The first-ever discovery of an extraterrestrial radioactive isotope on Earth has scientists rethinking the origins of the elements on our planet. The tiny traces of plutonium-244 (Pu-244) were found in ocean crust alongside radioactive iron-60. The two isotopes are evidence of violent cosmic events in the vicinity of Earth millions of years ago.
Star explosions, or supernovae create many of the heavy elements in the periodic table, including those vital for human life, such as iron, potassium and iodine. To form even heavier elements, such as gold, uranium and plutonium it was thought that a more violent event may be needed, such as two neutron stars merging.
A Complex Picture
However, a study led by nuclear physicist Anton Wallner with The Australian National University (ANU), who also holds joint positions at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Technical University Dresden, suggests a more complex picture. “The story is complicated – possibly this plutonium-244 was produced in supernova explosions or it could be left over from a much older, but even more spectacular event such as a neutron star detonation,” lead author of the study, Professor Wallner said.
“We can’t say much about neutron star mergers close-by to the solar system in the past few million years using this work,” wrote Wallner in an email to The Daily Galaxy about the possible impact on Earth’s evolution.
“If we could exclude supernovae as the source of the recent interstellar Pu-244 influx, then our data would tell us that within a few half lives of Pu-244, this is within the past few hundred million years, a rare event such as a neutron star merger could have been the source of this Pu-244” added Wallner in his email. “Note, however, this time period is approximately the time it takes for our solar system to complete one journey around the centre of our galaxy. And the solar system moves relative to other objects in the interstellar medium as well; this means production of the Pu-244 could have been at a location/distance that was substantially different to now/or the time when the Pu-244 entered the solar system and became deposited on Earth.”
Series of Massive Supernova Explosions Near Our Solar System
In 2016, Wallner was a member of an international team of scientists that found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris. The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans, concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms, said research leader Wallner.
“We were very surprised that there was debris clearly spread across 1.5 million years,” said Wallner in 2016. “It suggests there were a series of supernovae, one after another. It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”
The 2016 team also found evidence of iron-60 from an older supernova around eight million years ago, less than 300 light years away, close enough to be visible during the day and comparable to the brightness of the Moon, coinciding with global faunal changes in the late Miocene. Some theories suggest cosmic rays from the supernovae could have increased cloud cover.
Any plutonium-244 and iron-60 that existed when the Earth formed from interstellar gas and dust over four billion years ago have long since decayed, so current traces of them must have originated from recent cosmic events in space.
The dating of the sample confirms two or more supernova explosions occurred near Earth. “Our data could be the first evidence that supernovae do indeed produce plutonium-244,” Professor Wallner said. “Or perhaps it was already in the interstellar medium before the supernova went off, and it was pushed across the solar system together with the supernova ejecta.”
The VEGA accelerator at Australian Nuclear Science and Technology Organisation, (ANSTO) in Sydney was used to identify the tiny traces of the plutonium-244.
The image at the top of the page shows a “pair-instability” supernova, with 100 times more energy than an ordinary supernova. In the past year two teams of astronomers have found one, revisingthe limit of how big a explosion can be in our universe. NASA/CXC/SAO/ESA/ASU/JPL/CALTECH/UNIVERSITY OF MINNESOTA