Stardust in eternal ice: The isotope iron-60 is considered an indicator of nearby stellar explosions – but why is this exotic type of atom found even in fresh Antarctic snow? A possible answer to this mystery is now provided by an ice core from Antarctica, whose layers go back up to 80,000 years. Analyzes reveal that the increased influx of iron-60 only began around 40,000 years ago. The exciting thing about it: Since around this time, the solar system has been passing through a local interstellar cloud. Is it the origin of the exotic iron isotope?
Many elements in our chemical periodic table were once formed inside massive stars. When they exploded at the end of their life cycle, the atoms were released and dispersed into the interstellar medium with gas and dust. This also includes the radioactive isotope iron-60 with a half-life of around 2.6 million years. Because this isotope does not occur naturally on Earth, the iron-60 detected in terrestrial materials must have reached our planet through particles from supernovae.
In fact, finds of iron-60 in marine sediments and fossils suggest that there may have been several nearby supernovae around 1.8 to 2.5 million years ago. Astronomical data also suggests that our solar system has been traveling through a local bubble formed by a chain of previous supernovae for around five million years. In recent times, however, there have been no nearby stellar explosions, so there should be no “fresh” supply of iron-60 on Earth – or so it was thought.
How does iron-60 get into fresh snow?
All the more surprising was the detection of this supernova indicator in freshly fallen Antarctic snow in 2019. In the snow samples, which were less than 20 years old, researchers detected five iron-60 particles that could neither have been caused by nuclear weapons tests nor by cosmic radiation. But where does this exotic iron isotope come from? In order to find out more about this, a team led by Dominik Koll from the Helmholtz Center Dresden-Rossendorf has now carried out additional analyses.
For their study, Koll and his colleagues analyzed samples from an ice core taken from the Antarctic plateau as part of the EPICA project. To do this, the ice samples in Dresden were first prepared and examined using mass spectrometry for isotopes generated by cosmic radiation, such as beryllium-10 and aluminum-26. To detect iron-60, the team brought their prepared samples to Australia to the Heavy Ion Accelerator Facility in Canberra. It is the only facility in the world that can detect even tiny amounts of this isotope. “It’s like looking for a needle in 50,000 football stadiums filled to the ceiling with hay. The machine finds the needle in an hour,” explains co-author Annabel Rolofs from the University of Bonn.
Ice core reveals surprisingly rapid change
The analyzes produced an interesting result: 40,000 to 80,000 years ago, less iron-60 reached the Earth’s surface than at the time of the supernovae around two million years ago – but also less than today. At around 0.22 iron-60 atoms per square centimeter per year, the deposition rate measured in the ice core is below that in fresh Antarctic snow and in marine sediments from the last millennia, as Koll and his team report.
This means that the deposited amount of this rare iron isotope has changed significantly within a few tens of thousands of years – on a cosmic time scale, this is remarkably fast, as the researchers explain. This suggests that the fresh deposits must have a cause other than just the waning influx of million-year-old supernovae or cosmic radiation.
Local interstellar cloud as a cosmic archive?
But how can this fresh influx of iron-60 be explained? “Our idea was that the local interstellar cloud contains iron-60 and can store it for longer periods of time,” explains Koll. This local cloud is one of the many denser clumps of dust and gas in the interstellar medium of the local bubble. Our solar system entered this local cloud around 40,000 to 64,000 years ago and is currently passing through its edge.
Astronomers have long been discussing whether the dust in this interstellar cloud represents a kind of cosmic archive in which gas and dust from past supernovae are enriched. “If the local interstellar cloud is such an archive, then the Earth’s transition from the less dense interstellar medium into this cloud should also be reflected in a changed influx of iron-60,” the researchers explain.
New insights into our cosmic environment
Koll and his team have now demonstrated exactly this change for the first time: The change in iron-60 concentrations they detected in the ice core occurred around 40,000 years ago – and therefore around the time when the solar system entered the local interstellar cloud. “The orbit of the solar system would therefore explain the experimentally determined time profile of iron-60 deposition,” the team writes. Koll adds: “This gives us the opportunity for the first time to get to the bottom of the origin of these clouds.”
According to the researchers, the local interstellar cloud is not a pure supernova remnant because the current iron-60 deposition rate is too low for that. Instead, they think it is more likely that this cloud consists of ancient interstellar dust that has been enriched with the isotopes by particles from nearby supernovae. Koll’s team is already planning further analyses, including of an even older ice core that was recently obtained in Antarctica as part of the “Beyond EPICA” project.
Source: Dominik Koll (Helmholtz-Zentrum Dresden-Rossendorf, Dresden) et al., Physical Review Letters, 2026; doi: 10.1103/nxjq-jwgp