Resolving the Inner Core Nucleation Paradox

The solid inner core, a ball of iron and nickel over 5000 km below the surface, is the most remote region of our planet and yet plays a crucial role in the Earth system. As the whole planet cools, the inner core grows outwards from Earth's centre by a few millimetres each year. Remarkably, this slow process is the dominant power source that sustains fluid motion in the outer core, which is responsible for generating Earth's magnetic field.

The magnetic field emanates from the core and threads through the whole Earth, shielding the surface environment and low-orbiting satellites from potentially harmful solar radiation and enabling continued planetary habitability. Without the power supplied by inner core growth, Earth's magnetic field would probably not still be active today. The very presence of the inner core fundamentally changes the dynamics of fluid flow in the liquid core, altering the field we observe at Earth's surface in a complex manner that is still debated and may influence the processes and characteristics of magnetic polarity reversals and excursions. Growth of the inner core also affects the structure and evolution of enigmatic regions at the top and bottom of the outer core observed by seismology, which are important because they apparently do not help to generate a magnetic field.

However, despite decades of study, recent work has uncovered a significant gap in our understanding of how the inner core formed. Astonishingly, the change is so significant that our most advanced models of Earth's evolution imply that the inner core should not have formed. Given that Earth has a solid inner core, this leads to a significant gap in our understanding of the evolution of our planet. The inner core nucleation paradox arises from the way that a liquid transforms into a solid as it cools through its melting temperature.

Below the melting temperature, the energy of the solid is lower than the energy of the same amount of liquid. Although this means the formation of the solid from the liquid would be favoured, in the absence of external surfaces (so-called homogeneous nucleation) some energy is required to form a solid-liquid interface; until this energy barrier is overcome the liquid state can persist even below the melting point. The size of the barrier decreases as the system is supercooled further below the melting temperature. We observe this effect in the atmosphere where supercooled water droplets persist in the liquid state until snow forms around dust particles or ice flash-freezes on aircraft wings.

These examples also illustrate the importance of heterogeneous nucleation, where a pre-existing solid (e.g. an aircraft wing) reduces the energy barrier and allows rapid freezing. Supercooling is the missing ingredient from current models of inner core formation. Recent work, including our own pilot study using atomic-scale simulations, suggests that the amount of supercooling required for homogeneous nucleation of iron under core conditions is very large: 700-1000 K is needed for the inner core to nucleate on the billion-year timescale available.

This is too large to be compatible with current theories of inner core growth. We thus cannot explain the presence of a solid inner core at the centre of the Earth, even though we know it exists. This is the inner core nucleation paradox. In this proposal, we will resolve the inner-core nucleation paradox by a multidisciplinary approach that combines simulation of nucleation at the atomic scale with models of Earth's evolution spanning the last 4.5 billion years.

We will determine whether the inner core nucleated homogeneously or heterogeneously and place robust bounds on the inner core age. These results will be incorporated into a new generation of core evolution models that will provide a coherent picture of deep Earth evolution and form the framework for interpreting fundamental magnetic and seismic observations of Earth's deep interior.