Chemistry of diamonds rain in icy giants – CHEMRAIN

The chemistry of icy giant planets such as Neptune and Uranus has recently attracted the attention of the scientific community. The mantle of these planets is mainly composed of a dense fluid mixture of water, methane and ammonia. The extreme pressure and temperature conditions (T>1000 K, and P>3 GPa) in icy giants involve complex reactions that dictate many of the chemical and physical properties of these planets. In this regard, the formation of diamonds and super-ionic water (diamond rain) from planetary mixtures has been invoked to rationalize the luminosity and anisotropic magnetic field of icy giants. CHEMRAIN aims to break the borders to the so far inaccessible extreme chemistry of icy giants' interior. CHEMRAIN will simulate for the first time the early stages of nanodiamond nucleation from a fluid mixture representative of the interior of icy giants. Our main objective is to understand the chemistry under extreme conditions of diamond nucleation at the nanoscale. This will be achieved by developing a protocol gathering an array of state of-the-art computational techniques : ab initio methods, molecular dynamics, enhanced sampling techniques (metadynamics, umbrella sampling), QM:QM multi embedded scheme, theoretical spectroscopy and topological electronic descriptors (e.g. electron localization functions (ELF) ). The combination of enhanced sampling techniques and ab initio molecular dynamics will allow for the exploration of the reactivity of the planetary mixture, overcoming current time limitations in modelling diamond formation. This will lead to the discovery of realistic transformation mechanisms and free energy barriers for nanodiamond nucleation in planetary mixtures, taking into account the effects of pressure, temperature, and solvation. In the second step, the application of ELF on the reactive species along the transition pathway, obtained by reactive molecular dynamics, will allow for the study of how the nature of the binding network, charges, and volume of the binding and non-binding doublets evolve during the nucleation of nano-diamonds. This will open the way for a complete identification of the nature of the chemical environment (molecular, ionic, superionic, etc.) at icy giant P,T conditions, shedding light on the effect of extreme conditions on the reactivity of planetary mixtures. Finally, CHEMRAIN will establish a connection between experiments and simulations through vibrational theoretical spectroscopy. The modeling of new phases of planetary mixtures, including diamond and super ionic H2O, in addition to theoretical spectroscopy will bridge the gap between the nature of planetary mixtures probed in Raman high-pressure experiments and in simulations.