The True Origins of Gold in Our Universe May Have Just Changed, Again

By MICHELLE STARR

When humanity finally detected the collision between two neutron stars in 2017, we confirmed a long-held theory – in the energetic fires of these incredible explosions, elements heavier than iron are forged.

And so, we thought we had an answer to the question of how these elements – including gold – propagated throughout the Universe.

But a new analysis has revealed a problem. According to new galactic chemical evolution models, neutron star collisions don’t even come close to producing the abundances of heavy elements found in the Milky Way galaxy today.

“Neutron star mergers did not produce enough heavy elements in the early life of the Universe, and they still don’t now, 14 billion years later,” said astrophysicist Amanda Karakas of Monash University and the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) in Australia.

“The Universe didn’t make them fast enough to account for their presence in very ancient stars, and, overall, there are simply not enough collisions going on to account for the abundance of these elements around today.”

Stars are the forges that produce most of the elements in the Universe. In the early Universe, after the primordial quark soup cooled enough to coalesce into matter, it formed hydrogen and helium – still the two most abundant elements in the Universe.

The first stars formed as gravity pulled together clumps of these materials. In the nuclear fusion furnaces of their cores, these stars forged hydrogen into helium; then helium into carbon; and so on, fusing heavier and heavier elements as they run out of lighter ones until iron is produced.

Iron itself can fuse, but it consumes huge amounts of energy – more than such fusion produces – so an iron core is the end point.

“We can think of stars as giant pressure cookers where new elements are created,” Karakas said. “The reactions that make these elements also provide the energy that keeps stars shining bright for billions of years. As stars age, they produce heavier and heavier elements as their insides heat up.”

To create elements heavier than iron – such as gold, silver, thorium and uranium – the rapid neutron-capture process, or r-process, is required. This can take place in really energetic explosions, which generate a series of nuclear reactions in which atomic nuclei collide with neutrons to synthesise elements heavier than iron.

But it needs to happen really quickly, so that radioactive decay doesn’t have time to occur before more neutrons are added to the nucleus.

We know now that the kilonova explosion generated by a neutron star collision is an energetic-enough environment for the r-process to take place. That’s not under dispute. But, in order to produce the quantities of these heavier elements we observe, we’d need a minimum frequency of neutron star collisions.

To figure out the sources of these elements, the researchers constructed galactic chemical evolution models for all stable elements from carbon to uranium, using the most up-to-date astrophysical observations and chemical abundances in the Milky Way available. They included theoretical nucleosynthesis yields and event rates.

They laid out their work in a periodic table that shows the origins of the elements they modelled. And, among their findings, they found the neutron star collision frequency lacking, from the early Universe to now. Instead, they believe that a type of supernova could be responsible.

These are called magnetorotational supernovae, and they occur when the core of a massive, fast-spinning star with a strong magnetic field collapses. These are also thought to be energetic enough for the r-process to take place. If a small percentage of supernovae of stars between 25 and 50 solar masses are magnetorotational, that could make up the difference.

“Even the most optimistic estimates of neutron star collision frequency simply can’t account for the sheer abundance of these elements in the Universe,” said Karakas. “This was a surprise. It looks like spinning supernovae with strong magnetic fields are the real source of most of these elements.”

Previous research has found a type of supernova called a collapsar supernova can also produce heavy elements. This is when a rapidly rotating star over 30 solar masses goes supernova before collapsing down into a black hole. These are thought to be much rarer than neutron star collisions, but they could be a contributor – it matches neatly with the team’s other findings.

They found that stars less massive than about eight solar masses produce carbon, nitrogen, fluorine, and about half of all the elements heavier than iron. Stars more massive than eight solar masses produce most of the oxygen and calcium needed for life, as well as most of the rest of the elements between carbon and iron.

“Apart from hydrogen, there is no single element that can be formed only by one type of star,” explained astrophysicist Chiaki Kobayashi of the University of Hertfordshire in the UK.

“Half of carbon is produced from dying low-mass stars, but the other half comes from supernovae. And half the iron comes from normal supernovae of massive stars, but the other half needs another form, known as Type Ia supernovae. These are produced in binary systems of low mass stars.”

This doesn’t necessarily mean that the estimated 0.3 percent of Earth’s gold and platinum traced back to a neutron star collision 4.6 billion years ago has a different origin story. It’s just not necessarily the whole story.

But we’ve only been detecting gravitational waves for five years. It could be, as our equipment and techniques improve, that we find neutron star collisions are much more frequent than we think they are at this current time.

Curiously, the researchers’ models also turned out more silver than observed, and less gold. That suggests something needs to be tweaked. Perhaps it’s the calculations. Or perhaps there are some aspects of stellar nucleosynthesis that we are yet to understand.

The research has been published in The Astrophysical Journal.

https://www.sciencealert.com/neutron-star-collisions-may-not-be-making-much-gold-after-all

Some of Earth’s Gold Came From Two Neutron Stars That Collided Billions of Years Ago

For about a century now, scientists have theorized that the metals in our Universe are the result of stellar nucleosynthesis. This theory states that after the first stars formed, heat and pressure in their interiors led to the creation of heavier elements like silicon and iron. These elements not only enriched future generations of stars (“metallicity”), but also provided the material from which the planets formed.

