The interior structure of Uranus is illustrated © MIPT Press office

Extraordinary new compounds could exist in gas giants

Scientists use advanced computer modelling systems to establish molecular structure of planets like Uranus and Neptune with incredible results.

Deep in the outer Solar System, the gas giants are slowly and gracefully orbiting our Sun, but despite missions such as Juno and Cassini expanding our knowledge of Jupiter and Saturn, there is still much to learn about all the other celestial bodies floating around out there. Now, a new study published in Scientific Reports from the Moscow Institute of Physics and Technology and the Skolkovo Institute of Science and Technology used computer modelling to establish which molecules we might discover deep in Uranus, Neptune and other icy moons – and the results have found that there could be some pretty exotic compounds out there.

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It all comes down to the three building blocks of the smaller gas giants, carbon, hydrogen and oxygen, and their fundamental properties. Under atmospheric pressure, all compounds of these three elements, bar methane, water and carbon dioxide, become unstable. But when you change the pressure things start to change drastically. Methane, for example, starts to form crystals with other compounds and elements when you crack the pressure above four gigapascals, and begins to break down completely to form hydrocarbons above 93 GPa (that’s 0.93 million atmospheres!). To put that in perspective, the pressure at the bottom of the Earth’s deepest point, the Mariana Trench is a measly 108.6 MPa, a thousand times lower.

Under pressure

The team of scientists, led by Professor Artem R. Oganov took up the task of finding out if there were any changes to the elements when the pressure was ratcheted up into the hundreds of gigapascals, the internal pressure the smaller gas giants can easily reach, by using the world’s most powerful algorithm for crystal structure and compound prediction (USPEX). In the past USPEX has discovered previously forbidden (in the classical-chemistry context) compounds that are stable at high pressures, which may exist in the interiors of exoplanets. What they discovered when they turned it to Neptune and Uranus was a host of potential new compounds.

“The smaller gas giants – Uranus and Neptune – consist largely of carbon, hydrogen and oxygen,” says Oganov. “We have found that at a pressure of several million atmospheres unexpected compounds should form in their interiors. The cores of these planets may largely consist of these exotic materials.”

Between 10-215 GPa, a type of crystal (called a clathrate) of molecular hydrogen and methane can be found – 2CH4:3H2 for all you chemistry bods out there. More significantly, at the relatively pleasant pressure of 0.95 GPA (ok, that’s still 10,000 times the atmospheric pressure on the surface of Earth) a highly unstable carbonic acid becomes thermodynamically stable.

“It is possible that the cores of Neptune and Uranus may contain significant amounts of a polymer of carbonic acid and orthocarbonic acid,” says Oganov.

So now, whenever you look in to the darkness of space and wonder what wonderful and exotic worlds are out there, you can also begin to ponder the incredible unknown compounds that could also exist in our very own Solar System.


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