Introductions to the periodic table often claim the naturally occurring elements only go as far as uranium. Everything beyond this must have been made by humans, the story goes. Atoms of plutonium-244 found in rocks formed millions of years ago prove that's not quite right, but we can't yet be certain what natural phenomenon was responsible.
Supernova explosions make many heavy elements, quite likely including those with more protons than uranium. However, since all isotopes of these “transuranic” elements are radioactive, with short – compared to the age of the Earth – half-lives, any present in the cloud from which the Solar System formed 4.5 billion years ago have long since decayed.
So when Professor Anton Wallner of the Australian National University found plutonium-244 atoms in rocks collected from 1,500 meters (4,920 feet) below the Pacific Ocean he knew they must have arrived on Earth more recently. The plutonium was found in association with iron-60 isotopes. Unlike the iron-56 we encounter every day, iron-60 is radioactive, with a half-life of 2.6 million years.
“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,” Wallner said in a statement.
In sediments deposited over the last 10 million years, Wallner found two pulses of iron-60, from around 6.3 million and 2.5 million years ago. The later one had 3-4 times as much iron-60 but both were accompanied by the presence of plutonium atoms beyond what could be attributed to contamination from nuclear tests.
Around 80 percent of the iron-60 in the older pulse should have decayed since the older rocks formed, around half for the younger ones. Plutonium-244 has a much longer half-life – 80.6 million years – so only a small proportion would have decayed. Yet the ratios of the two radioactive isotopes are similar for each pulse.
Looking at the iron-60 alone, Wallner and co-authors calculate the abundance is consistent with it having been produced in two or more supernovas that occurred near Earth, about 150-350 light-years away.
“Our data could be the first evidence that supernovae do indeed produce plutonium-244,” Wallner said. That would be an important enough finding, but Wallner acknowledges an alternative. “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.”
Since anything but the faintest traces of plutonium-244 would be gone in a few hundred million years, its pre-existing presence would require some other explanation. Whatever formed the plutonium would have to be in the most recent 10 percent of Earth's history. Perhaps, if life on Earth had happened a bit faster, we wouldn't have needed gravitational waves from distant galaxies to alert us to collisions between neutron stars; we might have witnessed one in our own part of the galaxy.