A smattering of plutonium atoms embedded in the Earth’s crust is assisting in determining the origins of nature’s heaviest elements. Scientists have long suspected that elements like gold, silver, and plutonium are created when stars explode. A new study suggests that typical supernovas cannot explain the abundance of heavy elements in our cosmic neighbourhood. This implies that other cataclysmic events must have played a significant role, according to physicist Anton Wallner and colleagues in the May 14 issue of Science.
The finding supports astrophysicists’ recent change of heart. Standard supernovas are no longer popular. Researchers believe that heavy elements are more likely to be formed in collisions between two dense, dead stars known as neutron stars or in certain rare types of supernovas, such as those formed by fast-spinning stars.
Heavy elements can be produced with a series of reactions that enhance and expand the atomic nuclei by swelling up neutrons rapidly. This reaction series is known as the “r-process,” which means “r.” But “We don’t know exactly where we’re located for the R-process,” says Wallner of the Australian National University in Canberra. It’s like having an invite list, but not the venue, so you know who’s there without knowing where it was.
Scientists thought they had found an answer after observing a neutron star collision producing heavy elements in 2017. However, heavy elements are found in very old stars that formed too early for neutron stars to collide.
If an event with an r-process had happened in the vicinity recently, some elements would have landed on Earth, leaving fingerprints in the crust of the Earth. Wallner and colleagues have used a particular accelerator to separate and count atoms, starting with a 410-g sample of Pacific Ocean crust. The scientists searched one piece of the sample for plutonium called plutonium-244, which is produced via the r-process. Since heavy elements are always produced in specific ratios during the r-process, plutonium-244 can be used as a proxy for other heavy elements. The team discovered approximately 180 plutonium-244 atoms in the crust within the last nine million years.
The plutonium count was compared to atoms with a known source. Supernovas produce Iron-60, but it is produced by fusion reactions within the star rather than part of the r-process. The team discovered approximately 415 atoms of iron-60 in another, smaller piece of the sample.
Plutonium-244 is radioactive, with an 80.6 million-year half-life. Iron-60 has a half-life of 2.6 million years, which is even shorter. As a result, the elements could not have existed when the Earth formed 4.5 billion years ago. This suggests that their origin is a recent event. When the iron-60 atoms were counted according to their depth in the crust, how long ago they were deposited, the scientists discovered two peaks approximately 2.5 million years ago and about 6.5 million years ago, implying that two or more supernovas occurred in the recent past.
The scientists are unable to determine whether the plutonium they detected originated from those supernovas as well. The researchers calculated, however, that if it were, there would be too little plutonium produced in these supernovas to explain the abundance of heavy elements in our cosmic environment. That suggests that regular supernovae cannot be the primary source of heavy components, at least in the vicinity.

Journal Reference

A. Wallner, M. B. Froehlich, M. A. C. Hotchkis, N. Kinoshita, M. Paul, M. Martschini, S. Pavetich, S. G. Tims, N. Kivel, D. Schumann, M. Honda, H. Matsuzaki, T. Yamagata 60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae. Science. Vol. 372, May 14, 2021, p. 742. doi: 10.1126/science.aax3972.

Main Image Credit: Jean Beaufort, Infrared space telescope image of RCW 86