More recent work has suggested that some of the heaviest elements could actually be the result of binary stars merging. In fact, a recent study by two astrophysicists found that a collision which took place between two neutron stars billions of years ago produced a considerable amount of some of Earth’s heaviest elements. These include gold, platinum and uranium, which then became part of the material from which Earth formed.

The research was conducted by Prof. Szabolcs Márka from Columbia University and Prof. Imre Bartos of the University of Florida. Their findings were published in a study titled “Nearby Neutron-Star Mergers Explain Actinide Abundance in the Early Solar System”, which recently appeared in the May issue of the scientific journal Nature.


An artist’s conception of two neutron stars, moments before they collide. Credit: NASA

According to the scientific consensus, asteroids and comets are composed of material left over from the formation of the Solar System. When bits of these come to Earth in the form of meteorites, they carry traces of radioactive isotopes whose decay is used to determine when the asteroids were created. The study of these space rocks can also shed light on what materials existed in our Solar System billions of years ago.

For the sake of their study, Bartos and Márka ran numerical simulations of the Milky Way and compared the results to the composition of meteorites that were retrieved on Earth. What they found was that a single neutron-star collision could have occurred within our cosmic neighborhood – ~1,000 light years from our Solar System – roughly 4.65 billion years ago.

At the time, our Solar System was still a massive cloud of dust and gas that would soon undergo gravitational collapse at its center, thus giving birth to our Sun. Roughly 100 million years later, the Earth and other Solar Planets would form from the proto-planetary debris disk that fell into orbit around our young Sun.

This single cosmic event, they estimate, gave birth to elements that would become part of this disk – and which now make up roughly 0.3% of the Earth’s heaviest elements. Most of these are in the form on iodine, an element which is essential to biological processes. In this respect, this event may have played a role in the emergence of life here in the Solar System as well.

To put this event in perspective, consider that the Milky Way galaxy is an estimated 100,000 light years in diameter. This collision and the resulting explosion, therefore, took place roughly 1/100th the distance away. In fact, the research team indicated that if a similar event happened at the same distance today, the resulting radiation would outshine every star in the sky.

What is especially interesting about this study is the way it provides insight into an event that was both unique and highly consequential in the history and formation of Earth and our Solar System. “It sheds bright light on the processes involved in the origin and composition of our Solar System, and will initiate a new type of quest within disciplines, such as chemistry, biology and geology, to solve the cosmic puzzle,” Bartos summarized.

And as Márka indicated, it also addresses some of the deeper questions scientists have about the origins of life as we know it:

“Our results address a fundamental quest of humanity: Where did we come from and where are we going? It is very difficult to describe the tremendous emotions we felt when we realized what we had found and what it means for the future as we search for an explanation of our place in the universe.”

It also reaffirms what Carl Sagan famously said: “We are a way for the universe to know itself. Some part of our being knows this is where we came from. We long to return. And we can, because the cosmos is also within us… The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

Some of Earth’s Gold Came From Two Neutron Stars That Collided Billions of Years Ago

Solid Gold: Precious Metals Discovered to be Concentrated in Human Fecal Matter

Instead of flushing millions down the toilet, humans could be mining their poop for gold.

That’s at least what some researchers with the U.S. Geological Survey (USGS) think. They’re looking for ways to squeeze metals like gold and silver out of solid waste.

When poop arrives at a wastewater treatment plant, it is separated into biosolids and treated water. Inevitably about half of the biosolids (3.5 million tons in the United States alone) is sent to landfills or incinerated, while the other half is used as fertilizer.

Kathleen Smith, a USGS geologist, thinks people could make more of these biosolids; they’re full of tiny particles of metals that find their way into waste through beauty products, detergents and even odor-resistant clothing.

There are two good reasons to try to pull these metals out of poop, according to Smith, who’s presenting her research on the subject at an American Chemical Society meeting this week.

“If you can get rid of some of the nuisance metals that currently limit how much of these biosolids we can use on fields and forests, and at the same time recover valuable metals and other elements, that’s a win-win,” Smith explained in a statement.

The same chemicals (called leachates) that miners use to pull metals out of rock could be safely used to pull metals from waste, Smith and her colleagues found. The researchers have examined waste samples from small towns in the Rocky Mountains, as well as in rural areas and cities. They detected some sizable concentrations of platinum, silver and gold when they looked at their samples under a scanning electron microscope, they reported.

“The gold we found was at the level of a minimal mineral deposit,” Smith said. In other words, if that level of gold were observed in rock, it would be considered a potential mining prospect.

It’s not just gold that could be mined and sold. Waste contains elements like vanadium and copper that could be used in devices such as cellphones and computers, the researchers said.

The economic value of poop mining is still unclear, but some recent projections have been promising. Earlier this year, another group of researchers published a paper in the journal Environmental Science & Technology, estimating that metals extracted from poop in a population of 1 million people could yield $13 million per year.

http://www.livescience.com/50235-solid-gold-poop-could-yield-precious-metals.html