Universe Today
Some of the strangest weather in the solar system doesn’t happen on Earth, or even Jupiter’s Great Red Spot - it happens in the interior of the Ice Giants like Neptune and Uranus. Specifically, scientists have long believed that, at certain pressure and temperatures, it literally rains diamonds inside of these planets. And for the first time, scientists have mimicked the process they believe creates that. A new paper by physicists at the Lawrence Livermore National Laboratory (LLNL), published in Nature Physics, resolves a 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 is advertised in its name. In this specific case, the laser vaporized the outer layer of a diamond sample - and the scientists were watching it with as many sensors as they could, including an ultra-fast technique called X-ray diffraction. That vaporization sent a huge shockwave through the interior of the diamond itself, compressing it to pressures more than three times that of Earth’s core, with resultant 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 have melted diamond. However, previous physical measurements of the melting point of diamond disagreed with computer models based on quantum mechanics by up to 20%. That might not sound like a lot, but in the context of these experiments, that 20% represented a more than 1,000 degree Kelvin difference in the theoretical and observed melting point.
Fraser talks about efforts to create diamond rain in a lab.Despite their best efforts, scientists couldn’t get the two numbers to match up - until now. Armed with the new X-ray diffraction data, the authors were able to confirm their measured melting point of diamond aligned 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 where 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 of a surprise for the researchers. And 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’s billions of tons of them hiding away in the deep interiors of planets in our solar system and beyond. And the most enthusiastic will tell you about diamond rain - where the temperature and pressure of a planet is so much that diamond literally melts and forms a liquid, dropping toward a planet’s core.
Fraser talks about what the interior of Uranus is made out of.The friction and heat these diamond raindrops generate is one of the primary drivers of Neptune's excess energy that it then radiates back out into space. And for the first time, the LLNL paper provides solid, experimental data on exactly the pressures and temperatures for such an awe-inspiring event to occur.
Not that we will be able to see it actually occurring anytime soon. But the paper does have a more practical impact on a related field that hits closer to home - nuclear fusion. LLNL is the home of 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 still 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. And, according to their calculations, 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 of 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.
Learn More:
LLNL - Melting diamond could unlock triple fusion gain and the secrets of ice-giant planets
M. Millot et al - Diamond melting in shock compression experiments at 1 TPa pressures
UT - Diamond Rain on Ice Giants Could Influence Their Magnetic Fields
UT - Hallelujah, It's Raining Diamonds! Just like the Insides of Uranus and Neptune.
