How Gravitational Waves Led Astronomers to Neutron Star Gold
By Ian O'Neill
October 17, 2017
The origin of the universe's heaviest elements has mystified scientists, but after Monday's (Oct. 16) historic announcement of the detection of gravitational waves produced by two colliding neutron stars, astronomers have struck gold — literally.
Researchers know that stars fuse light atomic nuclei to create heavier nuclei. Elements in the universe heavier than hydrogen (but lighter than iron) are created by a process known as stellar nucleosynthesis: nuclear reactions that occur deep inside stars' cores. But it has been a long-standing mystery as to where in the universe elements heavier than iron are synthesized, researchers said in a statement from the Max Planck Institute for Astrophysics (MPA) in Germany.
Though astrophysicists have theorized processes for how heavy elements like gold, platinum and lead are created in the cosmos, observational evidence has been scarce — until now.
[Gravitational Waves from Neutron Star Crashes: The Discovery Explained]
"The origin of the really heaviest chemical elements in the universe has baffled the scientific community for quite a long time," Hans-Thomas Janka, a senior scientist at MPA, said in the statement. "Now, we have the first observational proof for neutron star mergers as sources; in fact, they could well be the main source of the r-process elements," which are elements heavier than iron, like gold and platinum.
After black holes, neutron stars are the densest known objects in the universe. Each is the size of a city, with a mass greater than that of Earth's sun; a teaspoon of this dense material would therefore weigh a billion tons. Neutron stars are created after stars more massive than Earth's sun explode as supernovas, leaving behind superdense magnetized balls of spinning matter composed mainly of neutrons, neutral particles that, along with protons, are found inside atomic nuclei.
Neutron stars therefore contain some of the building blocks of atomic nuclei. If these neutrons are somehow released from a neutron star, they might undergo reactions that allow them to stick together, creating elements heavier than iron.
For this process to work, however, it must be rapid, the researchers said.
Newly formed particles will be highly unstable and will lose neutrons, radioactively decaying into lighter particles. But if the surrounding environment is dense in free neutrons, more neutrons can be captured before the nuclei will decay, so heavier and heavier elements can be formed. And if a neutron star smashes into another neutron star, clumps of neutrons are blasted into space and can rapidly synthesize heavy elements like gold via a mechanism called rapid neutron capture process, or "r-process," according to an article released Oct. 16 in the journal Nature.
So, when astronomers confirmed the detection of the gravitational wave signal GW170817 that emanated from the site of a gamma-ray burst in a galaxy 130 million light-years away, they realized they were looking at an intense cosmic collision called a "kilonova." This was a ripe environment for the r-process to take place, the researchers said. Kilonovas are powerful explosions that unleash gamma-rays and have been long theorized to occur when neutron stars collide.
By comparing observations made using the Hubble Space Telescope and Gemini Observatory with theoretical models, astronomers have now confirmed that the r-process occurs in kilonovas, observing the spectroscopic fingerprint of heavy elements being created in the explosion's afterglow.
Researchers are witnessing a distant heavy-element factory synthesizing "maybe hundreds of Earth masses' [worth] of gold and … maybe 500 Earth masses' worth of platinum," theoretical astrophysicist Daniel Kasen, of the University of California, Berkeley, said in a new video.
With the help of the new gravitational wave signal, researchers now estimate that neutron star collisions may be responsible for the creation of most of the r-process heavy elements, like gold, found in galaxies, the Nature article said.
So, to paraphrase famed astronomer Carl Sagan, while we may be made of "star stuff," the ring on your finger is made of "neutron star stuff."
https://www.space.com/38493-gravitational-waves-neutron-star-gold.html
From the “How The Earth Was Made” documentary series...
Check out the “Carlin Trend” at the 27:16 mark of the ~45 min doc.
Season 2, episode 13: America’s Gold
https://www.youtube.com/watch?v=6baHoiV6ltg
_____________________________________________________
Below is a transcript for this portion of the show...
Season 2, episode 13: America’s Gold: "Carlin Trend" approximately the 27:16 mark:
“Nevada’s hot-spring gold deposits yielded over 40 million ounces of gold and 500 million ounces of silver, but by 1920, Nevada’s gold seams were increasingly uneconomical to mine. The desert became littered with ghost towns. It looked like America had run out of gold.
But in 1961, a new type of deposit was found. But it couldn’t be seen or touched. It was invisible. It would become the biggest strike in the history of America’s gold.
America’s gold was concentrated by incredible mountain-building forces that formed California, and volcanic processes deep beneath Nevada’s hot springs. But as the deposits became exhausted, geologists frantically searched for new stashes of gold.
Then in 1961, geologist and gold prospector John Livermore noticed a suspicious [50-mile crack] in the middle of Nevada called the Carlin Trend and set out to investigate.
John Livermore came to these hills following up a theory that gold deposits would be aligned directly above a deep crack in the Earth’s crust. He’d come up to outcrops like this, and he would go ahead and want to look at them.
The rocks are a strange mixture of mud and quartz, a mineral created in hot fluid and a good clue that gold might be deposited nearby. Based on his experience, he knew a lot of hot fluid had come up this crack.
