Extraterrestrial plutonium atoms turn up on ocean floor

They say it was made in colliding stars and later rained down through Earth’s atmosphere as cosmic dust millions of years ago. Their analysis opens a new window on the cosmos. “It’s amazing that a few atoms on Earth can help us learn about where half of all the heavier elements in our universe are synthesized,” said Anton Wallner, the paper’s first author and a nuclear physicist. Dr. Wallner works at the Australian National University as well as the Helmholtz Center in Dresden, Germany.

Dr. Wallner and his colleagues reported their findings in Science on Thursday. Plutonium has a bad reputation, one that is well-deserved. The radioactive element fuelled the world’s first nuclear test explosion as well as the bomb that levelled the Japanese city of Nagasaki during World War II. After the war, scientists found the health repercussions of plutonium to be particularly deadly. If inhaled or ingested in minute quantities, it could result in fatal cancers. Small amounts also pack a bigger punch than other nuclear fuels, a quality that aided the making of compact city busters that nuclear powers put atop their intercontinental missiles.

The element is often considered artificial because it is so seldom found outside of human creations. In the periodic table, it is the last of 94 atoms characterised as naturally occurring. Traces of it can be found in uranium ores. Astrophysicists have long known that it’s also spontaneously created in the universe. But they’ve had a hard time pinpointing any exact sites of its origin. What makes the deep ocean a good place for the gathering of extraterrestrial clues is its extreme remoteness from the waves of change near the planet’s surface. It’s a sanctuary where things can sit undisturbed for millions of years. In this case, the scientists got lucky when they had a chance to study material from a Japanese expedition that sampled the seabed in the equatorial Pacific.

Stars in their cores turn light elements into heavier ones, creating elements as heavy as iron. The new discovery sheds light on the relative contributions of two different ways the universe is thought to go about making all of the elements that are heavier than iron, including many found in everyday life, like copper and zinc, mercury and iodine.

The exploding stars known as supernovas have long been seen as a main source. The abrupt gravitational collapse of a massive star turns much of its matter into heavy elements that shoot back into space as it rebounds in a colossal blast. These drifting elements ultimately get mixed with more common atoms to become the raw material for new stars and planets, or for life itself in the case of humans.

Much of the global seabed is rich in rocky crusts of terrestrial metals deposited over the eons. The retrieved sample was roughly an inch thick and 18 inches in area. The scientists looked for extraterrestrial plutonium in the deepest layers, using an extremely sensitive detector optimised for the discovery of tiny traces of plutonium. Nuclear weapon states have experimented with assorted isotopes of plutonium since the start of the atomic age but have found little of the 244 isotope on Earth. Dr. Wallner says he and his colleagues are hot on the cosmic trail.