Could diamonds drive Neptune and Uranus’ magnetic fields?

Washington, DC—Diamonds could form in the relatively shallow interiors of planets like Neptune and Uranus and travel downward, driving the ice giants’ magnetic fields, according to new research from an international team of scientists including Carnegie’s Alexander Goncharov and Eric Edmund.

Published in Nature Astronomy, the SLAC National Accelerator Laboratory-led team’s findings used the European XFEL facility to resolve longstanding disagreements about the temperature and pressure conditions under which diamonds form from short-lived hydrocarbons such as those expected to be found inside these icy bodies.

“Although we are not able to directly probe the physics and chemistry occurring in planetary interiors, sophisticated lab techniques are able to monitor how small samples of planetary building-blocks behave and rearrange at the extreme conditions found within these planets,” Edmund explained.

Lab-mimicry and theoretical modeling have given scientists a rough idea of the process by which diamonds form from short-lived hydrocarbon molecules in icy planet interiors. However, different lab techniques have yielded varying results, making it challenging to pin down the depth at which this phenomenon occurs.

“This knowledge helps scientists understand the complex interior dynamics of these bodies,” Goncharov added. “Although they are not considered habitable, this information can help us better understand our own Solar System’s architecture and the evolution of icy worlds.”

This long-standing disagreement is between experiments that compress hydrocarbons to bring them to pressure extremes and experiments that create these conditions by hitting samples with high-speed projectiles mimicking a meteorite strike.

Taking a new approach, the team—led by SLAC’s Mungo Frost—used an X-ray laser at the  XFEL facility in Germany to hit a compressed sample of polystyrene with ultra-short x-ray flashes, producing a kind of “goldilocks” method that resolves the tension between the two earlier approaches. The world’s largest x-ray laser at FuXFEL generates ultrashort flashes 27,000 times pers second to facilitate next-level research breakthroughs, including this diamond-formation work.

"Through this international collaboration, we have made great progress at the European XFEL and gained remarkable new insights into icy planets," Frost said in a statement.

They found that diamond formation is observed under pressures ranging from 188,000 to 266,000 atmospheres (or 19 to 27 gigapascals) and above 4,040 degrees Fahrenheit (or 2500 kelvin).

This means that on ice giant planets, diamond forms at shallower depths than previously thought. Because it is denser than the surrounding material, it sinks deeper—a phenomenon sometimes called “diamond rain”—providing an additional heat source, which could drive convection in the ice layer and contribute to these planets’ complex magnetic fields.

“What’s more, this process of relatively shallow diamond formation from hydrocarbons means that this could occur on smaller icy bodies too, including mini-Neptunes,” Goncharov concluded. “Although there are no objects of this planetary class in our own Solar System, they are one of the most common types of exoplanet in our galaxy and better understanding their interior dynamics will improve our knowledge of planetary system evolution.”