However, he couldn’t see any evidence of gold, no quartz veins, no visible gold.
But on a hunch, he sampled this rock, took it back to the assay lab to see if there was any gold in it.
In assay labs, rocks are crushed and blasted in a furnace to over 1,800 degrees. And as the liquid rock cools, the minerals begin to separate.
At the end of this process, something extraordinary has happened.
An ordinary-looking rock actually contained a grain of gold at a concentration of about 5,000 times you would normally see in the Earth [crust].
It doesn’t look like much, but this is what they mine every day.
Livermore’s hunch paid off. The Carlin Trend is now one of the largest mining districts in the world.
The vast man-made pit is big enough to be seen from space.
This is the BetzePpost pit, one of the world’s largest gold mines.
It contains 45 million ounces of gold. They have to mine to get one ounce out.
As you can see, it’s huge— Because gold is so valuable, the extraction of just a few thousand ounces a day pays for this extraordinary mining operation. And removing the gold ore requires drastic action.
[explosion] Wow. That’s what 400,000 pounds of explosives looks like.
Giant diggers work 24 hours a day excavating over, enough to cover Central Park in 55 feet of rubble.
But within this raw gold ore, not a speck of gold has ever been seen with the naked eye.
The mystery is, where’s the gold?
You can’t see it. The rocks look really ordinary. The clue to where this gold is hidden is in the internal structure of this rock.
Magnified 500 times, the gold is still invisible.
But this rock is an extraordinary lattice of quartz and mud perforated with strange cavities. It looks like a honeycomb, like something’s eaten away at it. Clearly some strange geologic process has concentrated and hidden the gold in the rock.
Scientists realized if they were going to solve the puzzle of how gold came to be hiding in these rocks, first they would have to understand where the rocks came from.
A clue was found in the 19th century by cattle rancher Absalom Lehman in the eastern reaches of Nevada.
In 1885, Absalom Lehman stumbled upon a hole in the ground, and with rope and lantern, he lowered himself in the Earth and discovered this beautiful cave system with some of the most spectacular [limestone] cave formations in the world.
The solid foundations of this cave are made of a messy mixture of mud and calcium carbonate shells.
They are the remains of tiny sea creatures, evidence that Nevada was once covered by an ancient tropical sea.
For millions and millions of years, creatures with shells composed of calcium carbonate began to rain down through the ocean and accumulate on the sea floor forming this calcareous ooze.
Later, this ooze hardened into a sedimentary rock called limestone.
These extraordinary structures in the cave were formed when water eroded the huge limestone bed that sits underneath Nevada.
And back over at the Carlin Trend, scientists noticed that the gold ore was made of a very similar type of rock.
They figured out that the Lehman Cave limestone and Carlin Trend gold ore must once have been the same rock.
Clearly something happened to change this rock into this spongy gold-bearing ore.
Well, the clue is in the chemical reactivity of this limestone.
If I put dilute hydrochloric acid on this limestone, note how it fizzes. Very reactive. The acid is eating away at the rock.
Now if I put the acid on the spongy gold ore, no reaction. The fluid is soaking into the rock like a sponge.
Scientists concluded the reason Carlin Trend gold ore did not fizz is that it had already been attacked by an acid.
Beneath the bed of limestone at Carlin is a gigantic vertical crack.
Geologists now believe that a blast of hot, acidic gold-rich fluid was once forced upwards from deep within the Earth.
It streamed through the crack, drenching the limestone.
The acid ate into it, leaving a sponge-like muddy framework behind.
And in the cavities, it dumped quartz and the most minute sprinklings of gold.
It’s only been very recently that scientists have been able to use even more sophisticated imaging equipment—microscopy— to image down to the scale of individual atoms.
Magnified 100,000 times, tiny specks of submicroscopic gold can be seen embedded in the rock.
Zooming in further a staggering 4 million times, the gold particle is finally revealed.
This fleck of gold is only one-millionth the size of a pinhead, and each tiny white dot is an individual gold atom.
These ordinary-looking rocks have produced 65 million ounces of gold from a single crack in the Earth known as the Carlin Trend, and it’s made the United States the fourth largest gold producer in the world.
Scientists investigating how Nevada’s gold ore formed have found a sponge-like rock structure, suggesting that something ate away at the limestone, and gold ore not reacting with acid, evidence that an acidic fluid had already attacked the limestone, showering it with minute particles of gold.
Similar Carlin-type deposits have since been discovered yielding a further [35 million ounces of gold]
Yet these discoveries may only be the tip of the iceberg.
Geologists are now using state-of-the-art equipment, hoping to unlock millions of ounces of American gold trapped deep beneath the Nevada desert.
Throughout world history, over 5 billion ounces of gold have been recovered by man, and almost 1/10th of this has been found in California and Nevada, adding up to a staggering $280 billion worth of bounty. How much remains is anyone’s guess. ...”
Read more: https://www.springfieldspringfield.co.uk/view_episode_scripts.php?tv-show=how-the-earth-was-made-2009&episode=s02e13
