Exotic Mixtures: X-Ray Scattering Enables Closer Scrutiny of the Interior of Planets and Stars

By using a new measuring method — the so-called X-ray Thomson scattering — an international team of researchers was able to prove that the high pressure inside planets like Neptune or Uranus dissolves hydrocarbon into its individual parts and converts the released carbon atoms into diamond structures. Thanks to the new measuring method, such conditions, which experts call warm, dense matter and which do not occur naturally on Earth, can be examined more closely.
Credit: HZDR / Sahneweiß

Recreating extreme conditions in the lab, like those in the interior of planets and stars, is very complex and can only be achieved for fractions of a second. An international research team led by the Helmholtz-Zentrum Dresden-Rossendorf(HZDR) has now presented a new, very precise method of evaluating the behavior of mixtures of different elements under high pressure with the help of X-ray scattering. The results hone previous measurements and reinforce the premise that the matter in planets like Neptune and Uranus can alter dramatically: the hot hydrocarbon mixture in the interior of the ice giants can produce a kind of diamond rain, as the researchers report in Nature Communications.

Neither solid, nor fluid, neither gaseous, nor a plasma: the matter inside planets and stars can take on a particular intermediate state, at a temperature of thousands of degrees, and compressed a thousand times more than our Earth’s atmosphere – experts call it warm dense matter. There is a lot we still don’t know about it. Lab experiments are set to change all that but are technically highly complex because this exotic state does not occur naturally on Earth. Which all means that both the crafting and study of artificial warm dense matter is a challenge for investigators and theoreticians alike. “But in the last resort, we have to understand the processes in warm dense matter if we want to model planets,” explains Dr. Dominik Kraus, lead author of the study and the mastermind behind the measuring method. “We now have a very promising new approach based on X-ray scattering. Our experiments are delivering important model parameters where, before, we only had massive uncertainty. This will become ever more relevant the more exoplanets we discover.”

Diamond showers – a planetary energy source

At SLAC National Accelerator Laboratory at Stanford University, the researchers studied the structure of the matter in mixtures that are typical for planets, in the case of ice giants, hydrocarbon, employing intense laser light. Standard plastic film served as a substitute for planetary hydrocarbon. An optical high-energy laser converts the plastic into warm dense matter: short, strong laser pulses generate shock waves in the film and compress the plastic to the extreme. “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,” says Kraus, illustrating the dimensions. What happens is that the laser shock waves also heat up the matter to approximately 5,000 degrees. To evaluate the effect, researchers shoot an extremely powerful X-ray laser at the sample. Depending on how the light is scattered as it passes through the sample, they can draw inferences about the structure of the matter.

The researchers observed that in a state of warm dense matter, what was formerly plastic produces diamonds. The high pressure can split the hydrocarbon into carbon and hydrogen. The carbon atoms that are released compact into diamond structures. In the case of planets like Neptune and Uranus this means that the formation of diamonds in their interior can trigger an additional energy source. The diamonds are heavier than the matter surrounding them and slowly sink to the core of the planet in a kind of diamond rain. In the process, they rub against their surroundings and generate heat – an important factor for planet models.

X-ray scattering enhances measuring precision

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