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C-H-N-O system is central for organic chemistry and biochemistry, and plays amajor role in planetary science (dominating the composition of "ice giants"Uranus and Neptune). The inexhaustible chemical diversity of this system atnormal conditions explains it as the basis of all known life, but the chemistryof this system at high pressures and temperatures of planetary interiors ispoorly known. Using ab initio evolutionary algorithm USPEX, we performed anextensive study of the phase diagram of the C-H-N-O system at pressures of 50,200, and 400 GPa and temperatures up to 3000 K. Eight novel thermodynamicallystable phases were predicted, including quaternary polymeric crystal C2H2N2O2and several new N-O and H-N-O compounds. We describe the main patterns ofchanges in the chemistry of the C-H-N-O system under pressure and confirm thatdiamond should be formed at conditions of the middle-ice layers of Uranus andNeptune. We also provide the detailed CH4-NH3-H2O phase diagrams at highpressures, which are important for a further improvement of the models of icegiants - and point out that current models are clearly deficient. Inparticular, in existing models Uranus and Neptune are presented to haveidentical composition, nearly identical pressure-temperature profiles, and asingle convecting middle layer ("mantle") made of a mixture H2O : CH4 : NH3 =56.5 : 32.5 : 11. Here we provide new insights shedding light into thedifference of heat flows from Uranus and Neptune, which require them to havedifferent compositions, pressure-temperature conditions, and a more complexinternal structure.

Unusual chemistry of the C-H-N-O system under pressure and implications for giant planets