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

地球液核中固体的形成在地球系统中起着至关重要的作用。目前，整个地球的冷却导致固体内核每年从地球中心增长几毫米。值得注意的是，这个缓慢的过程提供了维持液核运动的大部分动力，而液核又负责产生地球磁场。磁场从地核发出，穿过整个地球，保护地表环境和低轨道卫星免受潜在有害的太阳辐射，并使行星能够继续居住。如果没有内核生长提供的能量，地球的磁场可能不会在今天仍然活跃。在内核形成之前，产生磁场的过程效率较低。地球的磁场至少有35亿年的历史，然而内核被认为早在5亿年前就形成了。目前尚不清楚完全液态的地核冷却是否能够提供维持这段时间所需的能量，这意味着地球长期演化的基本模型充其量是不完整的，甚至可能是不正确的。这使得最近几项备受瞩目的研究提出了在内核形成之前形成固体的机制，尽管现在是在内核的顶部。其他形式的自上而下的结晶也被提倡用于水星，火星，木卫三和月球。在所有情况下，所有现有的模型都表明，结晶通过产生不同的（可能可观察到的）层，改变流体动力学并影响全球磁场的性质，深刻地改变了行星内部的长期热和化学演化。然而，尽管经过了数十年的研究，目前所有关于行星核心动力学和演化的模型都忽略了原子尺度的过程，这些过程阻碍了晶体的成核和生长，并最终决定了系统接近平衡的方式。最近的研究表明，这些非平衡过程对于确定地球的长期演化及其磁场的起源至关重要。这项工作表明，在核心中形成固液界面实际上需要大量的能量成本，这意味着液态可以在远低于系统熔化温度的情况下持续存在。当系统进一步过冷到低于熔化温度时，能垒的大小会减小，正如在大气中观察到的那样，过冷水滴保持液态，直到尘埃颗粒周围形成雪或机翼上的冰瞬间冻结。克服能量障碍所需的过冷是如此之大，以至于目前的模型预测地球的内核不应该形成，这指出了我们对行星核心成核的理解的一个根本问题。理解地核条件下的晶体成核和其他非平衡过程（例如晶体生长）面临两个主要挑战：1）阐明地核巨大压力和温度条件下的原子尺度物理学; 2）将这些结果纳入地核宏观过程模型的理论。在这个项目中，我们将进行范围研究，将建立一个使用实验和计算模型来回答挑战1的途径，利用项目合作伙伴Mike Bergman教授独特的实验设施。我们还将产生一个研究成果，一个适用于行星核心的非平衡结晶理论，这将回答挑战2。最后，我们将在利兹大学举办一个研讨会，制定这一重要研究领域的未来战略方向。

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.