RESOLVING THE LEDGER
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Under their thin upper atmospheres, our solar system neighbors Neptune and Uranus are literally raining diamonds in their high-pressure, high-temperature, middle atmospheres. Conditions there are so extreme that researchers could only replicate them in a test space less than a few thousandths of an inch (or a few hundred micrometers) thick —using test materials blasted with lasers creating high-pressure shock waves hotter than the surface of the Sun .
But that was nearly a decade ago. Now researchers at the Lawrence Livermore National Laboratory (LLNL) in northern California have managed to recreate the conditions that exist still deeper within the atmospheres of these ice giant planets, while also recording how this diamond rain behaves deeper in these even higher pressure regions.
“We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus—and still measure atomic structure, temperature, density and optical reflectivity,” the study’s first author Marius Millot, a physicist at LLNL, explained in a statement.
Beyond helping planetary scientists model Uranus and Neptune’s phenomenally inhospitable atmospheres, the experiment has also revealed something (potentially) useful here on Earth: These high-energy, laser-pulsed shock waves could one day be deployed in fusion power systems known as inertial confinement, where they could triple such device’s energy gain.
The team at LLNL has been working on inertial confinement fusion ever since construction began on their National Ignition Facility (NIF) in 1997. The research has been laborious and not infrequently met with dismissiveness and skepticism —even when its fusion experiments first successfully produced more energy than its ignition lasers had put into it back in 2022.
This is where Millot’s findings, published this month in the journal Nature Physics, might help. The inertial confinement fusion process begins with a miniscule diamond capsule of fuel imploded by similarly powerful shock waves generated by similar high-energy lasers. This melting, imploding diamond needs to remain a uniform fluid for this ignited fusion reaction to keep up its momentum—something Millot’s group has managed to do with less power than past tests.
“Our work indicates that we could use slightly slower initial shocks and still achieve full melting of the diamond in our NIF implosions,” according to Millot.
“This is exciting because such a slower shock would make the fusion fuel more compressible,” he added. “That in turn increases the maximum energy yield we could obtain with the same laser energy.”
Ultimately, Millot and his colleagues are investigating how carbon behaves when it’s crunched into diamond formations at extreme pressure ranges up in the terapascals, which is to say tens of million times the ordinary atmospheric pressure you’re experiencing reading this right now. So, perhaps it goes without saying that these are not exactly conditions astronauts or space probes could simply go visit and measure inside either Uranus or Neptune’s truly alien worlds.
The researchers noted that their results offer “atomic-scale benchmarks” for improving quantum simulations of how matter likely behaves under these ice giants’ extreme atmospheric conditions. Past computer models had implied that these diamonds melted via a theorized intermediate step—one that Millot’s team discovered did not appear to actually exist during their more precisely measured experiments.
It turns out that the carbon atoms stayed locked tight in their diamond alignment, right until the melting began.
“We think that is because the sample does not have time to change when it only experiences a single shock,” Millot opined. “It remains ‘trapped’ in the diamond structure.”
These findings had eluded researchers before because, frankly, taking precise measurements via X-ray diffraction gets a little challenging when you are simultaneously blasting a tiny object with this much power. “These measurements are extremely difficult because carbon is a small and lightweight atom,” Millot said. “It scatters very few X-rays, so the signal we needed to measure was quite faint.”
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