A rare version of the radioactive element plutonium below the deep sea embedded in Earth’s crust provides new ideas about how heavy metals form in the stars. According to recent research, the isotope, called plutonium-244, may arrive on Earth in tandem with iron-60. Iron-60 is a lighter metal known to form in supernovas, explosions that occur due to the death throes of many types of stars. This finding suggests that heavy metals are by supernovas, although other events are possible such as at least for some of the plutonium-244, the mergers of neutron stars are responsible.
According to Anton Wallner, a nuclear physicist at the Helmholtz Center Dresden-Rossendorf, a research center in Germany, and the Australian National University, understanding how heavy elements are formed is one of the top three central questions of physics. Heart of the stars, half of the elements heavier than iron, is built through a relatively well-understood fusion process. The other half requires a high density of free neutrons to form. This means they are created in a more explosive environment than a typical star core.
Along with collaborators in Europe, Australia, Japan, Wallner was interested in finding out fingerprints of these celestial events on Earth. Heavy metals have some radioactive versions that don’t occur naturally on the planet. The researchers were on the search for plutonium-244, to be specific. This means 80.6 million years are taken for radioactive decay to eat away at half of the initial plutonium produced. During Earth formation, any plutonium-244 originally present has long since decayed. Any atoms that the researcher could find out would have to be extraterrestrial in origin.
To find these rare forms of atoms, the researchers turned to samples of Earth’s crust from nearly 5,000 feet below the Pacific Ocean. Wallner said these rocks are formed so slowly that a millimeter of crust records 400,000 years of history. Past 10 million years, the sample covered. The samples for iron-60 the researchers then probed. The extraterrestrial version of iron that forms in for plutonium-244 and supernovas. They find both.
Previous researchers have already shown fluctuations in iron-60 levels in the crust and deep-sea sediments over time. There were two increases in iron-60, one that happened sometime before 7 million years ago and the other one that occurred between 4.2 million and 55 million years ago. According to Wallner, “The supernova that happened and produced the iron-60 must have been spectacular at the time. It must have been similar [in brightness] to the full moon so that you would see it even in the daytime.”