Iron-60 Found Again in Antarctic Ice Core Samples from 40,000 to 80,000 Years Ago

An international research team recently analyzed trace radioactive elements in ancient Antarctic ice layers and discovered an important clue recording the solar system's cosmic journey over the past 80,000 years. The researchers believe that the “stardust” buried deep in the ice proves that Earth has long been receiving material from interstellar space, and that this material may help scientists unveil the mystery of the formation history of the interstellar environment surrounding the solar system.

At the core of the research is a rare radioactive isotope called iron-60. Iron-60 is not produced in large quantities in Earth's natural environment; it is typically born inside massive stars and ejected into cosmic space when those stars undergo supernova explosions. Therefore, iron-60 has long been regarded as an important “cosmic fingerprint” for studying stellar death and supernova activity.

The research team had previously found traces of iron-60 in relatively recent Antarctic snow samples, but its origin had always been disputed. To further seek answers, the researchers obtained Antarctic ice core samples aged between 40,000 and 80,000 years and extracted and analyzed trace elements from them. The results showed that iron-60 also exists in these ancient ice layers, and its distribution pattern shows obvious variations.

By comparing with data from recent Antarctic snow samples and deep-sea sediments, scientists successfully reconstructed the approximate trajectory of changes in iron-60 reaching Earth over the past 80,000 years. The data show that the influx of iron-60 was not constant, but went through stages including lows, peaks, and gradual declines.

The researchers believe that this variation is best explained by one interpretation: the solar system is passing through an interstellar region known as the “Local Interstellar Cloud,” and iron-60 is precisely the ancient supernova remnant stored in this cloud. When the solar system travels through the Milky Way and enters this interstellar cloud, Earth continuously receives iron-60 particles carried within it.

The Local Interstellar Cloud is an extremely thin structure of gas and dust surrounding the solar system, part of a larger interstellar cloud complex near the Sun. Astronomers currently believe that the solar system is inside this cloud. The new study suggests that these clouds are very likely closely related to ancient supernova explosions, and iron-60 is precisely the evidence left behind by this history.

To complete this study, scientists processed hundreds of kilograms of ice core samples and used high-sensitivity mass spectrometers to search for extremely rare iron-60 atoms. At the same time, they also detected other isotopes such as manganese-53, hoping to distinguish different sources originating from inside and outside the solar system.

The analysis results found that iron-60 produced solely by cosmic rays striking dust within the solar system cannot explain the actual quantities observed. The research team therefore believes that the most reasonable source of the excess portion is dust in interstellar space, and the Local Interstellar Cloud is currently the explanation that best fits the evidence.

Scientists say that this discovery not only helps confirm the connection between the solar system and the Local Interstellar Cloud, but also provides a new tool for studying the origins of these interstellar clouds themselves. In the past, humanity mainly relied on telescopes to observe distant celestial bodies to study cosmic history, but now, tiny atoms buried in Earth's ice layers have also begun to tell the story of what happened in the Milky Way's past.

The researchers point out that if data covering a longer time scale and with higher precision can be obtained in the future, humanity is expected to further reconstruct the historical trajectory of the solar system traversing different regions of the Milky Way, and trace a series of ancient supernova events that influenced the formation of the solar neighborhood's environment.

For the astronomical community, this interstellar dust frozen in Antarctic ice layers is not only a relic from the moment of distant stellar death, but more like a cosmic letter spanning tens of thousands of years, recording the solar system's long and continuous journey through the Milky Way.