Resolving the Inner Core Nucleation Paradox

坚固的内核是地表下5000多公里的铁球和镍球，是地球上最偏远的地区，但在地球系统中扮演着至关重要的角色。随着整个地球的降温，内核每年都会从地球中心向外增长几毫米。值得注意的是，这个缓慢的过程是维持外核流体运动的主要动力源，而外核流体运动是产生地球磁场的主要原因。磁场从整个地球的核心和线条发出，屏蔽了地表环境和低轨道卫星的潜在有害太阳辐射，使其能够继续适合行星居住。如果没有内核增长提供的能量，地球磁场可能不会在今天仍然活跃。内核的存在从根本上改变了液核中流体流动的动力学，以一种仍有争议的复杂方式改变了我们在地球表面观察到的磁场，并可能影响磁极反转和漂移的过程和特征。内核的生长也影响了地震学观测到的外核顶部和底部神秘区域的结构和演化，这一点很重要，因为它们显然无助于产生磁场。然而，尽管经过了几十年的研究，最近的研究发现，我们对内核如何形成的理解存在着重大差距。令人惊讶的是，这种变化是如此显著，以至于我们对地球演化的最先进模型暗示，内核不应该形成。鉴于地球有一个坚实的内核，这导致我们对地球演化的理解出现了很大的差距。内核成核悖论产生于液体在融化温度下冷却成固体的方式。在熔化温度以下，固体的能量低于等量液体的能量。虽然这意味着从液体形成固体将是有利的，但在没有外表面(所谓的均匀成核)的情况下，需要一些能量来形成固-液界面；在这个能量障碍被克服之前，液体状态甚至可以持续到熔点以下。当系统的过冷温度进一步低于熔化温度时，势垒的尺寸会减小。我们在大气中观察到了这种效应，在那里过冷的水滴以液体状态持续存在，直到尘埃颗粒周围形成雪或飞机机翼上的冰迅速冻结。这些例子还说明了非均相成核的重要性，即预先存在的固体(例如飞机机翼)降低了能量障碍，并允许快速冻结。过冷是当前内核形成模型中缺失的因素。最近的工作，包括我们自己使用原子尺度模拟进行的初步研究，表明在核心条件下，铁的均匀形核所需的过冷量非常大：在可用的十亿年时间尺度上，内核成核需要700-1000K。这一规模太大，与当前的核心增长理论不相容。因此，我们无法解释在地球中心存在固体内核的原因，即使我们知道它的存在。这就是核心成核悖论。在这个提议中，我们将通过一种多学科的方法来解决内核成核悖论，该方法将原子尺度上的成核模拟与跨越过去45亿年的地球演化模型相结合。我们将确定内核是均匀成核还是非均匀成核，并为内核年龄设定可靠的界限。这些结果将被纳入新一代核心演化模型，这些模型将提供地球深部演化的连贯图景，并形成解释地球深部的基本磁力和地震观测的框架。

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 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 to 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 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.