Postgraduate research study
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PhD project: Phase relations in planetary ices from atomistic and free energy calculations

Project description

The interior structure of planetary bodies determines, via the physical properties of their constituent materials, global observables such as magnetic fields, plate tectonics, and sustained presence of atmospheres and surface water. Yet those physical properties are often not known or not well enough constrained, resulting in large uncertainties in planetary modelling, in particular when applied to exoplanets, planetary bodies outside our solar system. This creates an unacceptable mismatch between the large number of exoplanets currently detected and characterized – and our understanding of their formation and evolution to the present day.

Exoplanet water worlds are, apart from gas giants, the most commonly found types of planetary bodies, and the largest water reservoirs in the known universe - containing vast amounts of water, methane, and ammonia. How these planetary ices mix and arrange at planetary conditions is not well known. While ices can be probed individually at planetary mantle conditions, their phase relations have only partly been investigated and planetary applications were poorly made, which includes limits to miscibility, existence and extent of ionic and superionic phases, and melt lines. Melting/crystallising relations of a multicomponent system can be studied by chemical thermodynamics, which however requires many physical parameters for each constituent phase in the system. Due to technical difficulties, constraining the properties of icy materials under high pressure and temperature condition by laboratory experiment is very challenging. On the other hand, these physical properties including equation of state can be obtained from atomistic calculations.

In this fully funded project, you will study the ammonia-water system at high pressures and temperatures and establish pressure-temperature-composition phase relations grounded in atomistic simulations, which feed into thermodynamical calculations. You will be jointly supervised by Andreas Hermann (School of Physics and Astronomy), who will advise on first-principles crystal structure prediction and molecular dynamics simulations, and Tetsuya Komabayashi (School of GeoSciences), who will advise on mineral thermodynamics and free energy calculations of melting relationships of a multicomponent system.

The project aims to apply recent method developments in machine learning-accelerated structure searching to the ammonia-water system, which will result in a comprehensive first principles description of phase relations in these mixtures, initially in the ground state. Results will provide a starting point for molecular dynamics simulations to characterize high temperature states across a range of compositions, and to extract transport coefficients and thermal equations of state. These will feed into thermodynamic models for melting of ammonia-water at high pressures, and predict the fluid-solid layer boundary in the planets by calculating eutectic relations. This will establish de-mixing conditions, with consequence for stratification inside planets.

The University of Edinburgh provides an excellent environment for your project, where you can interact with our interdisciplinary Centre for Science at Extreme Conditions (CSEC), our Centre for Exoplanet Science, and our UK Centre for Astrobiology, amongst others.

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