On Neptune and Uranus, Diamonds Rain Down from the Sky

Photo credit: Greg Stewart/SLAC National Accelerator Laboratory Scientists have recreated conditions on Neptune and Uranus in a lab at Stanford’s SLAC National Accelerator Laboratory.

The team explored how sheets of diamond rain form on the ice giants using lasers.

The phenomenon may explain why Neptune’s core is strangely hot.

Scientists don’t know much about the ice giants on the other end of our solar system. They’re a constant source of mystery and intrigue.

Take the conundrum, for example, of how the chemical reactions inside of Neptune and Uranus may cause diamonds to rain down on the planets’ cores. Under immense pressure deep below the planets’ surfaces, carbon and hydrogen atoms are smushed together, forming the crystals.

Scientists first conducted an experiment to explore this phenomenon in 2017, but years later they’ve finally narrowed down exactly how these diamonds likely formed, publishing their results May 26 in the journal Nature Communication.

“Our experiments are delivering important model parameters where, before, we only had massive uncertainty,” physicist Dominic Kraus, of the Helmholtz-Zentrum Dresden-Rossendorf research institute in Germany, said in a press statement. “This will become ever more relevant the more exoplanets we discover.” Kraus and his team conducted the experiments at the SLAC National Accelerator Laboratory at Stanford University.

To better understand how this molecular magic happens, the researchers recreated the diamond rain within Neptune’s core in the lab. Instead of using methane, which would be found inside the ice giants, as their sample, the scientists used the hydrocarbon polystyrene (C8H8), known colloquially as Styrofoam.

Kraus and his colleagues applied heat and pressure to the polystyrene and then used an optical laser to generate shockwaves that rippled through the material. When those shockwaves met, temperatures soared to 8,540 degrees Fahrenheit. (Earth’s core, for reference, is about 10,800 degrees Fahrenheit.) Pressure within the material also skyrocketed.

“We produce about 1.5 million bars, that is equivalent to the pressure exerted by the weight of some 250 African elephants on the surface of a thumbnail,” Kraus said. Whew.

The scientists then used SLAC’s Linac Coherent Light Source (LCLS) instrument to direct X-rays at the sample and measure how light bounced off of electrons inside it. For the first time, they watched the chemical reaction inside the non-crystalline substance unfold. The hydrocarbons split apart; the carbon rapidly converted to diamond and sank while the hydrogen escaped.

Visit PP bags homepage for more details.

Kraus says the experiment may explain why Neptune’s core produces a perplexing amount of energy—more than twice the amount it absorbs from the sun. These sheets of diamonds, the researchers suspect, could generate gravitational energy and subsequently heat energy as they rain down on the planets.

Ultimately, the experiment could help scientists solve other mysteries here in our own solar system and in distant planetary systems.