“Diamond rain,” a long-hypothesized exotic type of precipitation on ice giant planets, could be more common than previously thought, according to a new study.
Researchers mimicked the extreme temperatures and pressures found deep inside ice giants Neptune and Uranus and, for the first time, observed diamond rain as it formed.
In a peer-reviewed study published in Science Advances, a research team discovered that the presence of oxygen makes diamond formation more likely, allowing them to form and grow at a wider range of conditions and on more planets.
The team was led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the University of Rostock in Germany, as well as France’s École Polytechnique in collaboration with scientists from the Department of Energy’s SLAC National Accelerator Laboratory.
The interior of ice giants is thought to be mainly composed of a dense, fluid mixture of water, methane and ammonia. Because of the high pressures and temperatures deep inside these planets, the material mixture will likely undergo chemical reactions and structural transitions.
An example of these reactions is the possible dissociation of hydrocarbons and subsequent phase separation, allowing for the formation of diamonds.
Thanks to the advent of x-ray free-electron laser (XFEL) facilities and increasingly mature experimental capabilities, probing the internal structure of materials under dynamic compression to mimic planetary interiors has seen tremendous progress in recent years.
Apart from providing a more complete picture of how diamond rain forms on other planets, the study could lead to a new way of fabricating nanodiamonds, which have a wide array of applications in drug delivery, medical sensors, noninvasive surgery, sustainable manufacturing and quantum electronics.
“The way nanodiamonds are currently made is by taking a bunch of carbon or diamond and blowing it up with explosives,” said SLAC scientist and collaborator Benjamin Ofori-Okai. “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," he said. "In some cases, the diamonds seem to be forming faster than 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.”
The researchers are planning similar experiments using liquid samples containing ethanol, water and ammonia – what Uranus and Neptune are mostly made of – which will bring them even closer to understanding exactly how diamond rain forms on other planets.
New research methods
The researchers used a high-powered optical laser at the Matter in Extreme Conditions (MEC) instrument at SLAC’s Linac Coherent Light Source to create shock waves in polyethylene terephthalate (PET) plastic. Then, they probed what happened in the plastic with X-ray pulses from the LCLS.
Using a method called X-ray diffraction, they watched as the atoms of the material rearranged into small diamond regions. They simultaneously used another method called small-angle scattering, which had not been used in the first paper, to measure how fast and large those regions grew.
Using this additional method, the researchers were able to determine that these diamond regions grew up to a few nanometers wide. They found that, with the presence of oxygen in the material, the nanodiamonds were able to grow at lower pressures and temperatures than previously observed.
“The effect of the oxygen was to accelerate the splitting of the carbon and hydrogen and thus encourage the formation of nanodiamonds,” according to Dominik Kraus, a physicist at HZDR and professor at the University of Rostock. “It meant the carbon atoms could combine more easily and form diamonds.”
“The earlier paper was the first time that we directly saw diamond formation from any mixtures,” said Siegfried Glenzer, director of the High Energy Density Division at SLAC.
“Since then, there have been quite a lot of experiments with different pure materials. But inside planets, it’s much more complicated; there are a lot more chemicals in the mix. And so, what we wanted to figure out here was what sort of effect these additional chemicals have.”
“Since then, there have been quite a lot of experiments with different pure materials. But inside planets, it’s much more complicated; there are a lot more chemicals in the mix. And so, what we wanted to figure out here was what sort of effect these additional chemicals have.”Siegfried Glenzer
Advances made due to prior research
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 those two elements, ice giants contain other ones, such as large amounts of oxygen.
In the more recent experiment, the researchers used PET plastic – often used in food packaging, plastic bottles and containers – to reproduce the composition of these planets more accurately.
“PET has a good balance between carbon, hydrogen and oxygen to simulate the activity in ice planets,” Kraus said.