Momentous Discoveries

For the Accretionary Wedge blog festival with the theme of ‘Momentous Discoveries in Geology’, Marion Ferrat discusses how a pioneering lady discovered what lies deepest inside our planet.

We know a lot about our planet today: its position in the solar system, its age, its composition and its internal workings and structure. Many laborious experiments, observations and hypotheses have helped scientists piece together its mysteries bit by bit.

Earth within the inner solar system - Source: NASA, Wikimedia Commons.

Mercury, Venus, Earth and Mars – Source: NASA, Wikimedia Commons.

One branch of Earth Science in particular has revolutionised geoscientists’ understanding of the interior of the Earth: that branch is seismology.

Seismology is the study of seismic waves. In other words, the study of the energy released by earthquakes. Once released, this energy travels in all directions, moving from the ‘source’ point (this can be a natural earthquake or a man-made detonation), through the interior of the Earth, and back up to the surface again.

Seismology is useful because the seismic waves travel differently, and at different speeds, depending on the material they travel through. When a wave reaches a boundary between two different materials or layers within the Earth, it will be deflected: it can either be transmitted to the layer below (but in a slightly different direction), it can travel along the boundary itself, or it can be reflected back to the surface. When a wave passes through the boundary and into the next layer, the amount and direction of the deflection will depend on whether the material below is more or less dense than that above.

Seismic waves travelling through a layer of the Earth – Source: Julia Schäfer, Wikimedia Commons.

These multitude of possible pathways mean that, by looking at how and where on Earth a seismic wave arrives back at the surface, scientists can take a good guess as to what it has travelled through. By building up this information for more and more waves, they can start to paint a good picture of what is going on beneath our feet. Studying seismic waves for geoscientists is a little bit like carrying out a CAT scan for doctors: it allows them to scan the interior of something they cannot see from the outside.

Seeing to the centre of the planet

For my ‘Momentous Discovery in Geology’, I chose to look at a huge moment in the history of seismology: the discovery of the Earth’s inner core. And along with a momentous discovery, comes a momentous discoverer: Danish seismologist Inge Lehmann.

Internal structure of the Earth – Source: Kelvinsong, Wikimedia Commons.

The Earth is a little bit like an onion, in that it has layers. The outermost layer, which we live on, is called the crust. It can be as thin as a 10 kilometres under the oceans and as thick as 70 kilometres under large mountains such as the Himalayas.

Below the crust is the mantle, which makes up over 80% of our planet’s volume. The mantle is mainly solid but can behave in a viscous way when deformed very slowly, over geological timescales.

At the centre of the Earth lies the dense, metallic core. It is predominantly made of iron and nickel. The outer part of the core is liquid and plays an important role in influencing the Earth’s magnetic field.

The core lives nearly 3000 km beneath the surface and has a temperature of nearly 6000°C. It is too deep, too hot and too far to explore with any kind of instrument. This is where seismology steps in.

A liquid ball of molten metal?

Towards the beginning of the 20^th century, seismologists realised that the core must be liquid, thanks to the precious seismic waves they were observing.

When an earthquake occurs, energy is released in the form of two distinct types of seismic waves. Surface waves travel, as their name suggests, along the surface of the planet. These are the waves that cause the damage to human life and infrastructure. Body waves, on the contrary, travel inside the Earth and get deflected by the different layers they travel through, depending on whether each layer is more or less dense than its predecessor.

P- and S-waves travelling through a medium – Source: Actualist, Wikimedia Commons.

Body waves can be further split into two types, distinguishable by the way in which they displace the medium they travel through: Primary waves, or P-waves, and Secondary waves, or S-waves.

These two wave types travel differently through the Earth. One of the important characteristics of S-waves is that they cannot travel through liquid. P-waves can do but slow down considerably when not travelling through solid material.

These properties are what alerted scientists that there was something molten down in the centre of the Earth:

When seismic waves are released from an earthquake, they travel in all directions and should therefore be able to reach back to the surface all around the planet. However, seismologists noticed that seismic waves generated by an earthquake somewhere on the surface of the planet were not being observed at every seismometer on the surface. This no-wave zone is what is called the P- or S-wave shadow zone, where no arrivals can be recorded for a given earthquake.

Paths of P- and S-waves through the Earth's core: the liquid outer core cases a shadow zone - Source: USGS, WIkimedia Commons.

Paths of P-waves through the Earth’s core: the liquid outer core causes a shadow zone – Source: USGS, Wikimedia Commons.

The presence of this shadow zone meant that our P- and S-waves must be affected by something liquid, deep inside the Earth. And so arose the hypothesis of a liquid core.

Something more to the story

In 1929, a large earthquake occurred near New Zealand. Seismologists were quick to study the seismic wave arrivals at seismic stations around the world but Inge Lehmann studied them a little more closely than her peers.

She was puzzled by what she saw: seismometers located within the P-wave shadow zone of the earthquake, where no arrivals should be recorded, were showing signs of the earthquake’s waves. If the core was one large ball of liquid material, this should not be possible.

Lehmann suggested that these waves had travelled some distance inside the liquid core before bouncing off some other, previously unknown, boundary. This bouncing deflected the waves in another direction and meant that they found themselves arriving within the shadow zone.

This hypothesis was the basis of careful studying by Inge Lehmann of more seismic arrivals around the world and she eventually published her results in her revolutionary 1936 paper P’ (or P-prime). Today, the boundary between the outer and inner core is commonly known as the ‘Lehmann discontinuity’.

Inge Lehmann’s theory was later confirmed with the development of more sensitive instruments.

