To determine what molecules may be present in the interiors of Uranus, Neptune and their icy satellites, Russian researchers decided to use computer modeling. The results of this study show that exotic molecular and polymeric compounds are formed at the high pressures typically found at the interiors of such planets. Many compounds are possible under these conditions, including orthocarbonic acid and carbonic acid. Uranus and Neptune are often referred to as the smaller gas giants.
Artem Oganov, professor of Skoltech and the head of MIPT’s Computational Materials Discovery Lab and lead author of the study, explained that planets of this type normally consist mainly of carbon, hydrogen and oxygen. Under extreme pressures exceeding several million atmospheres, the team’s model shows that unexpected compounds could form in their interiors. If this is the case, the planet’s cores may consist primarily of these exotic materials.
USPEX (Universal Structure Predictor: Evolutionary Xtallography) is the world’s most universal and powerful algorithm for crystal structure and compound prediction. The algorithm was originally developed by Professor Oganov and his team and scientists have used it in recent years to discover several substances that are “prohibited” in classical chemistry, but may be stable at extreme pressures. The list of substances includes exotic new oxides of silicon, magnesium and aluminum, as well as previously unknown variants of salt – NaCl7, NaCl3, Na3Cl2, Na3Cl andNa4Cl3. It is thought that some of these variant may exist in the interiors of super-Earths.
The carbon-hydrogen-oxygen system is an extremely important one as all organic chemistry is based on these three elements. As it is not entirely clear how they behave under extreme pressures and temperatures, Oganov and his co-author Gabriele Saleh from MIPT embarked on a study to determine their chemical behavior under high pressure. Oganov also notes that the elements play an essential role in the chemistry of the giant planets.
Most compounds of carbon, hydrogen and oxygen are thermodynamically unstable under atmospheric pressure. The exceptions are methane, water and carbon dioxide. When the pressure increases, carbon dioxide and water remain stable. When the pressure rises above 93 giga Pascals (0.93 million atmospheres), methane decomposes and forms heavy hydrocarbons – polyethylene, ethane and butane. At approximately 4 GPa, molecular hydrogen interacts with methane and co-crystals are formed. Co-crystals form where two molecules together create one crystal structure. At 6 GPa, co-crystals made of methane and water (called hydrates), are formed.
Oganov and Saleh set the goal of finding all stable compounds at pressures lower than 400 GPa (around 4 million atmospheres). Several new substances, including a methane 2CH4:3H2 and a clathrate (an inclusion compound and a type of co-crystal) of molecular hydrogen were found. The team also found that carbonic acid (H2CO3) becomes thermodynamically stable at a pressure higher than 0.95 GPa.
The authors noted that for a substance that is highly unstable under normal conditions it is unusual for it to stabilize under high pressure, as it can only exist in a vacuum at very low temperatures and strong acids are needed for its synthesis. Oganov concludes that the cores of Neptune and Uranus may contain significant amounts of a polymer of orthocarbonic acid and carbonic acid.
The results have been published in the journal Scientific Reports.