Traces of plutonium found in the depths of the Pacific Ocean are providing scientists with a new theory of how heavy metals are formed in the solar system.
It was always known that the world’s most dangerous element was formed in a supernova – the explosion created when a star dies – but researchers have now found that the merger of neutron stars could also be a likely source.
Neutron stars are the core of stars that have died. They have a mass 1.4 times of the Sun.
Scientists at the Australian Nuclear Science and Technology Organisation (ANSTO) led the study, which has been published in the Science journal.
They analysed the Pu-244 (plutonium) and Iron-60 that was found in the seabed crust, and discovered it most likely fell from space in the last 10 million years.
What is plutonium?
Plutonium is radioactive and is one of the most dangerous elements on Earth.
Most of it is produced in reactors and is heavily guarded – this kind is known as plutonium 239.
It was central to the atomic age and was used to make the nuclear bomb that was dropped at the Japanese city of Nakasaki by the United States in the final stages of the Second World War.
Plutonium has a long half-life of 80 million years, which normally means there is none left from the time the solar system formed billions of years ago.
However, this newly-discovered undersea source is younger, so researchers say it is likely to have come from debris from recent near-Earth supernovae.
What was recently discovered?
Scientists at ANSTO found that the materials – Iron-60 and Pu-244 – have rained down on Earth at least twice in the past 10 million years.
"The key new discovery is that Pu-244 is also present in this material. This suggests that heavy elements, including gold, uranium and plutonium form in supernova explosions," Dr Michael Hotchkis, principal research scientist, told The National.
“However, what we found was a very small amount. Such a small amount means that, while supernovae can form heavy elements, it is more likely that most heavy elements we have on earth must actually have come from another source.
“The most likely source, responsible for the majority of the heavy elements, is a neutron star merger event.”
The collision of neutron stars, each of which are the size of a city, is a massive event and creates a black hole and gamma-ray burst.
The discovery could help solve one of the most important questions in physics – how do heavy metals form in the solar system.
Dr Hotchkis said Pu-244 will help further the study as it one of the elements that is recently-formed.
“You might ask, why do we not just look for heavy elements like uranium or gold in our samples? The problem is that there are already those elements everywhere on earth, because they are still there from the beginning of the solar system,” he said.
“The element gold is stable, it lasts forever. Uranium has such a long half-life that there is still plenty left from the formation of the solar system.
“We cannot use these elements to learn about recently-formed heavy elements, as we cannot distinguish the recently-formed material from the old material. With Pu-244 it is different – it can only be recently-formed.”
How did they detect the elements?
The scientists used the Accelerator Mass Spectrometry (AMS) system to make the discovery.
The sensitivity of the instruments is at least 20 times better than ones used in previous attempts.
“It is based on small-sized high-voltage accelerator that operates at one million volts,” said Dr Hotchkis.
“In an AMS system, individual atoms are detected and we can identify which isotope each atom is.
“The Vega AMS system at my laboratory at ANSTO was custom-built to optimise its sensitivity for heavy elements including plutonium isotopes.”