Tiny traces of material from a giant cosmic explosion are still falling to Earth more than 100 million years after the event occurred, according to a new international study published in Nature Astronomy.
The evidence comes from an unusual source: a slow-growing chunk of ferromanganese crust collected from the floor of the Pacific Ocean.
Hidden inside this crust, scientists discovered a few hundred atoms of a rare radioactive form of plutonium called plutonium-244 (Pu-244).
Pu-244 has a half-life of 81 million years, meaning it decays very slowly.
Finding even a few atoms of it is remarkable because it suggests that the material was created by an enormous explosion in space and has been drifting through the galaxy for a very long time.
Researchers believe the most likely source was a kilonova, a rare and extremely powerful explosion that occurs when two neutron stars collide and merge.
Neutron stars are incredibly dense remnants of dead stars. When they smash together, they create one of the brightest events in the universe and forge many of the heavy elements found today.
Scientists think that these neutron star mergers are responsible for producing about half of the universe’s heavy elements, including precious metals and radioactive elements such as uranium, thorium, plutonium, and curium.
The ocean crust sample that revealed the discovery weighed nearly 2 kilograms and was recovered in 1976 from a depth of almost 4,830 meters in the Pacific Ocean.
The crust grows extraordinarily slowly, taking more than 10 million years to form just a few centimeters. This slow growth allows it to preserve a long record of material that has fallen to Earth from space.
Researchers carefully sliced the crust into layers, each representing about one million years of growth. They expected to find fewer than 100 atoms of Pu-244 in each layer.
To detect such tiny amounts, the team used one of the world’s most sensitive instruments for identifying rare isotopes. The equipment was capable of counting individual atoms of plutonium and another radioactive element called curium-247 (Cm-247).
Scientists expected the plutonium to appear in spikes, similar to iron-60, another radioactive isotope previously found in the crust and linked to nearby supernova explosions that occurred around 2 million and 7 million years ago.
Instead, they found something surprising. The plutonium was spread relatively evenly throughout the layers rather than concentrated in peaks. This suggested that Pu-244 has been arriving on Earth continuously as a gentle rain of interstellar dust, rather than being delivered by recent supernova explosions.
The researchers also looked for curium-247. According to theories of element formation, plutonium and curium should be created together in neutron star mergers. However, no convincing evidence of curium was found.
The absence of curium provides an important clue. Because curium decays much faster than plutonium, scientists concluded that the explosion that produced the material must have happened more than 100 million years ago. Over that immense span of time, the curium disappeared, while traces of plutonium survived.
The team now hopes to find more evidence of this ancient stellar explosion, perhaps hidden in old rock layers on Earth or even in undisturbed dust on the moon.
