H─O samples recorded by small-angle x-ray scattering and x-ray diffraction. Science Advances. Vol 8, Issue 35. DOI: 10.1126/sciadv.abo0617

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