Iron isotope reveals Earth’s close encounters with supernovae

In 1999 researchers in Germany discovered traces of iron-60 in crust from the Pacific Ocean. Because only supernovae are known to create abundant amounts of ⁶⁰Fe, which has a half-life of 2.6 million years, Gunther Korschinek at the Technical University of Munich and colleagues concluded that dust from a relatively recent and nearby supernova had littered Earth’s surface with the isotope. After only a smattering of supporting evidence over the past 17 years, a burst of ⁶⁰Fe research has now solidified the supernova interpretation and opened a new window for investigating how those close encounters affected Earth’s past climate and life.

Anton Wallner at the Australian National University in Canberra and colleagues expand the ⁶⁰Fe record by using accelerator mass spectrometry to measure the isotope’s prevalence in crusts (including the pictured sample) and sediment cores from the Pacific, Atlantic, and Indian Oceans. The distribution and concentrations of ⁶⁰Fe rule out a terrestrial or meteoritic origin; rather, the evidence points to multiple nearby (within about 325 light-years) supernova events over the past 10 million years. Meanwhile, after years of unsuccessful lobbying, Korschinek and colleagues finally got their hands on samples of Moon regolith. Sure enough, the scientists found traces of ⁶⁰Fe. The new studies present a consistent global and extraterrestrial record of past supernova activity that occurred uncomfortably close to home.

In a third chapter to the unfolding story of supernova archaeology, Dieter Breitschwerdt at the Technical University of Berlin and colleagues simulate the sequence of stellar explosions that could account for the Pacific deposits of ⁶⁰Fe. The researchers propose the demise of two nine-solar-mass stars between about 290 light-years and 325 light-years away; the more recent supernova would have occurred 1.5 million years ago, when early species of Homo were spreading over the planet. While recent supernovae didn’t have drastic, mass extinction–inducing consequences, Wallner and colleagues note that some ⁶⁰Fe deposits correspond to a period of global cooling that eventually ushered in the Pleistocene epoch. (A. Wallner et al., Nature 532, 69, 2016, doi:10.1038/nature17196 ; L. Fimiani et al., Phys. Rev. Lett. 116, 151104, 2016, doi:10.1103/PhysRevLett.116.151104 ; D. Breitschwerdt et al., Nature 532, 73, 2016, doi:10.1038/nature17424 .)