The interior structure of Uranus is illustrated. Credit: MIPT Press office
The depths of Uranus, Neptune and their satellites may contain extraordinary compounds, such as Carbonic and Orthocarbonic acids (aka Hitler’s acid). These gas giants consist mainly of hydrogen, carbon and oxygen, which are the 3 cornerstones of organic chemistry. Using computer modeling, chemists from MIPT and Skoltech found at high pressures, typical for the interiors of such planets, exotic molecular and polymeric compounds are formed. “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,” says Prof. Artem Oganov.
A team led by Professor Oganov developed the world’s most universal and powerful algorithm for crystal structure and compound prediction – USPEX (Universal Structure Predictor: Evolutionary Xtallography). In recent years, scientists have used this algorithm to discover several substances that are ‘forbidden’ in classical chemistry and that may be stable at high pressures. These include a number of previously unknown variants of salt — Na3Cl, NaCl3, NaCl7 and even Na3Cl2 and Na4Cl3, as well as exotic new oxides of Mg, Si, Al which may exist in the interiors of super-Earths.
Summary of stable phases in the C-H-O phase diagram and convex hull for the CO2-H2O system.
Hydrogen bonds in carbonic and orthocarbonic acids at 400 GPa.
Now Oganov and Gabriele Saleh from MIPT have decided to study the chemical behaviour of the CHO system under high pressure. The scientists knew that under atmospheric pressure all compounds of carbon, hydrogen, and oxygen, except for methane, water, and carbon dioxide, are thermodynamically unstable. With an increase in pressure, water and carbon dioxide remain stable, but at pressures above 93 gigapascals (0.93 million atmospheres) methane begins to decompose forming heavy hydrocarbons – ethane, butane, and polyethylene. At a ower pressure – approximately 4 GPa – methane and molecular hydrogen interact, forming co-crystals (2 molecules together create one crystal structure), and at 6 GPa, hydrates – CO-crystals made of methane and water – are formed. To put this into context, the pressure at the bottom of the Mariana Trench (the deepest part of the world’s oceans) is 108.6 megapascals, 1000X lower.
Variation of properties of valence ELF basins along the homodesmic reaction H2O + H2CO3 → H4CO4 at 400 GPa.
Oganov and Saleh took on the task of finding all stable compounds in the range up to 400 GPa and discovered several new substances. These included a clathrate (inclusion compound, a type of co-crystal) of molecular hydrogen and methane 2CH4:3H2, which is stable in the pressure range 10-215 GPa.
At above 0.95 GPa carbonic acid (H2CO3) becomes thermodynamically stable. This is very unusual for a substance that is highly unstable under normal conditions – strong acids are needed for its synthesis and it can only exist in a vacuum at very low temperatures, the authors write. “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. https://mipt.ru/english/news/russian_chemists_discover_that_there_may_be_exotic_carbonic_acid_inside_uranus_and_neptune
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