A study published May 13 in Physical Review Letters has confirmed that radioactive iron forged inside an exploding star has been falling on Earth for at least 80,000 years — and is still falling today. Led by Dr. Dominik Koll of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, the international team detected the isotope iron-60 — a radioactive form of iron that can only be produced inside a dying massive star — in Antarctic ice cores spanning 40,000 to 81,000 years old, proving that our Solar System has spent tens of thousands of years threading through the lingering wreckage of an ancient supernova.
The finding matters beyond the spectacular: it transforms Antarctic ice into a geological archive of the Solar System's galactic journey, gives astrophysicists their first direct evidence of how the Local Interstellar Cloud — the wispy region of gas and dust our Sun currently moves through — has varied in density over time, and opens a new window onto whether interstellar environments have ever left a detectable mark on Earth.
Study Published Days Ago Closes a Puzzle That Has Stood Since 2019
The paper appeared online on May 13, 2026, making it one of the most closely watched results in nuclear astrophysics this week. The mystery it resolves dates to 2019, when Koll and colleagues found iron-60 in freshly fallen Antarctic snow less than 20 years old. Because no nearby stellar explosion has been recorded in recent history, that discovery had no obvious source. The new data, from a much older ice core, finally supplies one: the Local Interstellar Cloud itself, a vast diffuse structure that astronomers believe was seeded by supernova activity and that the Solar System entered several tens of thousands of years ago.
"Our idea was that the Local Interstellar Cloud contains iron-60 and can store it over long time periods," Koll said in the HZDR press release. "As the Solar System moves through the cloud, Earth could collect this material. However, we couldn't prove this at the time." The new ice core record proves it.
Iron-60 Cannot Form on Earth — Its Presence Means One Thing
Iron-60 does not occur naturally on Earth. Its half-life of 2.6 million years — confirmed across the scientific literature, including the Physical Review Letters paper — means that any iron-60 present when our planet formed has long since decayed. Detecting it today points unambiguously to a recent (in cosmic terms) interstellar source. Geological records had already shown that Earth received two pulses of iron-60 from nearby supernovae millions of years ago, preserved in deep-sea ferromanganese crusts on the Pacific floor. What the new study adds is a continuous, ongoing signal — iron-60 arriving not from a specific recent explosion, but carried persistently inside the interstellar cloud our Solar System is traversing right now.
The Local Interstellar Cloud is a region of slightly higher hydrogen density within the much larger, low-density Local Bubble that surrounds our corner of the galaxy. The Solar System is estimated to have entered the cloud several tens of thousands of years ago and is expected to exit it within a few thousand years, according to the HZDR team. Astronomers have long suspected the cloud's origins lie in supernova activity; the Koll et al. study is the first to supply a direct chemical fingerprint confirming that connection.
295 Kilograms of Ice, a Few Hundred Milligrams of Dust, Seven Iron-60 Atoms
The detective work required industrial-scale processing of Antarctic ice. The team transported roughly 300 kilograms of ice — sourced from the Alfred Wegener Institute in Bremerhaven via the European EPICA ice-drilling project — to Dresden, where it was chemically processed down to a few hundred milligrams of dust. From that dust, the team had to isolate individual iron-60 atoms. The paper specifies 295 kilograms of ice from the EDML core covering 40,000 to 81,000 years ago.
To guard against measurement error, the team cross-checked samples against two other radioisotopes — beryllium-10 and aluminium-26 — whose expected concentrations in Antarctic ice are already well characterized. Any accidental loss of iron-60 during processing would have produced a corresponding drop in those reference markers, giving the team a built-in audit.
The final measurement was performed at the Heavy Ion Accelerator Facility (HIAF) at the Australian National University in Canberra — currently the only facility on the planet sensitive enough to detect iron-60 at these concentrations. Using cascaded electric and magnetic filters, the machine sorted through roughly ten trillion atoms, discarding all but a handful of iron-60. The result: seven detector events against zero in the background measurement — statistically sufficient to confirm the signal.
"It's like searching for a needle in 50,000 football stadiums filled to the roof with hay," said Annabel Rolofs of the University of Bonn, a co-author of the study. "The machine finds the needle in an hour."
Lower Iron-60 in Older Ice Rules Out Simpler Explanations
The key finding is not merely the presence of iron-60, but how its concentration changed over time. According to the published paper, the iron-60 deposition rate in the 40,000-to-81,000-year-old ice was roughly five times lower than in more recent Antarctic snow and marine sediment samples from the last 40,000 years. That variation tracks the Solar System's gradual entry into the denser inner region of the Local Interstellar Cloud.
"This suggests that we were previously in a medium with lower iron-60 content, or that the cloud itself exhibits strong density variations," Koll said. The changing concentration also rules out competing theories — such as the slow-fading remnant of a supernova that exploded millions of years ago — because such a source would produce a smooth, gradual decline rather than the sharper variation the data shows.
"This means that the clouds surrounding the Solar System are linked to a stellar explosion," Koll added. "And for the first time, this gives us the opportunity to investigate the origin of these clouds."
A New Kind of Geological Record — and Questions About Earth's Past
The discovery sits at the intersection of nuclear astrophysics, glaciology, and solar system science. For nuclear astrophysicists, Antarctica's ice sheet — which began accumulating layer by layer from falling snow around 35 million years ago, as noted by ScienceAlert — now functions as a chemical logbook of the Solar System's path through the galaxy. Each layer captures particles from the atmosphere, freezing them in a vertical time capsule.
The broader implications reach further still. Elevated fluxes of cosmic rays — energetic particles accelerated by supernova remnants — have been theorized to influence Earth's climate and, over longer timescales, evolutionary pressures on life. The iron-60 flux detected in this study is extremely low and poses no biological risk. But the framework it establishes raises the question of whether past periods of more intense interstellar cloud traversal, documented in earlier deep-sea manganese records, left deeper marks on Earth's history. ScienceBlog notes that the Local Interstellar Cloud is one of roughly fifteen warm cloudlets drifting through the immediate galactic neighbourhood, each embedded within the Local Bubble, a void probably carved by a succession of supernovae in the Scorpius-Centaurus association starting around 10 to 15 million years ago.
Prof. Anton Wallner of HZDR, who co-led the study, described the significance of the method: "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."
Beyond EPICA Ice Core to Test Whether the Signal Disappears Before We Entered the Cloud
The HZDR team is already planning follow-up measurements using ice from the Beyond EPICA — Oldest Ice project, a European programme that recently drilled a 2,800-metre core from Little Dome C in Antarctica and in January 2025 reached ice older than 1.2 million years — far predating the Solar System's entry into the Local Interstellar Cloud. If a sample from before that entry shows a clean absence of iron-60, it would serve as the ultimate control, cementing the case that the cloud is the source rather than some other interstellar process.
If it does, Antarctic ice will have graduated from Earth's climate archive to an archive of the Solar System's journey through the galaxy — one frozen centimetre at a time.
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