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

TL;DR

This study explores the high-pressure chemistry of the C-H-N-O system relevant to planetary interiors, predicting new stable phases and providing insights into the internal structures of Uranus and Neptune.

Contribution

It is the first comprehensive ab initio study of the C-H-N-O system at planetary interior conditions, revealing novel stable compounds and implications for ice giant models.

Findings

Eight new stable phases predicted at high pressures.

Diamond formation confirmed in Uranus and Neptune conditions.

Existing planetary models are insufficient to explain observed differences.

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 C2H2N2O2 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 CH4-NH3-H2O 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 H2O : CH4 : NH3 = 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, pressure-temperature conditions, and a more complex internal structure.