Lehmann was a pioneer in the world of seismology and among women scientists, establishing a new theory about the Earth in a very much male-dominated world.

In 1971, the American Geophysical Union awarded her the William Bowie medal, its highest honour. Inge Lehmann went on to live to the age of 105 and published her last paper in 1987, at the age of 99.

A momentous discoverer and scientist indeed.

I first came across the intriguing world of nanoparticles when I saw an awe-inspiring talk by nano-extraordinaire Professor Michael Hochella from Virginia Tech at the Geological Society. He wove a fascinating tale about the world at nanoscale, the special properties, the infinite uses and the potential environmental impacts as well as outlining the need for caution, scrutiny and intensive research from the scientific community in the wake of an exploding nanotechnology industry. I’ve decided to re-visit the area of nanoscience for the ‘Momentous Discoveries in Geology’ blog festival.

What’s so special about Nanoparticles?

Solutions of gold nanoparticles of various sizes. The size difference causes the difference in colors. Source – Aleksander Kondinski, Wikimedia Commons.

Nanoparticles refer to particles between 1 and 100 nanometers in size and can be found in nature, inadvertently produced by humans or most recently, manufactured as part of the boom in nanotechnology. Their geo-relevance comes from both their behaviour in nature and potential nanotoxicity but also in their manufacture for engineering processes such as environmental tracers and remediation materials. Nanoparticles are unique and special in their physical properties, which can vary greatly from macro-scale properties. Nanotechnology exploits these unusual properties to improve the efficiency and sustainability of already existing processes. They are highly mobile, have enormous specific surface areas, unexpected optical properties and can exhibit what are known as quantum effects. An example being: superparamagnetism (magnetization that can randomly flip direction under the influence of temperature) a characteristic found in ferromagnetic materials smaller than 10nm. Nanoparticles can also have unusual optical properties: gold nanoparticles for example appear deep red to black in solution, depending on their size. They also melt at much lower temperatures (~300 °C for 2.5 nm size) than gold slabs (1064 °C). Their first known use for their colour-changing properties was back in roman times (30BC – 640AD), using gold nanoparticles, which were impregnated into the glass of

The Lycurgus Cup – a 4th-century Roman glass cage cup , which shows a different colour depending on whether or not light is passing through it; red when lit from behind and green when lit from in front due to the incorporation of nanoparticles – You can go and see it in the British Museum! Source – Johnbod, Wikimedia Commons.

goblets to give a colour change from green to blood red when lit from behind. The requirement of the exact mixture of sizes to manifest this effect suggested that the Romans knew what they were doing!

Early history of nanoparticles

The discovery of nanoparticles has not so much happened as a single momentous discovery but as a series of moments over a ~100 year period. As with many discoveries, they are as a result of breakthroughs in instrumentation and technology which breakdown the barrier to discovery. In addition to the early use in roman times, nanoparticles were also used in the medieval (500-1450AD) and renaissance (1450-1600 AD) periods, again for their colour-changing properties for use in stained glass windows in medieval times and for colouring ceramics during the renaissance. The deep reds you see are caused by the incorporation of gold nanoparticles in the glass and the deep yellows are caused by silver nanoparticles. We also see this effect used in Ceramics. In the Islamic world, where the incorporation of gold in artistic representations was not allowed, the solution was using dense nanoparticulated layers of glaze to generate a golden metallic shine. This colour changing capacity is caused by the size of the materials that are incorporated. Photography is also an early example of nanotechnology which is reliant on the production of silvernanoparticles that are sensitive to light.

When were they discovered?

Michael Faraday – 1861. He first described the optical properties of gold nanoparticles in his classic 1857 paper. Source – Wikimedia Commons.

The earliest and most significant breakthrough came during Michael Faraday’s pioneering experiments and his seminal paper in 1857 where he described the optical properties of nanometer-scale metals for the first time. He prepared the first metallic colloids (fine particles that suspend in solution, in between dissolved and settling particles, size range between 2-500 nm). He saw that they had special electronic and optical properties and stated – “It is well known that when thin leaves of gold or silver are mounted upon glass and heated to a temperature which is well below a red heat (~500 °C), a remarkable change of properties takes place, whereby the continuity of the metallic film is destroyed. The result is that white light is now freely transmitted, reflection is correspondingly diminished, while the electrical resistivity is enormously increased.”.

348px-Ernst_Ruska_Electron_Microscope_-_Deutsches_Museum_-_Munich-edit

First Electron Microscope with resolving power higher than that of a light Microscope designed by Ernst Ruska in 1931 with a magnification of around 12,000 times. Source – J Brew, Wikimedia Commons

While the unusual and fascinating properties of nanoparticles had been described, they were still too small to be seen and it took another 84 long years before electrical engineer Max Knott and physicist Ernst Ruska constructed the prototype electron microscope in 1931. This breakthrough put a spotlight on the “small world” and was an important step in allowing research at the nanoscale. This was followed by Erwin Mueller’s field-ion microscope which allowed the viewer, for the first time in history to observe individual atoms and their arrangement on a surface. This was a landmark invention allowing magnification of more than 2 million times. These technological developments formed the foundation for many other nano breakthroughs to come such as the ‘tunneling phenomenon’, the development of the field of molecular electronics, the development of Surface Enhanced Raman Spectroscopy (SERS), instrumental in the field of nanotechnology, the buckyball, quantum dots (which have implications for how solar energy is collected) and carbon nanotubes.

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This post is for a geoscience blog carnival called The Accretionary Wedge, which is being hosted by Matt Herod and you can see the call for posts here.

Momentous Discoveries

A momentous discovery deep below: Earth’s inner core