Future TechnologyA Pacific Ocean rock just confirmed an ancient neutron star collision from 100 million years ago
Key takeaways
- Plutonium-244 found in a Pacific Ocean ferromanganese rock dates to a kilonova that occurred roughly 100 million years ago
- Pu-244 is only produced in neutron star collisions; its presence confirms extraterrestrial origin, not nuclear contamination
- The debris fell through Earth's atmosphere and was slowly incorporated into deep-sea crust over millions of years
- Published in Nature Astronomy; one of the clearest geological records of a nearby kilonova event yet found
Scientists studying a lump of ferromanganese rock pulled from the Pacific Ocean floor in 1976 have found something that shouldn't be there: plutonium-244. Not the kind left over from nuclear weapons testing, and not from nuclear power stations. The isotopic signature is wrong for human-made plutonium. This Pu-244 was forged in space.
The study, published in Nature Astronomy, traced the isotope's origin to a kilonova: the violent merger of two neutron stars. These collisions are among the most energetic events in the universe, lasting fractions of a second but briefly outshining entire galaxies. During that fraction of a second, extreme neutron flux causes rapid neutron capture (the r-process), which is responsible for forging the heaviest elements in existence, including gold, platinum, uranium, and plutonium.
How stardust ends up on the ocean floor
Pu-244 has a half-life of 81.3 million years. The amount found in the rock is consistent with a kilonova that occurred roughly 100 million years ago, which places the explosion well before the asteroid impact that ended the Cretaceous period 66 million years ago. The debris from the collision spread outward through the galaxy, some of it eventually reaching our solar system, some of it drifting down through Earth's atmosphere and settling slowly onto the seafloor, where it was incorporated into ferromanganese crust as the rock formed over millions of years.
Ferromanganese crusts are extremely slow-growing mineral formations, accumulating at roughly 1-2 millimetres per million years. They're essentially geological tape recorders: studying their composition in layers allows researchers to read the cosmic environment Earth has moved through over tens of millions of years. Finding Pu-244 in a layer is a marker for a specific, datable cosmic event.
What this tells us
The discovery is one of the clearest pieces of direct physical evidence for a kilonova occurring within our galactic neighbourhood. Astronomers have observed kilonovae in other galaxies (the LIGO/Virgo collaboration detected the gravitational waves from one in 2017, designated GW170817), but having isotopic evidence of one's debris field in Earth's geological record is different: it's a local event, and a traceable one.
More practically, the finding adds to our understanding of how heavy elements are distributed through the galaxy after neutron star mergers. The debate about whether kilonovae alone account for the universe's gold and platinum, or whether other processes contribute, is still active. Studies like this provide data points for models that try to account for exactly where Earth's heavy elements came from.
The rock itself is still sitting in a laboratory. Fifty years after it was pulled from the ocean floor, it's turned out to contain evidence of events that happened before the last mass extinction.