Iron-60 discovery in Antarctic ice reveals: Local Interstellar Cloud leaves its mark

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 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

By Dominik Koll, from the Department of Nuclear Physics and Accelerator Applications (NPAA), and Institute of Ion Beam Physics and Materials Research (IBPMR) at Helmholtz-Zentrum Dresden-Rossendorf.

First published in The Conversation 14/5/26

When you think of outer space, you’re likely picturing stars, planets and moons. But much of space is filled with clouds of gas, plasma and stardust – known as interstellar clouds.

In the local parts of our galaxy alone there’s a complex of roughly 15 individual interstellar clouds. The Solar System is currently traversing one of them, aptly named the Local Interstellar Cloud. The origin and history of these clouds are believed to be tightly connected to the birth and death of stars. But we can see their imprints right here on Earth, in a place you might not expect – Antarctic ice.

My colleagues and I have been studying stardust trapped in old Antarctic snow and ice to trace the history of our solar neighbourhood, including the Solar System itself.

In a new study published in Physical Review Letters, we found a subtle clue that reveals our Solar System’s movement through the local interstellar environment over the past 80,000 years.

Astronomy usually looks outward. Telescopes collect light from distant stars and galaxies, allowing us to observe events across vast stretches of space and time. From these observations, we infer how stars live and die, how elements are formed, and how the universe evolves.

Our approach turns that idea on its head.

Instead of observing the light coming to us, we study the debris of exploding stars right here on Earth. As cosmic furnaces, stars forge many elements in their cores, from carbon and oxygen to calcium and iron. This includes rare isotopes (variants of chemical elements) such as iron-60.

When massive stars explode into supernovae at the end of their life, these elements are ejected into space and become interstellar dust.

Tiny grains of this dust then drift through the galaxy and occasionally find their way to Earth’s surface. Radioactive iron-60, a fingerprint of stellar explosions, is embedded within these grains. By searching for these atoms in geological archives on Earth, we can probe astrophysical events like supernovae long after their light has faded.

This is why Antarctica is so valuable. Its snow accumulates slowly and remains largely undisturbed, forming a layered record that stretches back tens of thousands of years. Each layer captures a snapshot of the material that was present in our cosmic neighbourhood at the time.

Finding stardust in Antarctic ice

When we studied 500kg of recent snow in Antarctica, we unexpectedly found this rare radioactive isotope. Where did it come from? There was no recent near-Earth supernova.

But our solar neighbourhood is filled with 15 clouds, with the Solar System currently traversing at least one of them. Is the stardust waiting in the clouds to be picked up by Earth? If yes, then the amount of stardust Earth collects should be related to their structure: the denser the clouds, the more iron-60 they contain. This was our educated guess in 2019.

Soon, other explanations were brought forward. Millions of years ago Earth received large showers of iron-60 from massive supernovae. Is the iron-60 in Antarctic snow the last remnant or an echo of this signal? A rain that became a drizzle?

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.

In recent years, we 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.

To find out, we analysed a 300kg section of Antarctic ice, dating from 40,000 to 80,000 years ago. The process is painstaking. The ice needs to be melted and chemically treated to isolate tiny amounts of iron, including the iron-60 from the stardust.

“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,” said 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,” said Professor Anton Wallner, from NPAA and IBPMR.

Then, using the sensitive atom counting technique of accelerator mass spectrometry at the Heavy-Ion Accelerator Facility at Australian National University, we counted individual atoms of iron-60.

The expectation was straightforward: based on previous measurements from surface snow of Antarctica and several thousand-year-old ocean sediments, we anticipated a certain steady level of iron-60 deposition.

Instead, we found less. Not zero, but noticeably lower than expected.

This result suggests that less interstellar dust was reaching Earth during that period. This is a remarkable change on a comparatively short astrophysical timescale and does not fit the long timescales of the iron-60 deposits that landed here millions of years ago. Instead, we needed to look for a smaller, more local source for the isotope.

Naturally, astronomers are also quite interested in the clouds around the Solar System. Last year, a study reconstructing the history of the clouds arrived at the conclusion that they most likely originated in a stellar explosion. Furthermore, they found the Solar System has been traversing the Local Interstellar Cloud from sometime between 40,000 and 124,000 years ago.

If that’s correct, we would expect that the amount of iron-60 collected on Earth should have changed sometime in the same time period – between 40,000 and 124,000 years ago.

This is exactly what our results showed in Antarctica.

The story doesn’t fit perfectly, though. If these clouds did originate directly from an exploding star, we would expect way more iron-60 than we actually see in Antarctic ice.

Nevertheless, these clouds are imprinted in Earth’s geological record. If we look deeper and analyse even older ice, we might soon unravel the mystery of these local interstellar clouds, revealing their full history and uncertain origins.

We 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.