Iron from outside the solar system has sprinkled down on Antarctica in recent years. Measurements of half a ton of snow turned up interstellar iron deposited within the last two decades, scientists report in a study accepted in Physical Review Letters. That iron comes from the explosions of massive stars, or supernovas, the team says.
Within the snow, the researchers isolated 10 atoms of iron-60, a radioactive variety, or isotope, of iron with a total of 60 protons and neutrons in its nucleus. Previous studies have found iron-60, an isotope spewed from supernovas, in ocean sediments and on the moon (SN: 7/10/99, p. 21). But those depositions were a few million years old, and are thought to be the result of ancient nearby explosions blasting waves of debris through space. The new study reveals that the Earth is still encountering the isotope in modern times.
Nuclear physicist Gunther Korschinek of the Technical University of Munich and colleagues transported the snow — still frozen thanks to careful packing and shipping — back to their lab. They melted, filtered and evaporated the snow, and used a technique called accelerator mass spectrometry on the remnants to identify iron-60.
Supernovas are a major birthplace of iron-60, but not the only one. For example, high-energy particles called cosmic rays can create the isotope when they slam into dust in the solar system. To eliminate that explanation, the team compared the amount of iron-60 in the snow with another isotope produced by cosmic rays, manganese-53. The ratio of iron-60 to manganese-53 found was much higher than expected if both isotopes were produced by cosmic rays. The iron-60 might also have been the result of past nuclear weapons tests, but similar logic ruled out that option.
“This gives us a clear indication that this stuff comes from outside of the solar system,” Korschinek says.
The result could help scientists better understand humankind’s place in space. The solar system resides within a low-density pocket of gas, known as the local bubble. It’s thought that exploding supernovas created shock waves that blasted out that bubble. But the solar system currently sits inside a denser region within that bubble, known as the Local Interstellar Cloud. The detection of recently deposited iron-60 suggests that this cloud may also have been sculpted by supernovas, the researchers say.
“This is actually quite a profound thing,” says astrophysicist Brian Fields of the University of Illinois at Urbana-Champaign, who was not involved with the research. “It’s telling us about the recent history of our whole neighborhood in the galaxy and about the lives and deaths of massive stars.”