Scientists melted a diamond and cracked a secret of ice giants

Some of the strangest weather in the solar system doesn’t happen on Earth or even in Jupiter’s Great Red Spot—it happens in the interiors of ice giants like Neptune and Uranus. Specifically, scientists have long believed that, at certain pressures and temperatures, it literally rains diamonds inside these planets. For the first time, scientists have mimicked the process they believe creates this phenomenon.

A new paper by physicists at Lawrence Livermore National Laboratory (LLNL), published in Nature Physics, resolves the 20-year-old scientific mystery and shows how the same physics that makes it rain diamonds inside Neptune could also help us triple our fusion energy output.

Let’s talk about the actual experiment first. The LLNL scientists set up their experiment at the University of Rochester’s Omega Laser Facility—which does exactly what its name suggests. In this case, the laser vaporized the outer layer of a diamond sample, and the scientists watched it with as many sensors as they could, including an ultrafast technique called X-ray diffraction. That vaporization sent a huge shock wave through the interior of the diamond itself, compressing it to pressures more than three times that of Earth’s core, with resulting temperatures equivalent to the surface of the sun—but only for a billionth of a second.

What they saw helped solve a 20-year-old mystery. This wasn’t the first time scientists had melted diamond. However, previous physical measurements of diamond’s melting point disagreed with computer models based on quantum mechanics by up to 20%. That might not sound like much, but in the context of these experiments, that 20% represented a difference of more than 1,000 kelvins between the theoretical and observed melting points.

Despite their best efforts, scientists couldn’t get the two numbers to match—until now. Armed with the new X-ray diffraction data, the authors were able to confirm that their measured melting point of diamond was consistent with modern quantum physics-based models. But that wasn’t their most interesting finding—they also discovered that, under the right conditions, diamonds would float on a sea of carbon.

As a press release from LLNL points out, we’re all familiar with one specific example in which the solid version of a material is less dense than the liquid version—ice and water. But finding that solid diamond is actually less dense than liquid metallic carbon was somewhat surprising for the researchers. It opened up the possibility of what, on paper, sounds like one of the coolest weather features anywhere in the solar system—diamond rain.

Ask a planetary scientist, and they will tell you that diamonds aren’t really all that uncommon. In fact, there are billions of tons of them hiding away in the deep interiors of planets in our solar system and beyond. The most enthusiastic will tell you about diamond rain—where a planet’s temperature and pressure are so great that diamond literally melts and forms a liquid, dropping toward the planet’s core.

The friction and heat these diamond raindrops generate are among the primary drivers of Neptune’s excess energy, which it then radiates back out into space. For the first time, the LLNL paper provides solid experimental data on the precise pressures and temperatures needed for such an awe-inspiring event to occur.

Not that we will be able to see it actually occurring anytime soon. But the paper has a more practical impact on a related field that hits closer to home—nuclear fusion. LLNL is home to America’s National Ignition Facility (NIF), where scientists are using inertial confinement fusion to try to create a power-positive fusion reaction. So far, they’ve gotten close, but that prize, “net positive” fusion reaction remains elusive.

As the authors explain in the paper, though, one notable outcome of their experiment, which used a diamond similar to the “target” in NIF experiments, is that they realized they don’t have to blast the diamond with as much power as had been the norm for NIF experiments so far. The authors believe a slower, gentler initial shock can still achieve the outcome of a perfectly melted diamond capsule. According to their calculations, it could result in three times the amount of fusion energy released by the fuel contained in that capsule.

That is a massive increase, but still not enough to push NIF over the “engineering breakeven” threshold (i.e., where it could be used as a power plant)—and, to be completely frank, it is still orders of magnitude off that level at this point.

But in the world of science, it’s sometimes fun to think about how a single experiment can have impacts as broad as increasing fusion energy yield and defining the gem-encrusted interiors of planets. Maybe someday we’ll use fusion-powered spaceships to visit the interior of Neptune and see the raining diamonds in real time—at which point the work published in this paper will have come full circle.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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