Using lasers and polystyrene, researchers say they have mimicked the high temperatures and pressures thought to cause diamond rain within ice giants

Diamond rain might sound like the stuff of poetry, but deep within the ice giants of our solar system it is thought to be reality – and now scientists say they have recreated the phenomenon.



The furthest flung true planets of our solar system, the ice giants Neptune and Uranus, are about 17 and 15 times the mass of Earth respectively.



While both have solid cores and atmospheres rich in gases including hydrogen and helium, the planets are largely made up a huge, slushy ocean of water, ammonia and substances known as hydrocarbons – molecules, such as methane, that are composed of hydrogen and carbon.

But researchers have long theorised that deep within these vast, blue planets something astonishing occurs: high temperatures and pressures act on the hydrocarbons deep in the oceans to produce diamonds that rain down, falling towards the planets’ interiors.

Now scientists say they have managed mimic conditions found within these planets to produce tiny diamonds in the laboratory. What’s more, the researchers were able to probe the structure of the material as it was made.

“You actually see the atomic structure of diamond,” said Dirk Gericke, co-author of the research from the University of Warwick.

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While scientists have previously tried a variety of techniques, including lasers, to explore the possibility of forming the “diamond rain”, the authors point out that many of them used pressures far below those predicted for the necessary conditions within the ice giants, while only indirect clues hinted that diamonds had been formed.



Writing in the journal Nature Astronomy, Gericke and an international team of researchers reveal how they fired lasers at humble polystyrene – a substitute for the hydrocarbons found within Neptune and Uranus.

The laser was used rapidly heat the surface of the polystyrene, causing it to expand and generate a shock wave. The team produced two shock waves, with the second faster than the first. When the shock waves caught up with each other, temperatures and pressures of about 5,000 K and 150 GPa respectively were produced – conditions similar to those found about 10,000km into the interior of the planets.

The conditions were enough to cause the bonds between the carbon and hydrogen within the polystyrene to break, with the carbon then joining together and giving rise to diamonds. The team were also able to “see” the diamonds being formed, using very short pulses of X-rays.

“The experimental time is very short,” said Dominik Kraus, first author of the research from the German research laboratory Helmholtz-Zentrum Dresden-Rossendorf. “That we saw this very clear signature of diamonds was actually very, very surprising.”

While the diamonds produced in the laboratory are tiny – just a few nanometres in diameter – the team say that within Neptune and Uranus, the diamonds produced could be far bigger.

“In the planet you have years, millions of years, and a long range of conditions where this actually can happen,” said Gericke, adding that that would give plenty of opportunity for the diamonds to grow.



In confirming the possibility that diamonds could indeed rain down inside Neptune and Uranus, Gericke added that the new research might shed light on the nature of proposed layers within the structure of the planets, as well as helping to solve the longstanding conundrum of why these ice giants are hotter than expected.

“These diamonds will sink down because they are heavier than the surrounding medium and when they sink down there will be friction with the surrounding medium, and at some point they will be stopped when they reach the core – and all this generates heat,” said Kraus.



But Gericke pointed out that the implications of the research go beyond the planets, with researchers now exploring whether the technique be used to produce diamonds for use in polishing and other industrial applications.

“There is a need to actually create artificial diamonds, even small ones, and the technique now is by explosives,” said Gericke. “The question is can you make the same thing by lasers, a little bit more efficiently.”