YES-27-D-24: Can Heterogeneous Nucleation Explain Earth’s Solid Inner Core?

The solid iron inner core at the centre of the Earth underpins fundamental processes that shape our planet. The boundary between the solid inner and liquid outer core is at its melting temperature, providing crucial information used to determine the temperature profile and cooling rate of the entire deep Earth. As the inner core slowly grows outwards, it releases latent heat and low-density fluids, driving outer-core convection which generates our global magnetic field. This field shields Earth’s atmosphere and surface from harmful space weather, making it a key component of the natural world and essential to the operation of satellites, telecommunications networks and power grids. Despite its importance, the origins of the inner core remain a mystery.

All liquids must cool below their melting temperature (undercool) before they can transform into solids. This is true of cloud droplets in Earth’s atmosphere (which can remain liquid to ~-40 °C), and it is true of the iron-rich liquids that solidified ~1 billion years ago to become Earth’s inner core. Atomic-scale simulations of how this solid nucleation process might occur in Earth’s core suggest that hundreds of degrees of undercooling are needed for inner-core nucleation, but this would produce a larger-than-observed inner core. In this PhD project, you will study how solid particles predating the inner core could have sparked its nucleation, helping to resolve a key event in the evolution of our planet’s deep interior.

Thus far, inner-core nucleation has been studied almost entirely through the lens of homogeneous nucleation, where a pure, undercooled liquid spontaneously forms a solid. In nature, heterogeneous nucleation prevails, where an existing solid surface (dust or pollen in the case of hail) provides an energetic shortcut to nucleation by allowing the new solid to form with less surface contact with the surrounding liquid. Pre-existing solids in the Earth’s core must be dense (to reach the centre where undercooling is greatest), resistant to melting and dissolution, and must significantly reduce the liquid-exposed surface area of newly formed iron-rich crystals.

You will use molecular dynamics simulations of ~100,000 atoms, performed on University of Leeds supercomputers (Aire and Calder), to test the melting, solubility and surface affinity of candidate heterogeneous nucleation materials under the conditions of Earth’s core (360 GPa, 5000 K). For viable candidate materials, you will identify the likely conditions of Earth’s core at the time of nucleation, paving the way for defining the state of the planet’s interior at this crucial stage. This will help to decipher what impact inner-core nucleation had at the time and whether it might still be detectable today in the palaeomagnetic record.

Profile: The ideal candidate will have a background in geophysics or a related field. Experience or interest in condensed-matter physics, thermodynamics and statistical mechanics will be beneficial. An aptitude for programming languages such as Python and Bash would be highly advantageous. The student will be based in the School of Earth, Environment and Sustainability at the University of Leeds, with access to a vibrant research community, interdisciplinary seminars, high-performance computing facilities and technical training.

Further reading:

• Wilson, et al., 2025. The formation and evolution of the Earth’s inner core. Nature Reviews Earth & Environment, 6(2), pp.140-154. https://doi.org/10.1038/s43017-024-00639-6
• Huguet et al., 2018. Earth’s inner core nucleation paradox. Earth and Planetary Science Letters, 487, pp.9-20. https://doi.org/10.1016/j.epsl.2018.01.018