Stardust trapped in Antarctic ice reveals tens of thousands of years of Solar System’s past, offering a unique window into the history of our cosmic neighborhood. This isn't just about the stars; it's about the clouds of gas, plasma, and stardust that fill space, known as interstellar clouds. These clouds are intimately linked to the birth and death of stars, and their imprints can be found right here on Earth, in the frozen depths of Antarctica.
My colleagues and I have been studying stardust trapped in old Antarctic snow and ice to trace the history of our solar neighborhood, including the Solar System itself. In a new study, we found a subtle clue that reveals our Solar System’s movement through the local interstellar environment over the past 80,000 years.
Instead of looking outward, we study the debris of exploding stars right here on Earth. Stars, as cosmic furnaces, forge elements like carbon, oxygen, calcium, and iron, including rare isotopes like iron-60. When massive stars die, they eject these elements into space, becoming interstellar dust. Tiny grains of this dust drift through the galaxy and occasionally reach 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 long after their light has faded.
Antarctica is invaluable for this research. 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 in our cosmic neighborhood at the time. 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.
Our solar neighborhood is filled with 15 clouds, and the Solar System is currently traversing at least one of them. We hypothesized that the stardust waiting in the clouds to be picked up by Earth. The amount of stardust Earth collects should be related to the clouds' structure: denser clouds, more iron-60. This was our educated guess in 2019.
However, other explanations emerged. 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? To find out, we analyzed a 300kg section of Antarctic ice, dating from 40,000 to 80,000 years ago. The process is painstaking, involving melting and chemically treating the ice to isolate tiny amounts of iron, including iron-60 from stardust.
Using accelerator mass spectrometry, we counted individual atoms of iron-60. The expectation was straightforward: based on previous measurements, we anticipated a steady level of iron-60 deposition. Instead, we found less, 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.
Astronomers are also interested in the clouds around the Solar System. A study reconstructing the history of the clouds concluded 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 the amount of iron-60 collected on Earth to have changed during that period.
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 analyze even older ice, we might soon unravel the mystery of these local interstellar clouds, revealing their full history and uncertain origins.
