PHOTON SCIENCE

An international team of researchers, including from DESY and led by Mungo Frost from SLAC National Accelerator Laboratory (U.S.), used the European X-ray free-electron laser (XFEL) to gain new insights into the formation and occurrence of diamond rain in ice giants such as Neptune, Uranus, or exoplanets outside of our solar system. The results, which have now been published in the scientific journal Nature Astronomy, also provide clues into the origin of their complex magnetic fields.

In earlier work using X-ray lasers, scientists had already discovered that diamonds can form from carbon compounds at the pressures and temperatures found inside large gas planets, confirming the possibility of diamond formation in ice giants which are primarily composed of water, ammonia, and hydrocarbons. Following their formation, diamonds are expected to slowly sink deeper into the planetary interior in response to gravitational forces, resulting in a ‘rain’ of precious stones from higher layers.

A new experiment at the European XFEL has now shown that the formation of diamonds from carbon compounds occurs at lower pressures and temperatures than previously assumed. In the case of icy Solar planets, this means that diamond rain can form at a shallower depth than initially thought, and may therefore have a stronger influence on the magnetic field. In addition, diamond rain should also be possible in gas planets that are smaller than Neptune and Uranus, the so-called "mini-Neptunes", which are one of the most common types of exoplanets found outside of the solar system.

After their formation, diamond particles can entrain gas and ice as they descend from the outer to the inner layers of the planet, causing currents of ice. The new results show that diamonds form above a layer of conductive ice which will be stirred as the diamonds fall through them. Currents of conductive fluids act as a kind of dynamo through which the magnetic fields of planets are formed. "Diamond rain probably has an influence on the formation of the complex magnetic fields of Uranus and Neptune," Frost said.

The group used a plastic film made from the hydrocarbon compound polystyrene as a carbon source, which was subjected to the extreme pressures and temperatures found deep in the interior of these icy planets. First, high pressures were generated by squeezing the foil between the tips of two diamonds using a so-called ‘diamond anvil cell’, in which the anvils function like a mini-vice. The foil was then exposed to multiple doses of high energy X-rays to heat it to more than 2200 degrees Celsius, imitating the extreme conditions experienced deep inside these planets.

Then the researchers also used the X-ray pulses produced by the European XFEL to observe when and how the diamonds formed during their experiments. The pressure and temperature at which diamonds were observed allows researchers to predict the depth they can be expected to form inside the planet.

The international research team also included scientists from several German institutions including DESY and the Helmholtz Centre Dresden-Rossendorf (HZDR), as well as researchers from the UK, France, and South Korea. DESY and HZDR are member institutes of the European XFEL user consortium HIBEF, which contributed significantly to this result. "Through this international collaboration, we have made great progress at the European XFEL and gained remarkable new insights into icy planets," says Frost.

"We are delighted that this important result for planetary research was made possible with the diffraction platform for high-pressure experiments designed by DESY for HIBEF and the AGIPD detector," says DESY scientist Cornelius Strohm, one of the authors of the publication.

(from DESY News)



Reference:
M. Frost et al., Diamond Precipitation Dynamics from Hydrocarbons at Icy Planet Interior Conditions, Nature Astronomy, 2024, DOI: 10.1038/s41550-023-02147-x