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A study of the Earth's crust suggests that supernovae are not gold mines

A flurry of plutonium atoms embedded in the earth’s crust are helping to solve the origins of nature’s heaviest elements.

Scientists have long suspected that elements such as gold, silver and plutonium are born during supernovae, when the stars explode. But typical supernovae cannot explain the amount of heavy elements in our cosmic neighborhood, a new study suggests. That means other cataclysmic events must have been the main contributors, physicist Anton Wallner and colleagues reported in the May 14 issue of Science magazine.

The result reinforces a recent shift in opinion among astrophysicists. Standard supernovae have fallen out of favor. In contrast, researchers think that heavy elements are more likely forged in collisions of two dense, dead stars called neutron stars or in certain rare types of supernovae, such as those formed from fast-spinning stars (SNs). : 5/8/19).

Heavy elements can occur through a series of reactions in which atomic nuclei swell more and more as they rapidly fatten neutrons. This series of reactions is known as the r process, where “r” means fast. But, says Wallner, of the Australian National University in Canberra, "we don't know for sure where the site for process r is." It’s like having the invitation list for a meeting, but not your location, so you know who’s there without knowing where the party is.

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Scientists thought they had their answer after a neutron star collision occurred producing heavy elements in 2017 (SN: 16/10/17). But heavy elements appear in very old stars, which formed too early for neutron stars to have time to collide. “We know there has to be something else,” says theoretical astrophysicist Almudena Arcones of the Technical University of Darmstadt, Germany, who did not participate in the new study.

If a recent r event event occurred nearby, some of the elements created could land on Earth, leaving footprints in the Earth's crust. Starting with a sample of 410 grams of crust from the Pacific Ocean, Wallner and colleagues used a particle accelerator to separate and count atoms. In one piece of the sample, scientists looked for a variety of plutonium called plutonium-244, which is produced by the process r. Since heavy elements always occur together in particular proportions in process r, plutonium-244 can serve as a proxy for other heavy elements. The team found about 180 plutonium-244 atoms, deposited in the crust over the past 9 million years.

piece of deep-water crustScientists analyzed a sample of the Earth's deep sea crust (shown) to look for plutonium and iron atoms with cosmic origins.Norikazu Kinoshita

The researchers compared the plutonium count to atoms that had a known source. Iron-60 is released by supernovae, but is formed by fusion reactions in the star, not as part of the process r. In another smaller piece of the sample, the team detected about 415 iron-60 atoms.

Plutonium-244 is radioactive, decomposing with a half-life of 80.6 million years. And iron-60 has an average lifespan of even less than 2.6 million years. Thus, the elements could not be present when the Earth was formed, 4.5 billion years ago. This suggests that its source is a relatively recent event. When the iron-60 atoms were counted according to their depth in the crust and therefore how long they were deposited, scientists saw two peaks about 2.5 million years ago and about 6.5 million years ago, suggesting two or there have been more supernovae in the recent past.

Scientists can't say if the plutonium they detected also came from those supernovae. But if it did, the amount of plutonium produced in those supernovae would be too small to explain the abundance of heavy elements in our cosmic proximities, the researchers calculated. This suggests that regular supernovae may not be the main source of heavy elements, at least in the vicinity.

That means other sources are still needed for process r, says astrophysicist Anna Frebel of MIT, who did not participate in the research. "Supernovae just don't cut it."

The measurement gives a snapshot of the r process in our corner of the universe, says astrophysicist Alexander Ji of Carnegie Observatories in Pasadena, California. "It's actually the first detection of something like that, so that's really very useful."

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