Little is known about the chemical transformations occurring within the interiors of Uranus and Neptune, the icy giants of our solar system. The extreme pressure and temperature conditions, reaching millions of atmospheres and thousands of Kelvin, make direct spacecraft exploration impossible. It is believed that a dense fluid composed mainly of water, methane, and ammonia lies between the atmosphere and rocky core, with pressures from 1 to 1000 GPa and temperatures between 1000 and 8000 K. Understanding the transformations underlying the chemistry of this mixture could provide valuable insights into some of the physical properties of these planets.
An intriguing hypothesis, the "diamonds in the sky" theory, suggests that diamond formation and superionic water could explain the unusual magnetic fields and luminosity of these planets. This hypothesis has been supported by numerous experimental works using Laser Heated Diamond Anvil Cells (LHDAC) techniques, which have probed nanodiamond formation from various C/H/O mixtures under extreme conditions. Additionally, theoretical studies based on ab initio molecular dynamics (aiMD) have provided insights into physical properties of these mixtures, such as lifetime of chemical bonds, bandgap, and Hugoniot curves. However, the limited accessible timescale in aiMD and the challenges in characterizing the process "in situ" from experiments have hindered the exploration of the mechanisms and free energies underlying the transition from C/H/O planetary mixtures to nanodiamonds.
In this talk, I will present the results of our computational investigation into the chemical pathways governing the transition from water/methane mixtures to nanodiamonds under extreme conditions. Our approach combines aiMD, enhanced sampling techniques and machine learning interatomic potentials. This approach enabled us to explore the reactivity of the planetary mixture while accounting for the direct effects of pressure and temperature on reaction mechanisms and associated free energies.
Our study reveals that the early stages of diamond formation in water above 3000 K and 10 GPa are dictated by a “super-acid” chemistry. Ionized water behaves as a super acid solution, protonating alkanes and leading to the formation of carbocations, whose high instability triggers the carbon growth process. For the first time, a link between the reactivity of hydrocarbons in extreme conditions and in super acidic solutions at mild conditions is established.

Flavio Siro Brigiano

Appearances