In the pure white snow of Antarctica, scientists have found rare isotopes that don't occur naturally on Earth. The isotope, known as iron-60, is usually forged in the crucible of supernova explosions, and the researchers believe it fell to Earth as our solar system passed through an interstellar gas cloud.
Iron-60 is an extremely rare isotope here on Earth, thanks to the fact that there are no ongoing sources. While it may have been produced during the formation of the planet, those atoms would have decayed billions of years ago. That means that any traces found on Earth today originated from the stars, where they're produced in supernovae.
But these traces can only really be detectable if they sit somewhere relatively undisturbed for long periods of time. In the past, iron-60 has been found in moon dust brought back by the Apollo missions, and in ancient seabed deposits. They tell the tale of stars in our cosmic neighborhood going supernova millions of years ago and showering Earth with higher levels of radiation – which may even have wiped out the mighty Megalodon.
Another suspected location of iron-60 is the pristine snow of Antarctica, which can build up for decades, centuries and even millennia. And that was the focus for this study, involving researchers from the Technical University of Munich and the Helmholtz Association.
The team collected 500 kg (1,100 lb) of Antarctic snow from around the Kohnen Station, shipped it to Munich, melted it down, and analyzed it. The solid components were separated out of the meltwater and processed using a few different chemical methods. A few milligrams of iron was found, and tested in an accelerator lab. And sure enough, within that iron were five atoms of iron-60.
"Our analyses allowed us to rule out cosmic radiation, nuclear weapons tests or reactor accidents as sources of the iron-60," says Dominik Koll, an author of the study. "As there are no natural sources for this radioactive isotope on Earth, we knew that the iron-60 must have come from a supernova."
The location of the isotopes gives further clues to their origin. The snow they were found in was at most 20 years old, and the researchers reasoned that they couldn't have come from too far away in the cosmos or they would have dissipated. The team says that the most likely source is the interstellar gas clouds that our solar system has been passing through for the last 40,000 years or so.
"If the gas cloud hypothesis is correct, then material from ice cores older than 40,000 years would not contain interstellar iron-60," says Koll. "This would enable us to verify the transition of the solar system into the gas cloud – that would be a groundbreaking discovery for researchers working on the environment of the solar system."
The research was published in the journal Physical Review Letters.
Source: Technical University of Munich