Rules of formation of H–C–N–O compounds at high pressure and the fates of planetary ices

• Published in: Proceedings of the National Academy of Sciences Vol. 118; no. 19; pp. 1 - 7
• Main Authors: Conway, Lewis J., Pickard, Chris J., Hermann, Andreas
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 11.05.2021; Proceedings of the National Academy of Sciences
• Subjects: Ammonia; Chemical effects; Crystal structure; Decomposition; Density functional theory; Diamonds; High pressure; Magnetic fields; Neptune; Phase transitions; Physical Sciences; Planetary evolution; Planetary interiors; Pressure; Pressure effects; Solar system; Stability analysis; Thermal evolution; H–C–N–O chemistry; structure search; high pressure; planetary ices
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.2026360118

The solar system’s outer planets, and many of their moons, are dominated by matter from the H–C–N–O chemical space, based on solar system abundances of hydrogen and the planetary ices H₂O, CH₄, and NH₃. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle–core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, groundstate density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets’ interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H–C–N–O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H–C–N–O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO—isoelectronic to diamond—emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures.