Massive stars are like gargantuan element foundries. For millions of years, they react atomic nuclei together into increasingly heavier chemical elements, before exploding as supernovae and scattering those elements out into the cosmos. Many of the elements created in supernovae are essential to life on Earth, and now for the first time ever, life on Earth has provided some rather unusual evidence for a supernova 2.2 million years ago.
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A study carried out by researchers at the Technische Universitaet Muenchen (TUM), published in Nature, concerns a particular kind of iron loving bacteria known as magnetotactic bacteria. These particular microbes live in ocean sediments, where they metabolise iron to create tiny crystals of iron oxide - a particular type of iron oxide, known as magnetite (Fe3O4).
The magnetite crystals made by these bacteria are quite uniform, each just 80 nanometres (80 billionths of a meter) in size. The iron they use comes from dust in Earth's atmosphere, which finds its way into the ocean. And every so often, they metabolise iron, which originally came from a supernova.
When a supernova explodes, nuclear fusion goes wild. Elements fuse together haphazardly, creating unusual and unstable radioactive isotopes. This is the only natural way in which certain elements, such as uranium, can be created. One particular isotope formed this way is iron-60.
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In fact, iron-60 is created almost exclusively in supernovae. With a half life of 2.6 million years, any iron-60 which was on Earth back when it formed is long gone, so finding any of this form of iron on Earth is excellent evidence for a nearby supernova sometime in the (relatively) recent past.
Shawn Bishop, a nuclear astrophysicist, and his colleagues analyzed sediment cores drilled from the floor of the Pacific Ocean. In those sediment samples, they found fossilized bacteria - and in those fossils, they found iron-60.
Studying sediment samples aged between 1.7 and 3.3 million years, they looked at intervals every 100,000 years, chemically extracting the bacterial fossils to analyze the magnetite crystals created at the time. They then tested those fossils at the Maier Leibnitz Laboratory in Garching, Munich, using an ultra sensitive mass spectrometer. The result was a glimpse of iron-60 in the fossils, providing the first ever evidence for a near-Earth supernova given directly by Earth life.
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What's more, the bacterial iron-60 was dated to approximately 2.2 million years ago. This coincides perfectly with a previous study in 2004, where researchers found the same iron isotope in Earth's crust. This was the first time the isotope had been found on our planet, and was also found buried under the Pacific Ocean floor.
While this is still a preliminary (and somewhat tantalizing) result, it's certainly an impressive one. It also confirms that there was very likely a supernova near to our solar system roughly 2.2 million years ago, and that ash from that supernova made its way onto the surface of our planet.
Emboldened by their discovery, Bishop and his team are now preparing to analyze a second sediment core to try and verify their findings. The second sample contains over 10 times as much material as the first, and should be able to provide evidence either for or against their results. If they do find further iron-60, Bishop plans to map out the way the supernova-forged iron made its way into the ecosystem over time.
Image: A composite image of N49, optically the brightest supernova remnant in the Large Magellanic Cloud. Credit: X-ray: NASA/CXC/Caltech/S.Kulkarni et al.; Optical: NASA/STScI/UIUC/Y.H.Chu & R.Williams et al.; IR: NASA/JPL-Caltech/R.Gehrz et al.