What's in the center of Neptune

Helmholtz Center Dresden-Rossendorf Neptune has a heart of diamonds

The inner workings of ice planets like Neptune or Uranus always look similar: A solid core in the middle is surrounded by dense layers of "ice". This cosmic "ice" consists of hydrocarbons, water and ammonia. This is why astrophysicists have long suspected that the hydrocarbons will be separated - because there are extremely high pressures around 10,000 kilometers below the surface of such planets. In the process, diamonds are formed, which then sink further into the interior of the planet.

X-ray laser helps with simulation

"So far, however, this glittering precipitate has not been directly observed experimentally," says Dr. Dominik Kraus from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). This is exactly what the head of a Helmholtz young investigator group with a team of German and US scientists was able to do change: Using an ultra-powerful X-ray laser, among other things, they simulated conditions like those inside the cosmic giants. This enabled the researchers to observe the splitting of hydrocarbons and the conversion of carbon into diamond in real time for the first time. Their results were published in the journal "Nature Astronomy" .

Shock waves bombarded samples

For their experiments, the researchers used a special form of plastic as a sample: Polysterol. It consists of a mixture of carbon and hydrogen. These samples were exposed to conditions that resemble the inner workings of Neptune and Uranus - with the help of two shock waves that the scientists shot through the polystyrene. The researchers generated these shock waves with an extremely powerful optical laser in combination with the world's most powerful X-ray laser, the Linac Coherent Light Source (LCLS).

The Linac Coherent Light Source (LCLS) is an X-ray laser source at the SLAC National Accelerator Laboratory in Stanford - a research facility of the US Department of Energy, operated by Stanford University. The X-ray pulses from the LCLS are 80 femtoseconds long and have wavelengths between 0.15 and 15 nanometers. For example, they can be used to measure atomic and molecular processes with high time resolution. In the LCLS, electrons are accelerated over a distance of around one kilometer and then deflected. The $ 420 million laser is the first for hard X-rays and is considered the most powerful X-ray laser in the world.

The result of the experiment: The plastic was compressed with a pressure of around 150 gigapascals at a temperature of around 5,000 degrees Celsius and diamonds actually formed. Since this only takes a fraction of a second, the researchers used ultra-fast X-ray diffraction - a type of X-ray camera - to take snapshots of the formation of the diamonds and the chemical processes.

The first, smaller and slower wave is overtaken by the stronger, second wave. The moment the two waves intersect, most of the diamonds are formed. The experiments show that almost all carbon atoms combine to form nanometer-sized diamond structures

Dr. Dominik Kraus, Helmholtz Center Dresden-Rossendorf

Thousands of years of diamond rain

In the experiment, only tiny diamonds that cannot be seen with the naked eye were formed. But based on the results, the researchers suspect that the diamonds on Neptune and Uranus form much larger structures and possibly sink slowly into the planet's core over thousands of years. But these results not only help to better understand the planets in our solar system, explains Helmholtz researcher Kraus.

"We can also use our findings to gain information to better understand the structure of exoplanets." Because with these planets outside of our solar system, researchers can mainly measure two core quantities: the mass, which results from fluctuations in the position of the parent star, and the radius, which astronomers deduce from the shadow that forms when the planet passes a star. The relationship between the two quantities then provides the researchers with clues about the chemical structure of the planet - that is, whether it is made up of light or heavy elements.

And the chemical processes inside, in turn, tell us aspects about the decisive properties of the planets. This enables us to improve the tarpaulin models. As our investigations show, simulations are not so far exact here.

Dr. Dominik Kraus, Helmholtz Center Dresden-Rossendorf

Nano diamonds for industry

But the research of the astrophysicists could also be of very practical use on Earth. Such nano-diamonds, like those from the experiments, would be used, among other things, for electronic instruments and medical procedures, but also as cutting materials in industry. Previously, they had to be made with the help of blasting. According to the HZDR researchers, the laser method could enable a process that is cleaner and easier to control.

on the radio | 08/21/2017 | 6:00 p.m.