Using a computer model, researchers from the Moscow Institute of Physics and Technology and the Skolkovo Institute of Science and Technology discovered which molecules may be hiding deep within Uranus and Neptune.

Artem Oganov and Gabriele Saleh found that, at high pressures, which is common for the planets' interiors, extraordinary polymeric and molecular compounds like carbonic and orthocarbonic acid are formed. Their findings were released in Scientific Reports.

According to the researchers, Uranus and Neptune are mostly made up of oxygen, hydrogen and carbon. And under atmospheric pressure, all the compounds of oxygen, hydrogen and carbon are thermodynamically unstable, save for carbon dioxide, water and methane.

Carbon dioxide and water will have no trouble remaining stable when the pressure increases, but once beyond 93 gigapascals is reached, methane starts decomposing, forming heavy hydrocarbons like polyethylene, butane and ethane.

For context, the pressure at the bottom of the deepest portion of the world's deepest oceans, the Mariana Trench, is 108.6 megapascals. This is about a thousand times lower than the pressures the researchers used for their study.

With another team, Oganov had previously developed a powerful, most universal algorithm for predicting crystal structures and compounds called the Universal Structure Predictor: Evolutionary Xtallography. This algorithm has been used by other researchers to discover stable substances at high pressures and "forbidden" in classical chemistry, like salt variants previously unknown and exotic new aluminum, silicon and magnesium oxides.

Now, Oganov and Saleh used the same algorithm to specifically observe the chemical behavior of oxygen-hydrogen-carbon systems under high pressure. They say the system is important because the entirety of organic chemistry is based on oxygen, hydrogen and carbon.

"And until now it had not been entirely clear how they behave under extreme pressures and temperatures," added Oganov.

The researchers took it upon themselves to uncover all compounds stable at up to 400 gigapascals and saw a handful of new substances, like a clathrate of molecular hydrogen, as well as methane 2CH4:3H2.

At 0.95 gigapascals, they found that carbonic acid is thermodynamically stable, which is highly unusual as the substance is extremely unstable under normal environments. For starters, it requires strong acids for synthesis and can exist only at very low temperatures in a vacuum.

At 44 gigapascals, carbonic acid turns into a polymer that stays stable under a minimum of 400 gigapascals of pressure.

At 314 gigapascals, carbonic acid enters an exothermic reaction with water, producing orthocarbonic acid, which is also known as "Hitler's Acid" for it has a molecular structure resembling a swastika.

Given these conditions, the researchers deduced that it's possible for Uranus and Neptune to feature cores with significant levels of carbonic and orthocarbonic acid.