Periodic Reporting for period 1 - xICE (Exploring interfaces in ice giant planets using multi-scale molecular dynamics simulations)
Reporting period: 2020-09-01 to 2022-08-31
Most of the 5000 known exoplanets today have a radius between 1.0 and 4.0 Earth radii, i.e. they appear to be large versions of Earth, so-called super-Earths, and small versions of Uranus or Neptune, so-called mini-Neptunes. Both prototype planets are as different as one can imagine: Earth, the rocky planet supporting life with an atmosphere and a magnetic field shielding it from solar radiation, and Neptune, the ice giant predominantly made of the planetary ices water, ammonia, and methane with a peculiar magnetic field structure. While we have a fairly good understanding of Earth’s interior, Neptune and its similar neighbor Uranus remain puzzling and have been therefore the main subjects studied within this fellowship. The magnetic fields of the ice giants are highly non-dipolar and their axes have large tilts with respect to the planets’ spin axes. To date, there are no conclusive models for their magnetic dynamos based on valid interior structure models, that reproduce the correct age of the planets (4.56 billion years). The key to solve this problem is to identify the regions, where the magnetic dynamos are driven, which could be either in a thin outer shell or in the deep interior inside the ice-rich region atop the rocky core.
The main objective of the project xICE was the characterization of materials that are predicted to be relevant for the deep interiors of our Solar System’s ice giant planets Uranus and Neptune, as well as their exoplanetary cousins. Uranus and Neptune are typically modeled as adiabatic planets that consist of three differentiated layers, i.e. a hydrogen-helium-rich outer mantle, a ice-rich inner mantle, and a rock-rich core. Typical thermodynamic conditions span thousands of Kelvin in temperature and several Mbar in pressure. Such models, however, have been unable in the past to explain the thermal evolution, interior structure, and magnetic field geometry in a consistent way. Therefore, recent models have proposed to consider thermal boundary layers, compositional gradients, and interface effects to match observational constraints. Of special importance for this endeavor of improvement is the understanding of the core-mantle boundary and the rock/ice mixing behavior. Specifically, it was the project’s goal to investigate physical processes at the core-mantle boundary that could provide hints on how thermal boundary layers and interfaces could be potentially formed and sustained in ice giants.
The main objective of the project xICE was the characterization of materials that are predicted to be relevant for the deep interiors of our Solar System’s ice giant planets Uranus and Neptune, as well as their exoplanetary cousins. Uranus and Neptune are typically modeled as adiabatic planets that consist of three differentiated layers, i.e. a hydrogen-helium-rich outer mantle, a ice-rich inner mantle, and a rock-rich core. Typical thermodynamic conditions span thousands of Kelvin in temperature and several Mbar in pressure. Such models, however, have been unable in the past to explain the thermal evolution, interior structure, and magnetic field geometry in a consistent way. Therefore, recent models have proposed to consider thermal boundary layers, compositional gradients, and interface effects to match observational constraints. Of special importance for this endeavor of improvement is the understanding of the core-mantle boundary and the rock/ice mixing behavior. Specifically, it was the project’s goal to investigate physical processes at the core-mantle boundary that could provide hints on how thermal boundary layers and interfaces could be potentially formed and sustained in ice giants.
The first project part focused on the calculation of equations of state, phase diagrams, optical and transport properties of materials relevant to the interior of ice giant planets. In particular, we performed extensive density functional theory molecular dynamics (DFT-MD) simulations for hydrogen, carbon, water (H2O), ammonia (NH3), and hydrocarbons (C-H mixtures). We also performed smaller set calculations for the core materials magnesium oxide (MgO), perovskite (MgSiO3) and their mixtures with hydrogen. In the second part of the project, we developed numerical potentials that use the accurate DFT-MD results obtained in the first part. The potentials were required to perform large-scale simulations that allowed us to construct proper thermodynamic potentials and study demixing and interface effects. One highlight of our results are the study of the phase behaviors of water and in particular to study the melting line of the the so-called superionic phase, which is made of mobile hydrogen atoms diffusing through a lattice of oxygen atoms. Additionally, we studied the metallization, melting, and electrical conductivity of ammonia together with an experimental group and finally we investigated diamond formation in C/H mixtures.
Overall, the project has already led to 6 peer-reviewed publications and a white paper for the Planetary Science and Astrobiology Decadal Survey 2023-2032. Furthermore, three manuscripts are still under review and three further publications are currently prepared for submission. In particular, the described results for ammonia, water, and in particular the C-H mixtures are in itself a major achievement and will contribute to develop more realistic models for Uranus and Neptune. The studies were presented at several conferences, workshops, and seminars.
Overall, the project has already led to 6 peer-reviewed publications and a white paper for the Planetary Science and Astrobiology Decadal Survey 2023-2032. Furthermore, three manuscripts are still under review and three further publications are currently prepared for submission. In particular, the described results for ammonia, water, and in particular the C-H mixtures are in itself a major achievement and will contribute to develop more realistic models for Uranus and Neptune. The studies were presented at several conferences, workshops, and seminars.
The new method of combining DFT-MD simulations with machine learning potentials that can be exploited in large-scale simulations is a major step forward and allows to tackle scientific problems now that were entirely out of reach before. The already finished simulations on water and C-H mixtures go significantly beyond the state-of-the-art approach in the field of warm dense matter.
All results have been made available to the public and in particular the planetary modeling community. The two most impactful results of the project for planetary modeling are the diamond formation dependent on pressure-temperature conditions and C/H ratio, and the melting lines and conductivities of ammonia and water. The C-H mixture study revealed a depleting zone where diamond formation is possible regardless of the C/H ratio. That means for all conditions at roughly below 3500 K and above 130 GPa, diamond will always be present. This zone is below the conditions of current adiabatic Uranus and Neptune models, but might be relevant for new models and ice-rich exoplanets. Additionally, ammonia was found to have much higher conductivity than water at similar conditions. This effect is of particular importance for non-adiabatic models that are significantly higher in temperature than the traditional adiabatic ones. Also the melting line of ammonia was found to be much more shallow than that of water, which could be important for potentially ammonia-rich regions in ice giants.
All results have been made available to the public and in particular the planetary modeling community. The two most impactful results of the project for planetary modeling are the diamond formation dependent on pressure-temperature conditions and C/H ratio, and the melting lines and conductivities of ammonia and water. The C-H mixture study revealed a depleting zone where diamond formation is possible regardless of the C/H ratio. That means for all conditions at roughly below 3500 K and above 130 GPa, diamond will always be present. This zone is below the conditions of current adiabatic Uranus and Neptune models, but might be relevant for new models and ice-rich exoplanets. Additionally, ammonia was found to have much higher conductivity than water at similar conditions. This effect is of particular importance for non-adiabatic models that are significantly higher in temperature than the traditional adiabatic ones. Also the melting line of ammonia was found to be much more shallow than that of water, which could be important for potentially ammonia-rich regions in ice giants.