Diffusion in the DAC: Probing the physical state of the Earth's inner core

内核是地球内部最深、最难接近的一层。它是一个由固体铁组成的球体，由一些镍和一种或多种轻元素(如硅、硫和碳)合金化而成，直径2500公里，每年增长1毫米。它内部的条件是难以想象的极端，压力高达地球表面压力的360万倍，温度接近6000摄氏度，类似于太阳表面。核心是地球系统的中心。当它结晶时，它会产生潜热，帮助驱动其上方液态铁外核的对流，从而驱动地球的热机。由于一些轻元素，如氧，更喜欢液态铁而不是固体，内核的生长也改变了外核的化学，这可能反过来影响整个地球的化学。许多地球地球发电机的数值模型--对流的外核产生地球磁场的机制--出于几个原因需要内核的存在，包括它的增长产生的额外热量。地磁场使我们免受太阳风的有害影响。因此，内核对于我们星球上生命的发展方式以及今天维持地球表面的温和条件都是重要的。然而，关于它的结构和演化，我们还有很多不了解的地方。我们所知道的很多东西都来自于对穿过内核的地震波的研究。这些研究告诉我们，这是一个复杂的地方，地震波在两极之间的传播速度比穿过赤道的速度还要快。还有一种所谓的“半球二分法”，即内核表面的地震波在东半球的传播速度快于西半球。也有证据表明，有一个直径约1000公里的“最内核”，其地震特征与内核的其余部分不同。一些研究甚至表明，整个内核的自转速度可能比地球其他部分更快。理解和解释这些令人惊讶的发现需要了解制造内核的铁合金的物理性质。其中最重要的性质之一是粘度，目前还没有对其进行直接测量。这样的测量正是这项研究的目的。粘度可以通过测量铁原子在铁晶体中扩散的速度来确定。这将通过两种方式完成。首先，使用布里斯托尔大学地球科学学院的激光加热钻石砧座。这种设备由两个相对的宝石级平顶钻石组成，在这两个钻石之间，小于人类头发直径(约100微米)、约5微米厚的铁盘被压缩到高达2亿巴的巨大压力。这些圆盘将被一层富含其中一种同位素的铁涂层作为示踪剂。在高压下，使用红外激光将样品加热到高达4000摄氏度的温度，使示踪原子在铁中扩散。在爱丁堡大学NERC离子微探测器设施中使用一种名为二次离子质谱仪(SIMS)的技术，我们可以一次剥离几个原子层的铁，测量每个阶段的示踪剂浓度。示踪原子在特定加热时间内扩散的距离可以告诉我们扩散速度，由此可以确定粘性。即使使用这种技术，也很难达到内核的极端条件，所以我们将使用第二种方法，伦敦大学地球科学系的从头计算机模拟。在这种方法中，使用量子力学方法在计算机中模拟了一盒铁原子。假设的示踪原子在盒子中扩散时可以被跟踪，这也允许我们计算扩散速度和粘度。

The inner core is the deepest and most inaccessible layer within the Earth. It is a sphere of solid iron, alloyed with some nickel and one or more 'light' elements (such as silicon, sulfur and carbon) and is 2500 km in diameter and grows by 1 mm every year. The conditions within it are unimaginably extreme, with pressures up to 3.6 million times the pressure at the surface of the Earth, and temperatures near 6000 C, similar to the surface of the sun. The inner core is central to the Earth system. As it crystallises it produces latent heat that helps drive convection in the liquid iron outer core above it, which drives the Earth's heat engine. Because some light elements such as oxygen prefer liquid iron to the solid, the growth of the inner core also changes the chemistry of the outer core, which may in turn effect the chemistry of the whole Earth. Many numerical models of the Earth's geodynamo - the mechanism whereby the convecting outer core produces the Earth's magnetic field - require the presence of the inner core for several reasons, including the extra heat produced by its growth. The geomagnetic field shields us from the harmful effects of the solar wind. Thus the inner core is important to the way in which life has developed on our planet and for maintaining the clement conditions on its surface today.Yet there is much we do not understand about its structure and evolution. Much of what we do know comes from the study of seismic waves that pass through the inner core. These studies tell us that it is a complex place, with seismic waves travelling faster from pole to pole than they do through its equator. There is also a so-called 'hemispheric dichotomy' where seismic waves in the surface of the inner core travel faster in the eastern hemisphere than the western hemisphere. There is also evidence that there is an 'innermost inner core' around 1000 km in diameter with a different seismic signature to the rest of the inner core. Some studies have even suggested that the whole inner core may be rotating faster than the rest of the Earth. Understanding and interpreting these surprising discoveries requires knowledge of the physical properties of the iron alloy from which the inner core is made. One of the most important of these properties is viscosity, for which no direct measurements have yet been made. Just such a measurement is the aim of this research.Viscosity can be determined by measuring how fast iron atoms diffuse through crystals of iron. This will be done in two ways. Firstly, using the laser-heated diamond anvil cell at the School of Earth Sciences, University of Bristol. This equipment consists of two opposing gem-quality diamonds with flat tips, between which discs of iron, less than the diameter of a human hair (around 100 microns) and around 5 microns thick, are compressed to enormous pressures up to 200 million bar. The discs will be coated with a layer of iron, enriched in one of its isotopes, to act as a tracer. While at high-pressure, the sample is heated to temperatures up to 4000 C using infrared lasers and causing the tracer atoms to diffuse through the iron. Using a technique known as secondary ion mass spectrometry (SIMS) at the NERC ion microprobe facility, University of Edinburgh, we can strip away the iron, a few atomic layers at a time, measuring the tracer concentration at each stage. How far the tracer atoms managed to diffuse during a certain heating time tells us the diffusion rate, from which the viscosity can be determined.Even with this technology, it will be difficult to reach the extreme conditions of the inner core so we will use a second method, ab initio computer simulation at the Department of Earth Sciences, UCL. In this method, a box of iron atoms is simulated within a computer using quantum mechanical methods. Hypothetical tracer atoms can be followed as they diffuse through the box, again allowing us to calculate the rate of diffusion and the viscosity.