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.