Our Solar System is currently passing through the Local Interstellar Cloud, a region of highly diluted gas and dust between the stars. On its path, Earth continuously accumulates iron-60, a rare radioactive isotope of iron produced in stellar explosions. This has now been confirmed by an international research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) through the analysis of Antarctic ice tens of thousands of years old. From the steady but time-varying influx, the researchers conclude that the radioactive isotope has been stored within the cloud since a long-past stellar explosion. The results have been published in the journal Physical Review Letters (DOI:10.1103/nxjq-jwgp).
Iron-60 is formed in the interiors of massive stars and is ejected into space when they explode. Geological archives show that our Solar System was hit twice by iron-60 from supernovae millions of years ago. In more recent times, however, there have been no nearby stellar explosions – and thus no direct supply of iron-60. When scientists discovered iron-60 in Antarctic surface snow less than twenty years old a few years ago, the question of its origin arose.
"Our idea was that the Local Interstellar Cloud contains iron-60 and can store it over long time periods. As the Solar System moves through the cloud, Earth could collect this material. However, we couldn't prove this at the time," explains Dr. Dominik Koll from the Institute of Ion Beam Physics and Materials Research at HZDR.
In recent years, the team led by Koll and Prof. Anton Wallner analyzed additional samples, including deep-sea sediments up to 30,000 years old. Iron-60 was also found there, but competing theories remained. The new Antarctic ice samples date back 40,000 to 80,000 years. Their analysis now makes it clear: the Local Interstellar Cloud is the likely source. "This means that the clouds surrounding the Solar System are linked to a stellar explosion. And for the first time, this gives us the opportunity to investigate the origin of these clouds," says Koll.
Local Interstellar Cloud stores iron-60 from stellar explosions
Our Solar System entered the Local Interstellar Cloud several tens of thousands of years ago and will leave it again in a few thousand years. At present, we are located near its edge.
For their study, the researchers analyzed an ice core from the period around the suspected entry into the cloud. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) provided a sample from the European ice drilling project EPICA. Comparing the iron-60 content with earlier deep-sea and snow samples showed that between 40,000 and 80,000 years ago, less iron-60 reached Earth than today and in more recent times. "This suggests that we were previously in a medium with lower iron-60 content, or that the cloud itself exhibits strong density variations," explains Koll.
The iron-60 signal thus changes over just a few tens of thousands of years – remarkably fast on cosmic timescales. With this insight, the researchers were able to rule out alternative explanations for the source of the iron-60 influx, such as the gradual fading of million-year-old stellar explosions.
Searching for traces in Antarctic ice
For the measurements, the team transported around 300 kilograms of ice from AWI in Bremerhaven to Dresden, where it was chemically processed – a lengthy procedure that ultimately left only a few hundred milligrams of dust. Step by step, they isolated the iron-60, taking great care to avoid losses at every stage.
At the DREsden Accelerator Mass Spectrometry (DREAMS) laboratory at HZDR, they therefore checked the sample after chemical preparation using two other radioisotopes: beryllium-10 and aluminium-26. The expected concentrations of these isotopes in the ice are well known. Any loss of iron-60 would have been accompanied by a reduction in their amounts as well. The team was able to rule this out.
Finding a needle in a haystack
For the final measurement, the team used the Heavy Ion Accelerator Facility (HIAF) at the Australian National University – currently the only facility in the world capable of detecting such tiny quantities of iron-60. Using electric and magnetic filters, they separated unwanted atoms according to their mass until only a handful of iron-60 atoms remained out of an initial 10 trillion atoms.
"It's like searching for a needle in 50,000 football stadiums filled to the roof with hay. The machine finds the needle in an hour," explains Annabel Rolofs from the University of Bonn.
"Through many years of collaboration with international colleagues, we have developed an extremely sensitive method that now allows us to detect the clear signature of cosmic explosions that occurred millions of years ago in geological archives today," summarizes Wallner.
The team is already planning further measurements. The goal is to analyze an even older ice core dating from before the Solar System entered the Local Interstellar Cloud. The AWI is a key partner in the Beyond EPICA – Oldest Ice project, which aims to recover ice cores of such age.
Publication:
D. Koll, A. Rolofs, F. Adolphi, S. Fichter, M. Hoerhold, J. Lachner, S. Pavetich, G. Rugel, S. Tims, F. Wilhelms, S. Zwickel, A. Wallner: Local Interstellar Cloud Structure Imprinted in Antarctic Ice by Supernova 60Fe, Physical Review Letters, 2026. (DOI:10.1103/nxjq-jwgp)
Additional information:
Dr. Dominik Koll | Prof. Anton Wallner
Institute of Ion Beam Physics and Materials Research at HZDR
