‘Gold arrived on our planet as Earth was forming 4.5 billion years ago — it holds an extraordinary history'
Jun Korenaga
Jun Korenaga
is Professor of
Earth
and Planetary Sciences at
Yale University
. He tells
Srijana Mitra Das
at TE about how
gold
got to Earth:
It's a little surprising to connect to Jun
Korenaga
, not least because the scientist is sitting against the backdrop of a planetary surface that could be — but doesn't have to be — Mars. Speaking with purple rock and feathery clouds in a sunless sky behind him, Korenaga explains the origins of gold — and Earth.
'My work focuses on estimating early Earth's history. In the last few years, I've worked on the Hadean Eon which was about 4.5 billion years ago — this is the most mysterious part of our planet's history because we don't have a rock record for it. I work on the theoretical side and try to reconstruct what early Earth looked like.' Gold is part of early Earth's story, although in unexpected ways. A symbol of stability now, gold had quite a dramatic past. Supernovae or cataclysmic stellar explosions and star collisions occurred in the universe. The extreme pressure of such imploding stars was so high, subatomic protons and electrons got pushed into their core — these formed neutrons. Rapid neutron capture by iron then created heavy elements like uranium, lead, silver and gold. Interestingly, this process occurred very, very swiftly — and then, these elements were expelled into space.
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Thus, metals like gold and platinum arrived on Earth while our planet was still forming. Korenaga explains, 'About 4.5 billion years ago, Earth was hit by a Mars-sized rock and the moon formed as the debris from this collision went into an Earth-orbiting disk. More bombardment followed — there were plenty of leftover rocks orbiting the sun as well and several fell on young Earth. The landing of these objects is known as 'late accretion', comprising about the last 1% of planetary growth. In this period, some of the rocks which fell on Earth had metallic components like gold.' Importantly, gold and platinum are among highly siderophile elements (HSEs) — these are metals with an extremely strong affinity for iron. Korenaga smiles slightly and says, 'Now, if Earth was created with no funny twists in its story, we actually wouldn't have any gold on our planet's surface because, sticking to iron, this was heavy and should have gone straight down into the core which we cannot access. But we do have gold on the surface, which shows that part of Earth's mantle can retain metallic components.' Korenaga's research, conducted with
Simone Marchi
, posits the notion that there is a thin or 'transient' part of the mantle where shallow areas melt away and a deeper region stays solid. This part could hold falling metallic components and reach them to the mantle. In the simulations the scientists conducted, as a rock crashed onto Earth, it hit a localised liquid magma ocean where heavy metals sank to the bottom. As these reached the partially molten area underneath, the metal would start sinking further down — then, the molten mantle solidified, capturing the metal there.
But how did this re-emerge to the part of Earth's surface humans could access? As Korenaga says, 'The part of the mantle which contains this metallic component is heavier and more chemically dense than the rest — to bring it up, you have to offset that density by being hotter than normal as hotter materials usually have lower density. Thus, thermal currents from Earth's core outweighed that compositional density and made these materials move up from the solid mantle to Earth's surface.' This process is called 'mantle convection', when hot mantle material rises as colder material sinks. Earth's mantle is almost totally solid — yet, over long geologic periods, it acts like a pliable material which can mix and move things within it. Those include the HSEs — like gold — which came to Earth from massive collisions billions of years ago and then reached the planet's surface through these enormous, yet intricate internal processes.
Is gold found on other planets as well? Korenaga says, 'Gold is found on the moon — but its abundance there is much lower than on Earth. It is found on Mars too. Of course, we don't have direct samples from Mars but we have so-called Martian meteorites. These are found on Earth but because of their isotopic features, they are traced back to Mars. Analysing these rocks shows us the presence of highly siderophile elements there — again though, the abundance of these, like gold and platinum, is much lower than on Earth.' Do siderophiles contain a larger story of the formation of our solar system — and universe? Korenaga comments, 'We understand planetary formation in terms of silicate rocks and iron which makes up most of the core. Iron is a major element while silicate rocks are made of silicon, oxygen, magnesium, iron, etc. Highly siderophile elements exist in very small abundances — their presence by itself doesn't drive any major planetary formation processes but they stick with iron and by measuring such trace elements, we can study more details of planetary formation.' These abundances thus help us decipher the paths planets once took.
The Golden Moon: This too has gold on it
THE GOLDEN MOON:
This too has gold on it
Given its incredible history — arriving on Earth 4.5 billion years ago, seeping deep into its mantle, pushed to the top by extraordinary forces operating from within our planet — how should we think of gold the next time we see it? Korenaga replies, 'Gold is, of course, widely available as jewellery and other items from shops but when we look at it, we should actually think about its extraordinary origins — we shouldn't take gold for granted. Its presence helps us understand crucial details of the very existence of this metallic Earth and the formation of its unique atmosphere which is 78% nitrogen and 21% oxygen — why did our planet develop in this way? A simple everyday item like gold can hold big answers to that.'
Korenaga concludes by remarking, 'Of course, my work explains why Earth's mantle has some amount of gold or platinum at the surface level but for humans to access pure gold, you need a concentrated form in a mine,' He adds, with a scientist's exactness, 'It is extremely inefficient otherwise to extract gold from rocks — but to understand the formation of gold mines, you need deeper knowledge about very local processes. And that is another story.'
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