Radioactive stardust remnants from stellar explosions have been found trapped in ice in Antarctica. These cosmic remains serve as clues that help uncover the history of our solar system, researchers have found in a new study.
Across the universe, we find colossal interstellar clouds of gas, dust, and plasma that lie between stars. Our own solar system is currently passing through such a cloud, known as the Local Interstellar Cloud, nicknamed the "Local Fluff." These clouds can accumulate matter as they float through the cosmos, and that matter can sometimes end up on Earth as our planet passes through the Local Fluff. In a new study of some of this matter, researchers have found a byproduct of ancient supernovas, a radioactive iron isotope known as iron-60 (60Fe), that got stuck in the cloud and ended up embedded in Antarctic ice.
So, how did researchers know that this iron isotope found in Antarctic ice was hitchhiking on an interstellar cloud from an ancient stellar explosion?
In 2019, Koll was part of a research team that detected 60Fe atoms in Antarctic snow. "We didn't know where it came from," he said. "So we continued working on it tracing the influx back … and we got the answer that it is related to the local interstellar cloud."
The team analyzed over 661 pounds (300 kilograms) of samples of ice from Antarctica dating back 40,000 to 80,000 years. This is the timeframe during which the team suspects the supernova took place, blasting the material into space only to be embedded in the cloud. After melting and chemically treating the ice and then using a technique known as accelerator mass spectrometry, which essentially speeds up ions, allowing researchers to separate isotopes from one another, they were able to look at and count individual atoms of this isotope in their samples.
"We looked for single atoms of the radioactive isotope 60Fe," Koll said. "This isotope is a fingerprint of exploding stars. Our hypothesis was that 60Fe might be within the local interstellar cloud if it originates from stellar explosions (that was postulated by modellers)."
The team compared the amount of the iron isotope that they found in recent snow to the amount in this newly-sampled ancient ice. And they found less 60Fe in the much older samples, suggesting that less iron-60 reached Earth between 40,000 and 80,000 years ago than in more recent times.
"This result suggests that less interstellar dust was reaching Earth during that period," Koll said. "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."
And this mystery source? These researchers think that it's most likely a stellar explosion that took place in the region of the Local Interstellar Cloud
"This means that the clouds surrounding the solar system are linked to a stellar explosion," Koll said in a statement. "And for the first time, this gives us the opportunity to investigate the origin of these clouds."
Our solar system has been traveling through the Local Interstellar Cloud for between 40,000 and 124,000 years, researchers have suggested, though it will only be another few thousand years until we are through.
This team aims to conduct more investigation to further confirm and expand their results by studying ice samples from even farther back in time, to a time before the solar system began traveling through this cloud.
This work was described in a study published May 13 in the journal Physical Review Letters.
