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 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.

The proposed international collaboration seeks to address fundamental problems with current theories that describe the evolution of the Earth and planetary cores and so the most immediate beneficiaries of the research will be academics involved in geosciences and planetary science. Also, while we expect substantial broader impact from longer term results that will emerge from this area of research, there is a significant scoping element to the proposed work where the immediate wider impact is hard to predict. However, we do believe that the subject area is of significant interest to the general public, especially those with an interest in science or scientific education. By engaging with these groups, it may be possible to derive indirect societal and economic benefit by the increase of technical literacy and the study of STEM subjects. In particular, we have identified three groups who we believe are particularly likely to engage with our work, and who we will target during the impact activities associated with this project. These are:

1. Scientifically inquisitive members of the general public

2. Teachers of STEM subjects in secondary education

3. Primary and secondary students studying science or mathematics

To engage members of the general public we will undertake a range of activities (described in the Pathways to Impact document) that will ensure that our research is disseminated to the widest possible audience. These activities include writing popular articles, presenting public talks on the broader implications of our science, and participating in university-led outreach events. We expect the immediate benefits to this group will probably be cultural (enjoyment and understanding of science) rather than economic. However, we should not understate the longer-term social and economic benefits that may accrue. The timescale for benefits from this activity is probably quite short, similar to the length of the project.

To engage STEM teachers we will piggy-back on an existing programme of collaborative activities, including annual workshops, at the University of Leeds. These workshops are designed to inform teachers of A-level and GCSE science of the range of opportunities for their students in the Earth sciences, and to provide activities that can be delivered by the teachers in a classroom setting. This activity will yield a direct benefit to education (and consequentially to the economy). In addition, many students (many more than we could reach directly) will benefit from this activity. The timescale for the direct benefits to teachers is quite short: material can be delivered imminently after each annual workshop. However, benefits to students reached by these activities may last a lifetime.

Further engagement with students will be via existing activities at Leeds. These activities reach a very large number of secondary students Yorkshire. The aim is to drive interest in STEM subjects with consequential benefits to students (who can expect better life-time economic outcome) and society in general. The timescale for the benefits of these activities is long, with benefits probably not accruing until students reach working age.