B. Schröder / HZDR / NASA / Goddard / Adler / University of Chicago/Wesleyan
Trajectory of the Solar System through the Local Interstellar Cloud. The cloud profile is preserved as an interstellar “fingerprint” in the Antarctic ice
Our solar system entered the Local Interstellar Cloud several tens of thousands of years ago, and is currently at its edge. As the Solar System moves through this stellar cloud, Earth is accumulating iron-60.
Our Solar System is currently crossing the Local Interstellar Clouda region of highly diluted gas and dust between stars. Along its path, the Earth continually accumulates ferro-60a rare radioactive isotope of iron produced in stellar explosions.
This discovery has now been confirmed by an international research team led by the HZDR (Helmholtz-Zentrum Dresden-Rossendorf), through the analysis of Antarctic ice tens of thousands of years old.
From the constant but time-varying flow, researchers concluded that the radioactive isotope has been stored in the cloud since a stellar explosion that occurred a long time ago. The results were in the magazine Physical Review Letters.
Iron-60 forms inside massive stars and is ejected into space when they explode. Geological records show that our Solar System was hit twice by iron-60 from supernovae millions of years ago.
However, in more recent times, there were no star explosions nearby — and therefore no direct supply of iron-60. When scientists, a few years ago, on the surface of Antarctica, snow was under twenty years oldthe question of its origin arose.
“Our idea is that the Local Interstellar Cloud contains iron-60 and can store it for long periods. As the Solar System moves through the cloud, Earth could collect this material. However, we were unable to prove this at the time”, explains the Dominic Kolldo Institute of Ion Beam Physics and Materials Research (IIM) do HZDR.
In recent years, the team led by Koll and professor Anton Wallner analyzed additional samples, including deep-sea sediments up to 30,000 years old. Iron-60 was also found in these samplesbut competing theories continued to exist.
The new ice samples from Antarctica date back 40,000 to 80,000 years. Their analysis now makes it clear: the Local Interstellar Cloud is the likely source.
“This means that the clouds surrounding the Solar System are connected to a starburst. And, for the first time, this gives us the opportunity to investigate the origin of these clouds”, says Koll.
Our Solar System entered the Local Interstellar Cloud several tens of thousands of years ago and will come out of it again in a few thousand years. We are currently located close to its shore.
For their study, the researchers analyzed an ice core from the period around the suspected cloud entry. AWI (Alfred Wegener Institute Helmholtz Center for Polar and Marine Research) provided a sample from EPICA (European Project for Ice Coring in Antarctica).
Comparison of iron-60 content with previous deep-sea and snow samples revealed that from 40,000 to 80,000 years ago, less iron-60 arrived on Earth than today and in more recent times. “This suggests that previously we were in an environment with lower iron-60 contentor that the cloud itself presents strong variations in density”, explains Koll.
The iron-60 signal thus varies over just a few tens of thousands of years – a remarkably fast pace on cosmic scales of time.
With this finding, the researchers were able to rule out alternative explanations for the origin of the iron-60 flux, such as the gradual weakening of million-year-old stellar explosions.
“Through many years of collaboration with international colleagues, we have developed an extremely sensitive method that now allows us to detect, in current geological archives, the clear signature of cosmic explosions that occurred millions of years ago,” summarizes Wallner.
“It was like look for a needle in 50,000 football stadiums filled to the ceiling with hay. The machine finds the needle in one hour,” he explains. Annabel Rolofsfrom the University of Bonn.
