Non-equilibrium thermodynamics in Earth's core -- the agenda for the next decade

The formation of solids in Earth's liquid core plays a crucial role in the Earth system. At the present day, cooling of the whole planet leads to the growth of the solid inner core from Earth's centre by a few millimetres each year. Remarkably, this slow process provides most of the power that sustains motion in the liquid core, which is in turn responsible for producing 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. Before the inner core formed, the process of generating the magnetic field was less efficient. Earth's field is at least 3.5 billion years old and yet the inner core is thought to have formed as early as half a billion years ago.

It is currently unclear whether cooling of an entirely liquid core could have provided the power needed to sustain the field for this time period, implying that the fundamental model of Earth's long-term evolution is at best incomplete and possibly incorrect. This has led several recent high-profile studies to propose mechanisms for forming solids prior to inner core formation, though now at the top of the core.

Other forms of top-down crystallization have also been advocated for Mercury, Mars, Ganymede and the Moon. In all cases, all current models show that crystallization profoundly alters the long-term thermal and chemical evolution of planetary interiors by producing distinct (and possibly observable) layers, changing the fluid dynamics and influencing the properties of global magnetic fields. Yet, despite decades of study, all current models of the dynamics and evolution of planetary cores ignore the atomic-scale processes that hinder the nucleation and growth of crystals and ultimately determine the systems' approach to equilibrium.

Recent work has left no doubt that these non-equilibrium processes are crucial for determining Earth's long-term evolution and the origin of its magnetic field. This work showed that there is actually a substantial energy cost for forming a solid-liquid interface in the core, meaning that the liquid state can persist far below the melting temperature of the system. The size of the energy barrier decreases as the system is supercooled further below the melting temperature, as observed in the atmosphere where supercooled water droplets remain liquid until snow forms around dust particles or ice flash-freezes on aircraft wings.

The supercooling required to overcome the energy barrier is so large that current models predict that Earth's inner core should not have formed, pointing to a fundamental problem with our understanding of nucleation in planetary cores. Understanding crystal nucleation and other non-equilibrium processes (e.g. crystal growth) at core conditions faces two main challenges:

1. elucidating the atomic-scale physics at the immense pressure and temperature conditions of Earth's core;
2. a theory for incorporating these results into a model of the macroscopic processes in Earth's core.

In this project we will conduct a scoping study that will establish a pathway for using experimental and computational models to answer challenge 1, utilising the unique experimental facilities of project partner Prof. Mike Bergman. We will also produce a research output, a theory of non-equilibrium crystallization that is suitable for application to planetary cores, that will answer challenge 2. Finally, we will hold a workshop at the University of Leeds that will map out the future strategic direction of this important research area.