Colliding neutron stars

A long time ago in a galaxy far, far away: the origins of gold

Image credit: Science Photo Library

Observations of world-shattering events that took place millions of years ago in distant galaxies are providing scientists with clues about how gold is created and why it’s easier to obtain than it should be.

It’s hard to tell accurately how much gold there actually is on Earth. Despite the fact that more than 90 per cent of what’s in circulation has been mined since the California Gold Rush of the mid-19th century, people have been digging it up for thousands of years before that with no monitoring, so there’s no way of putting a precise figure on what there is in bank vaults and jewellery boxes, and how much is left unearthed.

That hasn’t prevented various people with an interest in the value of gold from making an educated guess. Estimates vary widely though, from just over 150,000 tonnes up to 2.5 million tonnes depending on whom you ask.

In the middle ground are the World Gold Council, which reckons 187,200 tonnes of gold have been mined, and Thomson Reuters GFMS, whose annual gold survey estimates 171,300 tonnes.

Perhaps predictably, investors whose wealth is boosted by scarcity suggest lower amounts are available, arguing that until the Middle Ages mining technology was simply too primitive to account for the quantities that would have to have been extracted to justify higher figures. There’s also the problem of how much to allow for to compensate for illegal and unreported mining today.

What’s more important for the jewellery industry and other sectors like electronics that are the principal users of gold is how much there is left to be removed. The key issue here is that reserves which may not be economic to search for or excavate now might become economically viable as their value increases. At current rates though, the US Geological Survey predicted at the end of 2017 that 54,000 tonnes remain to be tapped.

Whatever the true figure, geologists and astrophysicists alike continue to make new discoveries in their attempts to work out how the Earth’s gold reserves got there in the first place.

For a start, there’s much more of it that’s relatively accessible – with a little effort – than there should be. During the Earth’s formation billions of years ago, molten iron sank to its core, taking with it the vast majority of the embryonic planet’s precious metals. It’s reckoned there are enough elements like gold and platinum at the centre of the planet to cover its surface with a layer four metres deep.

That’s no help for the foreseeable future. Fortunately though, the same metals are up to thousands of times more abundant in the Earth’s topmost mantle layer than this theory would predict.

One explanation for this anomaly is that a massive meteorite shower bombarded the Earth after its core was formed, adding gold and other elements like sprinkles in the icing on a cake. Evidence for this comes from a 2011 analysis of rocks from Greenland that are nearly four billion years old. Scientists at Bristol University’s School of Earth Sciences didn’t measure gold content, but were able to make deductions from the first assessment of the isotopic composition of tungsten in rocks of this age.

Their research, reported in Nature, found a 15 parts per million decrease in the relative abundance of isotope 182W between the Greenland samples and recent rocks, an effect which suggests that the excess of accessible gold is a by-product of meteorite bombardment.

Bristol’s Dr Matthias Willbold said this is evidence of collisions that would have been literally earth-shattering at the time, but proved to be fortuitous in the long-term.

“Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material,” he explained. The impacting meteorites were stirred into the Earth’s mantle by a convection process acting on a massive scale before subsequent geological processes that formed the continents concentrated gold and other precious metals into the ore deposits that are mined today.

Alluvial gold mine in Guyana

Alluvial gold mine in Guyana

Image credit: Image Source

Research on Earth can help explain how gold got here. Where it came from, and how it may have been created in the first place, is another part of the story that is still being confirmed by observations of events that took place millions of years ago.

The first successful detection of gravitational waves last year was a big breakthrough that has been described as marking a new chapter in astrophysics. Among other things it’s a significant step in confirming the origins of gold and other elements.

The initial observation in August 2017 by LIGO, the USA’s Laser Interferometer Gravitational-Wave Observatory, was followed two seconds later by the detection of a short duration gamma-ray burst by the Fermi Space Telescope satellite.

The subsequent flurry of observations identified the source as the distant NGC 4993 galaxy. Light reaching Earth now gives an idea of what happened in the galaxy 130 million years ago when it was relatively old and no longer forming many stars. A pair of dense neutron stars – each as heavy as our Sun yet only 10km across – drew towards each other, rotating increasingly quickly until they were spinning around each other 500 times per second.

As well as sending ripples through space that scientists are seeing today, their eventual collision appears to have resulted in the formation of huge amounts of gold, platinum and other heavy elements through the process of supernova nucleosynthesis. It’s estimated that gold alone equivalent to the mass of the Earth was pumped out into the universe.

Professor Andrew Levan, whose team at the University of Warwick’s Astronomy & Astrophysics Group was the first to get observations of the source from the Hubble Space Telescope, said that as soon as they saw the data they realised they had found a new kind of astrophysical object. “This ushers in the era of multi-messenger astronomy,” he said. “It is like being able to see and hear for the first time.”

Levan’s colleague Dr Joe Lyman was at the European Southern Observatory in Germany and alerted the astrophysics community that the source was unlike any seen before. “The exquisite observations obtained in a few days showed we were observing a kilonova, an object whose light is powered by extreme nuclear reactions,” he said. “This tells us that the heavy elements, like the gold or platinum in jewellery, are the cinders, forged in the billion-degree remnants of a merging neutron star.”

The processes that take place in galaxy NGC 4993 can’t be replicated in the laboratory, but having observed them, scientists can improve their understanding of what’s going on by using sophisticated computer simulations.

Dr Andreas Bauswein from the Heidelberg Institute for Theoretical Studies has been awarded a €1.5m European Research Council grant for a five-year project that will use the gravitational wave observation as the basis for attempts to better understand what happens when neutron stars collide, including the process by which  metals like gold are created.

As well as working out how much matter is ejected, and how, GreatMoves (General relativistic moving-mesh simulations of neutron star mergers), will consider what the gravitational wave measurements can tell us about the fundamental properties of matter. The hope is that analysing their exact form may reveal more about the innermost building blocks of matter.

Have we reached peak gold?

Economy

Mining accounts for around three quarters of the world’s annual demand for gold, the remainder coming from recycling. But even with tens of thousands of tonnes still in the ground and more likely to be discovered, analysts have begun warning of ‘peak gold’ – the point at which extraction reaches a maximum and starts to decline towards the point where there’s none left to be economically added to reserves. Unlike fossil fuels, all the gold that’s ever been dug up is still in existence and available to be used by sectors like the jewellery industry, which is by far the biggest customer. Increasing demand from the technology sector is having a small but significant effect on world supplies, however. The British Geological Survey estimates that about 12 per cent of current world gold production finds its way into electronics, where it is often used in such small quantities that it may no longer be economical to recycle it and for the first time we are starting to gradually use up our large but finite supplies.

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