Unusual Chemistry of the C–H–N–O System under Pressure and Implications for Giant Planets

Abstract

C-H-N-O system is central for organic chemistry and biochemistry, and plays a major role in planetary science (dominating the composition of "ice giants" Uranus and Neptune). The inexhaustible chemical diversity of this system at normal conditions explains it as the basis of all known life, but the chemistry of this system at high pressures and temperatures of planetary interiors is poorly known. Using ab initio evolutionary algorithm USPEX, we performed an extensive 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 thermodynamically stable phases were predicted, including quaternary polymeric crystal C 2 H 2 N 2 O 2 and several new N-O and H-N-O compounds. We describe the main patterns of changes in the chemistry of the C-H-N-O system under pressure and confirm that diamond should be formed at conditions of the middle-ice layers of Uranus and Neptune. We also provide the detailed CH 4 -NH 3 -H 2 O phase diagrams at high pressures, which are important for a further improvement of the models of ice giants -and point out that current models are clearly deficient. In particular, in existing models Uranus and Neptune are presented to have identical composition, nearly identical pressure-temperature profiles, and a single convecting middle layer ("mantle") made of a mixture H 2 O : CH 4 : NH 3 = 56.5 : 32.5 : 11. Here we provide new insights shedding light into the difference of heat flows from Uranus and Neptune, which require them to have different compositions, pressuretemperature conditions, and a more complex internal structure.

Significance of the high-pressure properties and structural evolution of gas hydrates for inferring the interior of icy bodies

“…For example, Neptune emits more energy than it receives from the sun. The source of this excess heat has long remained a mystery, but it has been suggested to originate from the frictional heat generated by diamonds settling within the surrounding icy material (Naumova et al 2021;Kraus 2018). Additionally, if hydrogen, a by-product of diamond formation, is metalized under extreme pressures deep within the planet (e.g., Li et al 2021), it may contribute to the magnetic fields of Uranus and Neptune as a conductor, along with the superionic phases of water and ammonia (Kraus 2018).…”

Computational prediction of novel MAB phases

“…The energy landscape of the (MoxSc1-x)2AlB2 system is thus explored using a CE model in addition to the crystal structure prediction code USPEX. The USPEX code builds on implementing the use of evolutionary algorithms and has successfully predicted new stable materials in addition to exotic properties, such as hardness, the maximum bandgap, and magnetic configurations [64,[77][78][79].…”

Mixture of hydrogen and methane under planetary interior conditions

“…21–26 Note that more complex mixtures containing hydrogen, carbon, nitrogen, and oxygen were also studied. 27–29 In particular, evidence for diamond formation under conditions typical for the deep interior of Uranus has been found in laser-driven shock compression experiments on plastics like polystyrene. 28,30–32…”

“…[21][22][23][24][25][26] Note that more complex mixtures containing hydrogen, carbon, nitrogen, and oxygen were also studied. [27][28][29] In particular, evidence for diamond formation under conditions typical for the deep interior of Uranus has been found in laser-driven shock compression experiments on plastics like polystyrene. 28,[30][31][32] Molecular dynamics (MD) simulations combined with density functional theory (DFT) were used to calculate the equation of state (EOS) and analyze the dissociation, polymerization into carbon chains, and chemical bonding properties of methane.…”