A rare version of the radioactive element plutonium embedded in Earth's crust below the deep sea is providing new clues as to how heavy metals form in the stars.
The new research finds that the isotope, called plutonium-244, may arrive on Earth in tandem with iron-60, a lighter metal known to form in supernovas, explosions that occur during the death throes of many types of stars. This finding suggests that supernovas may create both heavy metals — although it's possible that other events, such as the mergers of neutron stars, are responsible for at least some of the plutonium-244.
Understanding how heavy elements formed is one of the top three most burning questions in physics, said Anton Wallner, a nuclear physicist at the Australian National University and the Helmholtz Center Dresden-Rossendorf, a research center in Germany. Half of elements heavier than iron are built in the hearts of stars through a fairly well-understood process of fusion. The other half, though, requires a high density of free neutrons to form. This means they must form in a more explosive environment than a typical star core — supernovas, perhaps, or massive events such as a neutron-star merger or a collision of a black hole and a neutron star.
Along with collaborators in Japan, Australia and Europe, Wallner was interested in finding out if he could discover fingerprints of these celestial events on Earth. There are some radioactive versions of heavy metals that don't occur naturally on the planet. In particular, the researchers were on the hunt for plutonium-244, a variation of plutonium with a half-life of 80.6 million years. This means it takes 80.6 million years for radioactive decay to eat away at half of the initial plutonium produced. Any plutonium-244 originally present during Earth's formation has long since decayed, so any atoms the researchers could find would have to be extraterrestrial in origin.
"Can we find plutonium-244 on Earth?" Wallner said. "Then we know it's coming from space."
To hunt for these rare atoms, the researchers turned to samples of Earth's crust from nearly 5,000 feet (1,500 meters) below the Pacific Ocean. These rocks form so slowly that a millimeter of crust records 400,000 years of history, Wallner told Live Science. The sample covered the past 10 million years.
The researchers then probed the samples for iron-60 — the extraterrestrial version of iron that forms in supernovas — and for plutonium-244. They found both.
It was no surprise to find iron-60, Wallner said, as previous research had already shown fluctuations in iron-60 levels in deep-sea sediments and crust over time. The findings confirmed what researchers had previously suspected: There were two increases in iron-60 — one that occurred between 4.2 million and 55 million years ago, and one that happened sometime before 7 million years ago. These influxes of the metal may have been the result of two fairly nearby supernovas, Wallner said.
"The supernova that happened and produced the iron-60 must have been spectacular at the time," he said. "It must have been similar [in brightness] to the full moon, so you would see it even in daytime."
In the past, the researchers did not have sensitive enough methods to accurately count the extremely rare atoms of plutonium-244 scattered in Earth's crust. But in the new study, using cutting-edge technology and methods, they did. The timing of this extraterrestrial plutonium's arrival on Earth is a bit harder to pin down, as the researchers had to search layers of crust corresponding to between 3 million and 5 million years of history. However, the influx of plutonium-244 did correlate with the influx of iron-60.
"The ratio of plutonium-244 to iron-60 seems to be constant," Wallner said. This suggests that both might come from a common origin.
Forged in stars
Although the coordinated arrival of plutonium-244 and iron-60 suggests that both could have come from supernovas, a lot of questions remain. Computer models that attempt to mimic the formation of elements within supernovas really struggle to generate heavy-element formation, Wallner said. The ratio of iron-60 to plutonium-244 found in the new study suggests that the plutonium-244 would be a lot less prevalent than iron-60 after the stellar explosion, perhaps just a small percentage of the total elements formed.
It's also possible, Wallner noted, that the plutonium-244 atoms discovered in the deep-sea crust didn't come from a supernova at all. The plutonium-244 could have been formed in an earlier event and may have been floating aimlessly in deep space when a blast of iron-60 whooshed through, pushing the heavier plutonium-244 along with it. In that situation, both elements would have arrived on Earth at the same time, but the plutonium-244 would be a lot older.
To explore that possibility, the researchers want to look at different classes of atoms with different half-lives. The half-lives act like a clock so that scientists can determine a range of estimates for the ages of the elements. If the plutonium-244 were found in concert with an element of a much shorter half-life, for example, it would suggest that both were younger and fresher. It would also suggest that the amount of plutonium-244 produced in a supernova was lower and that more of it may have come from other events, like a neutron-star merger.
The research team is already studying a piece of crust 10 times larger than the one in this research. Having a larger piece of crust will allow researchers to expand their search for plutonium-244 atoms and get a more precise timeline of when those atoms arrived on Earth.
"What is fascinating is that you find some six or 10 atoms which you can identify in the end as not from Earth but from space, and then you get some hints about where it had been produced and when it had been produced," Wallner said.
The research was published today (May 13) in the journal Science.
Originally published on Live Science.
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.