Where does gold come from? – David Lunney

In medieval times, alchemists tried to achieve
the seemingly impossible. They wanted to transform lowly lead
into gleaming gold. History portrays these people
as aged eccentrics, but if only they’d known that their
dreams were actually achievable. Indeed, today we can
manufacture gold on Earth thanks to modern inventions that those medieval alchemists
missed by a few centuries. But to understand how this precious metal became embedded
in our planet to start with, we have to gaze upwards at the stars. Gold is extraterrestrial. Instead of arising
from the planet’s rocky crust, it was actually cooked up in space and is present on Earth
because of cataclysmic stellar explosions called supernovae. Stars are mostly made up of hydrogen,
the simplest and lightest element. The enormous gravitational pressure
of so much material compresses and triggers nuclear fusion
in the star’s core. This process releases energy
from the hydrogen, making the star shine. Over many millions of years, fusion transforms hydrogen
into heavier elements: helium, carbon, and oxygen, burning subsequent elements faster
and faster to reach iron and nickel. However, at that point nuclear fusion
no longer releases enough energy, and the pressure from the core peters out. The outer layers collapse into the center, and bouncing back from this sudden
injection of energy, the star explodes forming a supernova. The extreme pressure
of a collapsing star is so high, that subatomic protons and electrons
are forced together in the core, forming neutrons. Neutrons have no repelling electric charge so they’re easily captured
by the iron group elements. Multiple neutron captures enable
the formation of heavier elements that a star under
normal circumstances can’t form, from silver to gold, past lead and on to uranium. In extreme contrast to the million year
transformation of hydrogen to helium, the creation of the heaviest
elements in a supernova takes place in only seconds. But what becomes of the gold
after the explosion? The expanding supernova shockwave
propels its elemental debris through the interstellar medium, triggering a swirling dance
of gas and dust that condenses into new stars and planets. Earth’s gold was likely delivered this way before being kneaded into veins
by geothermal activity. Billions of years later, we now extract
this precious product by mining it, an expensive process that’s compounded
by gold’s rarity. In fact, all of the gold
that we’ve mined in history could be piled into
just three Olympic-size swimming pools, although this represents a lot of mass because gold is about 20 times
denser than water. So, can we produce more
of this coveted commodity? Actually, yes. Using particle accelerators, we can mimic
the complex nuclear reactions that create gold in stars. But these machines can only construct gold
atom by atom. So it would take almost the age
of the universe to produce one gram at a cost vastly exceeding
the current value of gold. So that’s not a very good solution. But if we were to reach
a hypothetical point where we’d mined
all of the Earth’s buried gold, there are other places we could look. The ocean holds an estimated
20 million tons of dissolved gold but at extremely miniscule concentrations
making its recovery too costly at present. Perhaps one day, we’ll see gold rushes
to tap the mineral wealth of the other planets of our solar system. And who knows? Maybe some future supernova
will occur close enough to shower us with its treasure and hopefully not eradicate
all life on Earth in the process.

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