Diamond Rains on Neptune and Uranus Explained by New Research

The theory that the icy planets Neptune and Uranus could have diamond rains has been tested in new experimental research that shows how this could be possible.  

The hypothesis states that the severe heat and pressure thousands of kilometers underneath the surface of these ice planets should eject hydrocarbon pieces, with the carbon compressing into diamond and falling deeper towards the planetary nucleus.  

Two Poorly-Understood Planets  

The team of researchers used the SLAC National Accelerator Laboratory’s Linac Coherent Light Source (LCLS) X-ray laser for the most accurate measurements yet to understand how this ‘diamond rain’ process should take place, and found that carbon shifts directly into crystalline diamond.  

“This research provides data on a phenomenon that is very difficult to model computationally: the ‘miscibility’ of two elements, or how they combine when mixed,” explained plasma physicist Mike Dunne, director of the LCLS. “Here, they see how two elements separate, like getting mayonnaise to separate back into oil and vinegar.”  

Both Neptune and Uranus are the most poorly understood planets in our Solar System. They are relatively far away, and only one spacecraft, Voyager 2, has even been close to them, for a single flyby only, and not a dedicated long-term expedition.  

However, ice planets are incredibly common in the Milky Way, according to NASA, as exoplanets like Neptune are ten times more common than Jupiter-like exoplanets. Learning more about our Solar System’s ice giants is crucial to understanding planets across the galaxy. And to understand better, we have to grasp what happens beneath their exteriors.  

How Diamonds Form on Uranus and Neptune  

We already know that the atmospheres of both planets are mainly composed of hydrogen and helium, with a small amount of methane. Underneath these atmospheric layers, a super hot and super dense fluid of ‘icy’ materials, including water, methane, and ammonia, can be found around the planet’s core.  

Calculations and experiments carried out previously have shown that, with enough pressure and temperature, methane can be shattered into diamonds, implying that diamonds can take shape within this hot and dense material.  

Planet Neptune [Image: solarsystem.nasa.gov]
A previous experiment at SLAC led by physicist Dominik Kraus at the Helmholtz-Zentrum Dresden-Rossendorf in Germany utilized X-ray diffraction to prove it. Now, Kraus and his team have advanced with their research.  

“We now have a very promising new approach based on X-ray scattering,” Kraus said about their latest experiments. “Our experiments are delivering important model parameters where, before, we only had massive uncertainty. This will become ever more relevant the more exoplanets we discover.”  

It is rather difficult to replicate the inner parts of giant planets on Earth because you need a material that replicates the stuff inside that world.  

“In the case of the ice giants, we now know that the carbon almost exclusively forms diamonds when it separates and does not take on a fluid transitional form,” Kraus explained.  

Something’s Off On Neptune  

In addition, the team discovered that there’s something really peculiar about Neptune. Its interior is way hotter than it should be, emitting 2.6 times more energy than it takes from the Sun. If diamonds, which are denser than the material surrounding them, are raining down into the planet‘s inner part, they could be generating gravitational energy, which is transformed into heat produced by friction between the diamonds and the material around them.  

This experiment seems to state that we don’t have to find another explanation – at least, not yet. It also shows a technique (the method the team used to find an explanation) we can use to scan the interiors of the other planets in the Solar System.   

“This technique will allow us to measure interesting processes that are otherwise difficult to recreate,” Kraus said. “For example, we’ll be able to see how hydrogen and helium, elements found in the interior of gas giants like Jupiter and Saturn, mix and separate under these extreme conditions. It’s a new way to study the evolutionary history of planets and planetary systems, as well as supporting experiments towards potential future forms of energy from fusion.”  

The paper detailing the study has been published in Nature Communications. 

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