In the past, there was an energetic event at some distant point in the universe, which produced radioactive iron and plutonium. Recently, traces of plutonium were found in rocks at the bottom of the Pacific Ocean by an oil company in Japan, which donated the material to scientists – and now a team of researchers proposes that radioactive debris has reached our planet in the past 10 million years. years, and would have been formed by supernovae along with the fusion of neutron stars.
“Fresh” samples like these can help scientists understand how the universe produced elements heavier than iron, such as gold, uranium and plutonium. “These elements are still a mystery to us, as we don’t know exactly where they are produced or how much,” says physicist Anton Wallner, of the Australian National University. Today, astronomers consider that the heaviest elements can be generated in supernovae, the explosion that occurs in the “death” of very massive stars.
These explosions are extremely powerful and can launch elements in all directions – so much so that, if one of these occurs relatively close to us, “all the elements produced are fired here and rain on our heads, literally”, explains astronomer Brian Fields . He and other colleagues had previously proposed, in another study, that supernovae can create stable and unstable elements, the latter being radioactive forms that last a few million years before decaying. For them, these atoms can last long enough to reach Earth after the star dies, and could be identified.
In the end, the search for these elements resulted in the discovery of iron-60 in rocks under the sea, in Antarctic snow and even in lunar samples. This suggested that there was some stellar explosion – probably a supernova – in the Earth’s neighborhood about 3 million years ago, spreading radioactive iron. Now, the iron found in rocks at the bottom of the ocean confirms this, also suggesting that a new dose of the element came here 6 million years ago; that is: “there is not only one exploding star, but two,” said Fields.
In addition to iron, they also found plutonium-244, a radioactive element that does not occur naturally on Earth. They compared the observations with what the models predicted about the production of these elements by cosmic events, and realized that there must be some more process involved in addition to the supernova. It is possible, then, that the fusion of neutron stars is part of the production: “it is the supernova explosions that produce a part of these elements, but the fusion of neutron stars or other rare events as well,” explained Walter.
Hendrik Schatz, a physicist at Michigan State University who is not part of the study team, was surprised by the results. For him, these new discoveries add to previous evidence that shows that heavy elements, such as plutonium, cannot come only from old supernovae. “There is a lot of evidence that points to multiple sources,” he explains. “Neutron star fusions are probably one of the most important sources, but they don’t seem to be able to explain all the observations.”
The article with the results of the study was published in Science magazine.