Scientists studying a hunk of Earth’s crust gathered from deep in the Pacific Ocean have discovered traces of rare forms of plutonium and iron whose chemical makeup suggests they were forged in powerful collisions or explosions in outer space before falling to Earth, reports Nell Greenfieldboyce for NPR.
For many years, the predominant hypothesis was that heavy elements such as gold, silver, platinum, uranium and plutonium were created by the violent deaths of stars, called supernovae, reports Emily Conover for Science News. But a new study, published this week in the journal Science, suggests that ordinary supernovae aren’t enough to explain all the heavy elements researchers have identified in Earth’s vicinity.
Instead, the paper proposes that other interstellar events such as the collision of two super-dense collapsed stars, called neutron stars, or certain rare types of supernovae, according to Science News.
“It’s amazing that a few atoms on Earth can help us learn about where half of all the heavier elements in our universe are synthesized,” Anton Wallner, a nuclear physicist at the Australian National University as well as the Helmholtz Center in Germany and the paper’s first author, tells William J. Broad of the New York Times.
The researchers say the rare iron and plutonium isotopes, iron-60 and plutonium-244, respectively, rained down on Earth sometime in the last ten million years before sinking nearly a mile to the bottom of the equatorial Pacific. Researchers can say this with some confidence because of the rate of radioactive decay of the elements in question. As Stephanie Pappas reports for Live Science, any plutonium-244 that was baked into our planet’s crust during the formation of Earth would be long gone by now, implying that any plutonium-244 found today must have extraterrestrial origins.
By looking at the amounts of iron-60 and plutonium-244 in these deep sea rocks, the researchers used various models to predict what created the rare isotopes in the first place. These models corroborate earlier research using iron-60 that suggested a probable supernova in Earth’s “cosmic neighborhood” some three million years ago, writes Greenfieldboyce for NPR, but they also identified iron that may have come from a second nearby supernova roughly six million years ago.
On the other hand, the models suggested something else might be responsible for the plutonium-244: a merger of two dead neutron stars.
"Our data actually suggests that it might be that both scenarios are necessary," Wallner tells NPR. "It's both. It's supernovae explosions that produce a part of these heavy elements but also neutron star mergers or any other rare events."
Alexander Ji, an astrophysicist at Carnegie Observatories who was not involved in the research, tells Science News “it’s actually the first detection of something like this, so that’s really, really neat.”
Wallner tells the Times his team has already moved on to studying another, larger sample of plutonium-244 that may offer more clues into the origins of the heaviest known elements in the universe.