An isotopic record of nearby supernovae

Twenty years ago it was suggested that nearby supernova explosions could leave their mark here on Earth in the shape of geological isotope anomalies caused by direct deposition of debris or through cosmic-ray spallation in the atmosphere. So it proved, with the emergence of iron-60, found in deep-sea ferromanganese crusts, as a supernova indicator. Two papers in this issue add further detail to the picture, and suggest there have been multiple supernovae within a few hundred light years, over the past few million years. Anton Wallner et al. report that the deep-sea ⁶⁰Fe signal is global, extended in time and of interstellar origin from multiple events. Their results reveal ⁶⁰Fe interstellar influxes onto Earth 1.7–3.2 and 6.5–8.7 million years ago. Dieter Breitschwerdt et al. report calculations of the most probable trajectories and masses of the supernova progenitors, and hence their explosion times and sites. The ⁶⁰Fe signal arises from two supernovae at distances between 90 and 100 parsecs. The closest occurred 2.3 million years ago, and the second about 1.5 million years ago at 9.2 and 8.8 solar masses, respectively.