Scientists have discovered new evidence that Earth is traveling through clouds of ancient stellar debris left behind by stars that exploded long ago. By studying Antarctic ice that formed tens of thousands of years ago, researchers detected traces of iron-60, a rare radioactive form of iron produced in supernova explosions. This discovery supports the idea that the local interstellar cloud surrounding our solar system contains material from ancient stellar explosions.
The study was led by an international team from Dresden-Rossendorf-Helmholtzzentrum (HZDR) and the results were published in the following journals: physical review letter.
Rare supernova iron discovered in Antarctic ice
Iron-60 is formed deep inside massive stars and is ejected into space when those stars explode as supernovae. Previous research has found evidence that Earth was exposed to iron-60 from a nearby supernova millions of years ago. But scientists remain uncertain about the source of the iron-60, which was discovered in relatively young Antarctic snow several years ago, because there are no known recent stellar explosions close to Earth.
“Our idea was that the local interstellar cloud contains iron-60 and could store it for long periods of time. As the solar system moves through the cloud, Earth might be able to collect this material. However, we could not prove this at the time,” explains Dr. Dominic Coll of the HZDR Ion Beam Physics and Materials Laboratory.
To investigate further, Coll and Professor Anton Wallner studied additional geological samples in recent years, including deep-sea sediments that are up to 30,000 years old. These samples also contained iron-60, but scientists still could not completely rule out other explanations.
The newly analyzed Antarctic ice samples are much older, dating back between 40,000 and 80,000 years. According to the research team, this result strongly suggests that the local interstellar cloud is the source of the radioactive material.
“This means that the clouds surrounding our solar system are associated with stellar explosions, and for the first time we have the opportunity to investigate the origins of these clouds,” Coll said.
Solar system moving through an interstellar cloud
Scientists predict that our solar system entered the local interstellar cloud tens of thousands of years ago and will exit it again within the next few thousand years. Researchers say the solar system is currently located near the outer edge of the cloud.
The research team focused on ice cores that cover the period during which the solar system may have entered the clouds. The sample was provided by the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) through the European EPICA ice drilling project.
When researchers compared the ice core results with previous measurements of snow and deep-sea sediments, they found that less iron-60 reached Earth between 40,000 and 80,000 years ago than it does today.
“This suggests that we were previously in a medium with low iron-60 content, or that the cloud itself was exhibiting strong density changes,” Koll explains.
The researchers found that the iron-60 signal changed significantly over a period of just tens of thousands of years. This is relatively rapid on the cosmic time scale. This helped researchers rule out competing theories, such as the idea that the material came from an old supernova explosion that slowly disappeared over millions of years.
Extracting trace traces of iron-60
To carry out the study, the researchers transported around 300 kilometers of Antarctic ice from the AWI in Bremerhaven to Dresden for chemical treatment. After careful preparation, only a few hundred milligrams of dust remained.
Scientists then carefully separated the iron-60, making sure no material was lost during the process.
In HZDR’s DREAMS (DREAsden Accelerator Mass Spectrometry) laboratory, the team tested the prepared samples with two additional radioisotopes: beryllium-10 and aluminum-26. The expected levels of these isotopes in Antarctic ice are already well known, allowing researchers to ensure that no iron-60 was lost during preparation.
Detection of small numbers of atoms
For the final measurements, the scientists used the Australian National University’s Heavy Ion Accelerator Facility (HIAF). This facility is currently the only facility in the world capable of detecting such minute amounts of iron-60. The machine used electrical and magnetic filters to separate atoms by mass from the original sample of 10 trillion atoms until only a few iron-60 atoms remained.
“It’s like looking for a needle in 50,000 football fields 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 extremely sensitive methods that allow us to detect clear traces of cosmic explosions that took place millions of years ago in today’s geological archives,” summarizes Wallner.
The researchers now plan to study even older ice cores from the time before the solar system entered the local interstellar cloud. AWI is participating in the “Beyond EPICA – Oldest Ice” project, which aims to recover ice samples that extend back into Earth’s past.
