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

点击翻译按钮获取中文摘要

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.