Isotope ratios give insights into how heavy elements are born in space

Although many of the processes that make elements are well understood, Anton Wallner of the Helmholtz-Zentrum Dresden-Rossendorf calls how and where some heavy elements are formed “an open question.” One mechanism requires an explosive, neutron-rich environment, but several settings in space fit this description, which leads scientists to suggest different scenarios for heavy-atom formation. To try to pin down the answer, Wallner and colleagues used accelerator mass spectrometry to measure the abundance of iron and plutonium isotopes in a sample of oceanic crust (Science 2021, DOI: 10.1126/science.aax3972). The researchers found a layer of the crust enriched in ⁶⁰Fe and concluded that those atoms came to Earth from a supernova. But when the researchers measured ²⁴⁴Pu in the same section of crust, there was less than would be expected if this isotope was made in the same explosion. That doesn’t mean supernovas don’t make any ²⁴⁴Pu, Wallner says. But it suggests other neutron-rich explosions, such as those that occur when neutron stars collide, must also contribute to making heavy elements in our universe.