Analysis of deep-ocean archives reveals that a few per cent of fresh 60Fe has been captured in interstellar dust and deposited in Earth’s crust, indicating that many supernova events occurred over the past ten million years within a distance of up to 100 parsecs from Earth. 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. The rate of supernovae in our local Galactic neighbourhood within a distance of about 100 parsecs from Earth is estimated to be one every 2–4 million years, based on the total rate in the Milky Way (2.0 ± 0.7 per century1,2). Recent massive-star and supernova activity in Earth’s vicinity may be traced by radionuclides with half-lives of up to 100 million years3,4,5,6, if trapped in interstellar dust grains that penetrate the Solar System. One such radionuclide is 60Fe (with a half-life of 2.6 million years)7,8, which is ejected in supernova explosions and winds from massive stars1,2,9. Here we report that the 60Fe signal observed previously in deep-sea crusts10,11 is global, extended in time and of interstellar origin from multiple events. We analysed deep-sea archives from all major oceans for 60Fe deposition via the accretion of interstellar dust particles. Our results reveal 60Fe interstellar influxes onto Earth at 1.5–3.2 million years ago and at 6.5–8.7 million years ago. The signal measured implies that a few per cent of fresh 60Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ten million years at distances of up to 100 parsecs.

A nearby supernova (SN) explosion in the past can be confirmed by the detection of radioisotopes on Earth that were produced and ejected by the SN. We have now measured a well resolved time profile of the ${}^{60}\mathrm{F}\mathrm{e}$ concentration in a deep-sea ferromanganese crust and found a highly significant increase 2.8 Myr ago. The amount of ${}^{60}\mathrm{F}\mathrm{e}$ is compatible with the deposition of ejecta from a SN at a distance of a few 10 pc. The well defined time of the SN explosion makes it possible to search for plausible correlations with other events in Earth’s history.

We have made a new determination of the half-life of the radioactive isotope ${}^{60}\mathrm{Fe}$ using high precision measurements of the number of ${}^{60}\mathrm{Fe}$ atoms and their activity in a sample containing over ${10}^{15}$ ${}^{60}\mathrm{Fe}$ atoms. Our new value for the half-life of ${}^{60}\mathrm{Fe}$ is $(2.62\pm 0.04)\times {10}^6\mathrm{yr}$, significantly above the previously reported value of $(1.49\pm 0.27)\times {10}^6\mathrm{yr}$. Our new measurement for the lifetime of ${}^{60}\mathrm{Fe}$ has significant implications for interpretations of galactic nucleosynthesis, for determinations of formation time scales of solids in the early Solar System, and for the interpretation of live ${}^{60}\mathrm{Fe}$ measurements from supernova-ejecta deposits on Earth.

An ${}^{60}\mathrm{Fe}$ peak in a deep-sea FeMn crust has been interpreted as due to the signature left by the ejecta of a supernova explosion close to the solar system $2.8\pm 0.4\mathrm{Myr}$ ago [Knie et al., Phys. Rev. Lett. 93, 171103 (2004)]. In an attempt to confirm this interpretation with better time resolution and obtain a more direct flux estimate, we measured ${}^{60}\mathrm{Fe}$ concentrations along a dated marine sediment. We find no ${}^{60}\mathrm{Fe}$ peak at the expected level from 1.7 to 3.2 Myr ago. Possible causes for the discrepancy are discussed.