Elements of life under pressure: chemistry and crystal structures of H-C-N-O compounds in planetary interiors

Abstract

The interiors of icy planets such as Uranus, Neptune and several exoplanets contain mixtures of methane, ammonia and water. It is unclear how these mixtures will behave under the high pressures found near the core-mantle boundary. If the molecules are to decompose, what new materials will form? We use ab initio techniques with crystal structure prediction to discover new stable compounds in the quaternary H–C–N–O phase diagram. We show that, of several new compounds, the only stable quaternary compound at 500 GPa is HCNO, a 1:1:1:1 mixture of the elements. We discuss how these results have implications for the understanding of icy planetary interiors and the underlying chemical trends that determine stability at these pressures.

At lower pressures, around those of a typical upper-mantle, we show several new methane-hydrogen inclusion compounds wherein molecular methane and hydrogen coexist in a crystal structure. We review the stability of previously measured experimental compounds and, with complementary experimental mea- surements, update the methane-hydrogen phase diagram containing 1:2, 2:1, and 1:8.33 compounds. We then extend this to the complete carbon-hydrogen phase diagram, which includes higher hydrocarbons such as ethane and butane, up to 300 GPa and at finite temperatures using the harmonic approximation.

The role of inclusion compounds in planetary interiors is shown to be important. We continue on this theme by considering water-hydrogen mixtures, substituting water with ammonium fluoride, a compound with a structural similarity to water. We assess how far this similarity extends under pressure, where it forms unusual dense ionic compounds, and then consider ammonium fluoride-hydrogen inclusion compounds.

The results of the computational work are discussed where appropriate in comparison to experimental results. Throughout, we discuss how these results can be used by future high pressure shock-wave experiments and static diamond anvil cell experiments. Prior work has been restricted to binary or ternary mixtures; this work indicates a step towards modelling the correct chemistry in icy planet interiors by sampling a quaternary mixture of atoms and demonstrates the feasibility of large-scale sampling of diverse chemical spaces with ab initio methods.