Scientists studying ancient Antarctic ice have discovered new evidence that Earth is currently traveling through a cloud of material left behind by long-dead stars.
An international research team led by Helmholtz-Zentrum Dresden-Rossendorf found traces of a rare radioactive form of iron called iron-60 in Antarctic ice tens of thousands of years old.
The findings suggest that our solar system is moving through a region of space known as the Local Interstellar Cloud, which contains material from ancient stellar explosions.
The study was published in Physical Review Letters.
Iron-60 is extremely unusual because it is mainly created inside massive stars and released into space when those stars explode as supernovae.
Scientists have previously found evidence that Earth was showered with iron-60 from nearby supernova explosions millions of years ago.
But in more recent times, there have been no known nearby stellar explosions that could explain why small amounts of iron-60 are still reaching Earth today.
Researchers began investigating the mystery after traces of iron-60 were discovered in fresh Antarctic snow only a few years ago. They proposed that the Local Interstellar Cloud surrounding our solar system may be storing iron-60 from ancient explosions over very long periods of time.
As Earth travels through this cloud along with the rest of the solar system, tiny amounts of the radioactive material could slowly fall onto our planet.
To test the idea, scientists analyzed ice cores collected in Antarctica. These ice layers act like a frozen history book, preserving particles that settled onto Earth tens of thousands of years ago. The team studied ice dating from about 40,000 to 80,000 years ago.
The samples came from the European EPICA ice drilling project, supported by the Alfred Wegener Institute.
Researchers compared the ice results with earlier studies of ocean sediments and Antarctic snow. They found that iron-60 levels changed over time. Between 40,000 and 80,000 years ago, less iron-60 was reaching Earth than today.
According to the researchers, this suggests that the Local Interstellar Cloud may contain regions with different densities of iron-60, or that the solar system previously traveled through an area with lower concentrations of the material.
The discovery also helped scientists rule out another possibility — that the iron-60 simply came from very old supernova debris slowly fading over millions of years. Instead, the evidence strongly supports the idea that Earth is actively moving through a cloud already enriched with material from past stellar explosions.
The work required incredibly sensitive measurements. Scientists transported about 300 kilograms of Antarctic ice to Germany for processing. After extensive chemical treatment, only a few hundred milligrams of dust remained.
To detect the tiny amounts of iron-60 hidden inside that dust, the researchers used the Australian National University’s Heavy Ion Accelerator Facility, currently the only facility capable of making such measurements.
Researchers compared the challenge to finding a single needle hidden inside 50,000 football stadiums filled with hay.
The team now hopes to analyze even older Antarctic ice formed before the solar system entered the Local Interstellar Cloud. By doing so, they may learn more about the ancient supernova explosions that shaped our cosmic neighborhood long before humans existed.
