Earth is currently passing through a cloud of radioactive debris from an ancient supernova. Traces of iron-60, an isotope that forms only during the explosion of massive stars, have been found in Antarctic ice. This discovery shows that our solar system is now bathed in the ashes of a long-dead star.
To understand this oddity, it must be known that iron-60 is produced only in the cores of giant stars and ejected into space when they explode as supernovae. Until now, scientists thought that traces of this radioactive element found on Earth dated from explosions that occurred millions of years ago. However, recent measurements in fresh Antarctic snow have revealed the presence of iron-60, surprising researchers because no recent supernova has occurred nearby.
Path of the solar system through the local interstellar cloud. The cloud's profile is preserved as an interstellar imprint in Antarctic ice.
Credit: B. Schröder/HZDR/NASA/Goddard/Adler/U.Chicago/Wesleyan The international team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) then put forward a bold hypothesis: the local interstellar cloud, through which our solar system is moving, might have contained iron-60 for millennia. By crossing this cloud, Earth collects these radioactive particles. To test this idea, scientists analyzed Antarctic ice cores aged between 40,000 and 80,000 years.
The results of the study, published in
Physical Review Letters, show that the amount of iron-60 in ancient ice is lower than in recent snow. This indicates that our solar system entered the cloud a few tens of thousands of years ago and will exit it within a few thousand years. The fluctuations of the iron-60 signal over short cosmic timescales allow ruling out the hypothesis of remnants from ancient supernovae.
To reach this conclusion, researchers deployed considerable technical resources. Approximately 660 pounds (300 kg) of ice were transported from the Alfred Wegener Institute to Dresden for chemical analysis. After extensive processing, only a few hundred milligrams (roughly 0.02 oz) of dust remained. The scientists then carefully isolated the iron-60, verifying their methods using other isotopes such as beryllium-10 and aluminum-26, whose concentrations in ice are well known.
The detection itself required the heavy-ion accelerator at the Australian National University, the only instrument in the world capable of spotting minute quantities of iron-60. It is like searching for a needle in 50,000 football fields filled with hay: the machine finds the needle in one hour. This technical feat confirmed that the local interstellar cloud is indeed the source of the iron-60, thus linking our immediate cosmic environment to a stellar explosion.
Scientists now plan to analyze even older ice cores, dating from before the solar system entered the cloud. This could reveal the structure of the interstellar medium and the history of nearby supernovae. For the first time, we have the opportunity to study the origin of these clouds surrounding our solar system, charting our journey through the galaxy.