The hypothesis says that intense heat and pressure thousands of kilometers below the surface of the ice giants Neptune and Uranus should break down hydrocarbon compounds. In this case, carbon is compressed into a diamond and sank even deeper into the planetary nuclei. Now scientists have presented new experimental data showing how this could be possible. A new study was published in the journal Nature Communications.
The new experiment used the world’s first rigid free-electron X-ray laser (LCLS, Linac Coherent Light Source), developed by the SLAC national accelerator laboratory for the most accurate measurements of how this “diamond rain” process should occur. It was found that carbon passes directly into a crystalline diamond.
This study provides data on a phenomenon that is very difficult to model computationally.
Neptune and Uranus are the most poorly studied planets in the solar system. They are too far away – only one space probe, Voyager 2, was next to them. And this is only for the flight, and not for a special long-term mission.
But ice giants are extremely common in the Milky Way. According to NASA, exoplanets like Neptune are 10 times more common than exoplanets like Jupiter.
That is why understanding the ice giants of our solar system is vital to understanding the planets throughout the galaxy. And in order to understand them better, it is important for scientists to know what is happening under their calm blue shell.
Now the scientific community knows that the atmosphere of Neptune and Uranus consists mainly of hydrogen and helium with a small amount of methane. Under these atmospheric layers, the planet’s core is covered by a super-hot, super-dense liquid made of “ice” materials such as water, methane, and ammonia.
Both calculations and experiments, which took decades, have shown that, with sufficient pressure and temperature, methane can be broken into diamonds – assuming that diamonds can form within this hot, dense material.
During a previous experiment at SLAC, physicist Dominic Kraus and his team used X-ray diffraction. Now, researchers have taken another step forward.
We now have a very promising new approach based on X-ray scattering. Our experiments provide important parameters of the model, where before we had only enormous uncertainty.
Dominic Kraus, physicist SLACK
It’s hard to copy the parameters of giant planets here on Earth. Scientists need fairly intense equipment – and this is LCLS. Also needed is material that replicates substances inside a giant planet. For this, the team used hydrocarbon polystyrene instead of methane.
The first step is to heat and increase the pressure of the material to reproduce the conditions inside Neptune at a depth of about 10 thousand km. The pulses of an optical laser generate shock waves in polystyrene, which heats the material to about 5 thousand Kelvin (4,727 degrees Celsius). It also creates intense pressure.
We produce about 1.5 million bar, which is equivalent to the pressure that about 250 African elephants exert on the surface of a miniature model.
Dominic Kraus, physicist SLACK
In a previous experiment, X-ray diffraction was used to study the material. This works well for materials with a crystalline structure, but to a lesser extent for non-crystalline molecules, so the picture was incomplete. In a new experiment, the team used a different method, measuring how X-rays scatter electrons in polystyrene.
This allowed them not only to observe the conversion of carbon into diamond but also what happens to the rest of the sample – it is split into hydrogen. In the case of the ice giants, we now know that carbon forms diamonds when it is separated, which does not take a liquid transitional form, scientists say.
This is important because there is something really strange about Neptune. Its internal content is much hotter than it should be; in fact, it emits 2.6 times more energy than it absorbs from the Sun.
If diamonds are denser than the material around them, they enter the inner space of the planet, they can release gravitational energy, which is converted into heat generated by the friction between the diamonds and the material around them.
This experiment shows a method that scientists could use to “explore” the interiors of other planets in the solar system. It will allow researchers to measure various processes that are otherwise difficult to recreate.
For example, scientists will be able to see how hydrogen and helium, elements inside gas giants like Jupiter and Saturn, mix and separate under extreme conditions.