You’ve probably heard of a catalytic converter — it’s that thing in your car’s exhaust pipe that transforms nasty, unburnt hydrocarbons into less toxic emissions.
But not many of us would know that the origins of our catalytic converters are bedded in one of the most violent chapters of Earth’s history: when our young planet was under intense bombardment by huge asteroids.
Catalytic converters are made from platinum-group metals — the rarest metals on our planet, and they have an extra-terrestrial origin.
OK, back up. What are platinum-group metals?
The platinum-group metals — platinum, palladium, rhodium, iridium, osmium, ruthenium and rhenium — are the seven rarest metals found on the surface of our planet.
Together, they make up about 0.0000002 per cent or 0.2 millionths of the Earth’s crust.
These metals, which we’ll call PGMs for short, are prized not just for their scarcity but because of their exceptional catalytic properties and their resistance to chemical corrosion and heat.
And their demand is expected to grow in the coming years with the rise of clean energy technologies and the growing need for things like desalination plants.
And what does that have to do with asteroids?
Around 4 billion years ago, Earth had recently formed a solid crust and some primitive oceans and was about 40 degrees Celsius hotter than today.
At the time, it was under cataclysmic bombardment from space rocks.
We aren’t talking about a few falling stars, or even a couple of meteor showers, but a torrent of 50-kilometre-wide asteroids hitting our planet as often as once a century.
For reference, that’s roughly the same size as the asteroid that wiped out the dinosaurs hitting the Earth about once per human lifetime.
Some of these asteroid impacts were so violent they could have vaporised the early seas back into the atmosphere.
And they continued to pummel the Earth for a few hundred million years, bringing with them a precious cargo: platinum-group metals.
If they were pummelling the Earth, where did all the PGMs go?
As you’ve probably noticed, the surface of our planet is not littered with 50km-wide asteroids.
About half a billion years later, after the meteorite bombardment had calmed down, Earth “hit puberty” with the onset of plate tectonics.
Plate tectonics are the huge slabs of Earth’s crust that fit together like a jigsaw puzzle floating on the mantle.
Where those puzzle pieces collide, especially where one piece is denser than the other, the dense one is usually pushed down into the mantle.
Where they pull apart, molten lava forges its way to the surface, cools and forms new land.
Since the earliest formation of plate tectonics, the surface of our planet has been almost entirely recycled back into the mantle, taking the PGMs with it.
Once down there, the PGMs formed alloys with iron and nickel and rained down through the mantle to our dense core.
So how are there PGMs on the surface?
While there are still some PGMs down in our mantle, they have an extremely high melting point and need to be superheated to enter the magma and make their way to the surface.
In the past, this happened through magnesium-rich lavas called komatiites, which are known to have erupted at temperatures of more than 1,600C.
The only thing is, the Earth’s interior has been getting cooler and cooler over time, and these super-hot, magnesium-rich magmas no longer erupt.
So the only place we can find their remains, and the PGMs they brought to the surface, is in really old parts of the crust — away from plate tectonic boundaries — that haven’t been recycled into the mantle in a long time.
If that sounds familiar, you’re right. That pretty well describes a lot of Australia.
But while Australia has reserves of these metals in regions like the Yilgarn Craton in Western Australia, at present we only supply 0.1 per cent of the global demand for PGMs.
The biggest stash is in a patch of very old crust in South Africa.
South African stash
Near Pretoria, north of Johannesburg in South Africa, there’s a 7-kilometre-thick stack of ancient, frozen magma called the Bushveld Complex.
The base of this stack is made from komatiite magmas.
And in this deposit are crystals forming numerous delicate layers that extend over an area 25 times the size of the ACT.
While this was clearly a huge amount of magma, most of our global PGM supply is concentrated in only two layers — one 30 centimetres thick and the other 100 centimetres thick — within the 7-kilometre pile of crystals.
Scientists think it is likely that the layered rocks were formed by many little injections of magma, so that each spurt created its own thin, pancake-shaped layer of crystals.
This means that the platinum-group metals were likely concentrated in a complex plumbing system — with lots of pipes and little magma pools — that we are unable to see today.
What does this have to do with us?
If you are looking for these rare metals closer to home, you’ll find them in the catalytic converter of your car exhaust.
Your catalytic converter contains a mixture of platinum, palladium and rhodium in a honeycomb-like grid to maximise the surface area and enhance the exposure to your car exhaust.
These rare metals turn carbon monoxide and unburnt hydrocarbons into carbon dioxide and water.
While carbon dioxide is by no means an environmentally friendly car emission, it is certainly a lot safer to breathe than your untreated exhaust.
Each catalytic converter contains around 3 to 7 grams of PGMs — about the size of a 10-cent piece.
Although it’s the richest stash in the world, the most PGM-rich rocks in South Africa still only contain an average of 4 grams per tonne.
To get enough platinum-group metals for your catalytic converter, miners need to dig up about the equivalent of a small hatchback car worth of rock.
Which may seem like a lot of effort for such a small reward.
But then again, you’re basically driving around with a piece of 4-billion-year-old meteorite that has been down into the Earth’s mantle and come back up again!
Dr Dominique Tanner is a Lecturer in Geochemistry at the School of Earth, Atmospheric and Life Sciences, University of Wollongong. She is also one of the Top 5 scientists for 2019.