Deep in the hearts of Neptune and Uranus, it could be raining diamonds. Now scientists have produced new experimental tests that show how this could be possible.
The hypothesis is that the intense heat and pressure thousands of kilometers below the surface of these ice giants should separate the hydrocarbon compounds, with the carbon compressing into diamond and sinking further into the planetary nuclei.
The new experiment used the SLAC National Accelerator Laboratory’s Linac Coherent Light Source X-ray laser (LCLS) for the most accurate measurements of how this “ diamond shower ” process should occur, and found that carbon passes directly to a crystalline diamond.
“This research provides data on a phenomenon that is very difficult to computationally model: the ‘miscibility’ of two elements, or how they combine when they are mixed,” explained plasma physicist Mike Dunne, director of the LCLS, and is not listed as author on paper.
“Here they see how two elements are separated, like making the mayonnaise separate again in oil and vinegar.”
Neptune and Uranus are the least understood planets in the Solar System. They are prohibitively far: only a single space probe, Voyager 2, has even been close to them, and only for a flyby, not a dedicated long-term mission.
But ice giants are extremely common in the broader Milky Way: According to NASA, Neptune-like exoplanets are 10 times more frequent than Jupiter-like exoplanets.
Understanding the ice giants of our Solar System, therefore, is vital to understanding planets across the galaxy. And to better understand them, we need to know what’s going on underneath their serene blue exteriors.
We know that the atmospheres of Neptune and Uranus are mainly composed of hydrogen and helium, with a small amount of methane. Beneath these atmospheric layers, a super-hot, super-dense fluid of “frozen” materials like water, methane, and ammonia envelops the planet’s core.
And decades-old calculations and experiments have shown that, with sufficient pressure and temperature, methane can decompose into diamonds, suggesting that diamonds can form within this hot, dense material.
An earlier SLAC experiment led by physicist Dominik Kraus at Helmholtz-Zentrum Dresden-Rossendorf in Germany used X-ray diffraction to demonstrate this. Now Kraus and his team have taken their investigation one step further.
“We now have a very promising new approach based on X-ray scattering,” Kraus said of his latest efforts. “Our experiments are delivering important model parameters where, previously, we only had massive uncertainty. This will become increasingly relevant as we discover more exoplanets.”
It is difficult to replicate the interiors of giant planets here on Earth. You need a pretty intense team – that’s the LCLS. And you need a material that reproduces things within that giant planet. For this, the team used hydrocarbon polystyrene (C8H8) instead of methane (CH4 4)
The first step is to heat and pressurize the material to replicate conditions within Neptune to a depth of about 10,000 kilometers (6,214 miles): Optical laser pulses generate shock waves in the polystyrene, which heats the material to around 5,000 Kelvin (4,727 degrees Celsius, or 8,540 degrees Fahrenheit). It also creates intense pressure.
“We produce around 1.5 million bars, which is equivalent to the pressure exerted by the weight of about 250 African elephants on the surface of a miniature,” said Kraus.
In the previous experiment, X-ray diffraction was used to then probe the material. This works well for materials with crystalline structures, but not so much with non-crystalline molecules, so the image was incomplete. In the new experiment, the team used a different method, measuring how X-rays scattered the electrons in the polystyrene.
This allowed them to not only observe the conversion of carbon to diamond, but also what happens to the rest of the sample: it divides into hydrogen. And there are practically no carbon residues left.
“In the case of the ice giants, we now know that carbon almost exclusively forms diamonds when it separates and does not take on a fluid transitional shape,” Kraus said.
This is important, because there is something really strange about Neptune. Its interior is much hotter than it should be; in fact, it emits 2.6 times more energy than it absorbs from the sun.
If the diamonds, denser than the surrounding material, are raining inside the planet, they could be releasing gravitational energy, which is converted into heat generated by the friction between the diamonds and the surrounding material.
This experiment suggests that we don’t have to find an alternative explanation … not yet, anyway. And it also shows a method that we could use to ‘probe’ the interiors of other planets in the Solar System.
“This technique will allow us to measure interesting processes that would otherwise be difficult to recreate,” Kraus said.
“For example, we can see how hydrogen and helium, elements found inside gaseous giants like Jupiter and Saturn, mix and separate in these extreme conditions. It is a new way of studying the evolutionary history of planets and planets systems as well as experiments supporting possible future forms of fusion energy. “
The research has been published in Nature’s Communications.
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