Imagine Jupiter with a diamond core the size of Earth. That’s what science fiction author Arthur C. Clarke described in his novel (and movie) 2010: Odyssey 2. Now, imagine the same thing, but at Uranus and Neptune. In addition to a possible diamond core, diamond rain fills the interior. Scientists at the U.S. Department of Energy’s SLAC National Accelerator Laboratory think they know how these diamonds form on ice-giant planets.
“‘Diamond rain’ on icy planets presents us with an intriguing puzzle to solve,” said SLAC scientist Mungo Frost, who led the research. “It provides an internal source of heating and transports carbon deeper into the planet, which could have a significant impact on their properties and composition. It might kick off movements within the conductive ices found on these planets, influencing the generation of their magnetic fields.”
Modeling Diamond Rain
The interiors of Uranus and Neptune are completely alien to us here on Earth. Planetary scientists call them “ice giants” because their main components are mostly water and hydrocarbons—essentially ammonia, and methane. Those are commonly thought of as icy volatile compounds. We only see the exterior clouds and upper atmosphere—and each planet sports a slightly different shade of blue. But, once you get inside these worlds, the pressure of all that material compresses the lower layers. The temperatures inside can be as high as several thousand Kelvin.
The high pressure and temperature cause some weird effects. For example, the water exists in a strange kind of “supercritical fluid” state. That’s thanks to the fact that it’s being compressed. The same high temps and pressures, along with the presence of oxygen, combine to compress the hydrocarbons into diamonds.
So, how do you prove that’s what’s happening? Frost and his colleagues wanted to simulate the extreme conditions thought to exist in the ice giants in an experiment at SLAC. So, they placed polystyrene film (a stand-in for the hydrocarbons) into a chamber called a “diamond anvil cell.” Two diamonds inside acted as anvils. They held the film during bombardment by high-energy X-rays. Next, they heated the film to 2200 degrees Celsius, which imitated the environment inside the two planets. The result was diamonds forming from the film.
Next, they took the experiment a step further and extended the time of the bombardment. That let them figure out the depths inside the planets where these diamonds form. Heating the film over longer timescales showed them that the diamonds could be created at lower pressures and temperatures. Inside Uranus and Neptune, the diamonds form and “rain out” at very shallow depths inside the planet.
A Surprise Effect on the Magnetic Field
If diamonds do form inside the ice-giant planets at shallower depths, then they probably have an interesting side-effect on the planet’s magnetic field, which is already kind of weird. On Earth, the outer core generates our magnetic field. However, the ice giants don’t have the same core structure that we do. Instead, a thin layer of conducting material probably generates their magnetic fields. The result is an asymmetrical magnetic field that doesn’t extend from the poles as Earth’s does. The magnetic fields on both are tilted and dozens of times stronger than Earth’s.
So, how does this affect the fields? After the diamonds are created inside the planet, they “rain down”. As they do, they affect the creation of the planetary magnetic fields. Here’s how that happened. The diamonds drift down, dragging gas and ice along with them to the inner layers. Since the diamonds form above a layer of conductive ice, they end up “stirring” that layer. That activity creates currents in the layer, creating a “dynamo” that helps drive the creation of the magnetic field.
While there’s no way to dive into Uranus and Neptune to see these actions are taking place, the experiments are still groundbreaking. They provide much-needed insight into just what conditions are like inside those worlds. And, they could help us understand similar worlds around other stars. The next step now is for the researchers to create more in-depth experiments and models of those worlds. That work should shed new light onto exactly how diamond rain forms on and impacts the properties of those other distant worlds.