Scientists recreate Neptune's 'diamonds rain' conditions on Earth

The international team of researchers included collaborators from the Department of Energy's SLAC National Accelerator Laboratory at Stanford University, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the University of Rostock in Germany and École Polytechnique in France. They utilized a new material for their latest experiment, which includes chemicals closer to the compositions of the ice giants. The team found that oxygen facilitates the formation of diamonds, aiding their growth in a wide variety of situations and planetary hosts.

How did they do it?

For both their last and current experiment, the scientists focused on plastic made from a mixture of carbon and hydrogen, two key ingredients in the chemical composition of Uranus and Neptune. In the newest experiment, the researchers tried to improve upon the accuracy of the chemistry they were trying to mimic by using PET plastic.

You can find PET in many common items like food packaging, containers, plastic bottles, etc. Dominik Kraus, a physicist at HZDR and professor at the University of Rostock, shared in a press release that they used PET (Polyethylene terephthalate) because it provided the right balance between carbon, hydrogen, and oxygen, allowing the scientists to simulate the conditions on the ice planets.

The plastic was subjected to a high-powered optical laser at the Matter in Extreme Conditions (MEC) instrument at SLAC's Linac Coherent Light Source (LCLS). After the laser created shock waves in the PET, the scientists then treated the material with X-ray pulses from the LCLS to observe what happened.

Employing the techniques of X-ray diffraction and small-angle scattering, the researchers observed the atoms rearrange into small regions of diamonds, tracking how quickly the growth took place. They were able to pinpoint that these regions grew up to a few nanometers wide. The x-factor was the presence of oxygen, which allowed the nanodiamonds to grow at lower pressures and temperatures.

As Kraus explained in the press release, "The effect of the oxygen was to accelerate the splitting of the carbon and hydrogen and thus encourage the formation of nanodiamonds."

How do we benefit on Earth?

That's great, but what about us here on Earth? How do we benefit from extraterrestrial diamond rains? The scientists say that their discoveries could lead to new ways of making nanodiamonds, or diamond nanoparticles, with a size below 1 micrometer. These can have a tremendous number of potential applications, whether in medical sensors, surgery, or quantum electronics.

In an email interview with Interesting Engineering, Professor Kraus elaborated on the possible benefits stemming from the improvements in the techniques used to create nanodiamonds that could result from their work. "We have shown that nanodiamonds can be created very effectively by simple laser-driven shock-compression of cheap PET plastics," he stated, adding that nanodiamonds do not only already have current useful applications, in polishing and prosthetics, for example, but will be utilized even wider in the future. Kraus sees them being used in catalysis (for example, for "Sun-driven CO2 reduction reactions to recycle CO2 back to combustible gases" like methane). Other uses may include enhanced medical imaging, new forms of drug delivery, as well as "ultra-small and very precise quantum sensors for temperature and magnetic fields which may result in a plethora of applications," as shared by the scientist.

He explained that at this time, small nanodiamonds are generally produced by explosives in a dirty process that's hard to control. Their technique of employing laser-driven shock compression of inexpensive plastics could allow for very precise control of nanodiamond formation, leading to a greater ability to "optimize and control this process."

He also believes it may be soon possible to achieve industrial scaling of their process thanks to new generations of "energetic lasers which could produce relevant conditions more than ten times per second." That would be a "revolutionary increase," according to Kraus, which would result in the production of nanodiamonds in larger quantities.  

Giant diamonds in outer space

Another fascinating detail shared by the scientists is that the diamonds on Neptune and Uranus may not be like the tiny nanodiamonds they are creating. The diamonds on the ice giants could even reach millions of carats in weight. Over millennia, these diamonds likely sink through the ice of the planets, gathering into thick layers around the solid planetary cores.

They also think that along with the diamond formation, superionic water might be forming — a special phase of water described as "hot, black ice" which can only exist in extreme conditions of very high temperatures and pressures. The presence of this water, which can conduct electric current, could also be the reason for the unusual magnetic fields observed on Uranus and Neptune.

Further research

To test their findings and processes further, the scientists plan to carry out similar experiments with other ingredients commonly found on Neptune and Uranus, this time using liquid samples comprised of ethanol, water, and ammonia.

In an email conversation with Interesting Engineering, Siegfried Glenzer, director of the High Energy Density Division at SLAC, shared that the best next step is to incorporate the diamond formation process in planetary models. These models could then be tested through a comparison of the radiation emission on these planets against NASA measurements. "Importantly, a proposed NASA mission to Neptune would allow us to perform more detailed measurements of the mass distribution inside the planet, which could provide compelling evidence for the existence of diamond rain," he also shared, referring to the Neptune Odyssey orbiter mission, slated to launch in 2033.

Professor Kraus is also hopeful that further probes might provide even greater insights, especially a potential NASA probe to Uranus within the next decade. He believes that as a result of their work, "the scenario of diamond precipitation in giant planets now seems very realistic and may be present to a larger extent than thought before."

Check out their study in Science Advances.

Abstract

Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H2O systems, respectively. Here, we investigate a stoichiometric mixture of C and H2O by shock-compressing polyethylene terephthalate (PET) plastics and performing in situ x-ray probing. We observe diamond formation at pressures between 72 ± 7 and 125 ± 13 GPa at temperatures ranging from ~3500 to ~6000 K. Combining x-ray diffraction and small-angle x-ray scattering, we access the kinetics of this exotic reaction. The observed demixing of C and H2O suggests that diamond precipitation inside the ice giants is enhanced by oxygen, which can lead to isolated water and thus the formation of superionic structures relevant to the planets' magnetic fields. Moreover, our measurements indicate a way of producing nanodiamonds by simple laser-driven shock compression of cheap PET plastics.