The gleaming “spaceship Earth,” along with the Solar System it inhabits, is not static in the void but rather slowly and majestically revolves, participating in a galactic waltz around the core of the Milky Way.
It is often impossible for us to ascertain the Solar System’s exact location throughout this ancient journey—however, fresh clues, preserved for tens of thousands of years within Antarctic ice, have recently surfaced. The findings of this study are detailed in the journal Physical Review Letters.
There, a team of scientists led by nuclear astrophysicist Dominik Kroll from the Helmholtz-Zentrum Dresden-Rossendorf in Germany discovered rare iron isotopes that point to our planet’s recent passage through an interstellar dust cloud originating from supernovae—the remnants of long-dead stars.
In recent decades, the Antarctic ice sheet has become an invaluable repository of information concerning our planet’s history. It began to accumulate layer upon layer from snowfall approximately 35 million years ago, trapping atmospheric particles in each stratum as they froze.
Over time, this stratified accumulation, compressed by its own weight, transformed into a vertical time capsule, from which scientists can extract long ice cylinders to obtain a chronological record of atmospheric changes spanning millions of years.
In 2019, Kroll and his associates examined freshly fallen Antarctic snow and detected traces of an iron isotope termed $\text{Fe}^{60}$, or iron-60.
They have now located this iron-60 isotope frozen within ice cores dating back to the period between 40,000 and 81,000 years ago.
Iron-60 is unique because it can only be generated under extreme conditions not naturally occurring on Earth, such as those associated with supernova explosions.
It is entirely plausible that some amount of iron-60 was incorporated into Earth during its formation, but with a half-life of only 2.6 million years, it would have almost entirely decayed away in about 15 million years. Any iron-60 present when Earth formed 4.5 billion years ago is long gone.
This implies that any iron-60 content detected on our planet exceeding a certain background threshold must have arrived from space. There is no known terrestrial natural process capable of producing it in substantial quantities.
Furthermore, given how it is produced, scientists theorize that its presence on Earth, both in deep-sea sediments and in recent snowfalls, signifies that our Solar System traversed debris originating from a supernova—and perhaps is still passing through it.
After initially detecting iron-60 in surface snow, Kroll and his collaborators resolved to dig deeper, metaphorically speaking, to ascertain how far back the iron-60 record extended.
Utilizing samples procured through the European Project for Ice Coring in Antarctica (EPICA), the researchers sifted through 295 kilograms of Antarctic ice searching for the elusive iron-60 atoms.
They melted the ice, extracted the residue, and meticulously counted the quantity of iron-60 atoms they recovered.
The results demonstrated a higher concentration of iron-60 than could be accounted for by the very minor background contribution from cosmic rays bombarding Earth—suggesting that at least some of the iron-60 in the Antarctic ice must have originated from interstellar space.
And here is where the most compelling aspect emerges. The concentration of iron-60 in ice formed tens of thousands of years ago is distinctly lower than the concentration detected in snow from the last few decades.
The Solar System is currently moving through a region designated as the Local Interstellar Cloud, composed of gas, dust, and plasma, which scientists believe arose from supernova activity. Therefore, it is logical to infer that this cloud deposits a very faint rain of iron-60 onto Earth.
The research outcomes from Kroll’s team indicate that the Antarctic ice acts as a record of Earth’s journey through this cloud, thereby offering a granular view of the cloud’s structure. Moreover, this implies the existence of denser and less dense regions of dust containing iron-60 within the cloud.
This is truly a remarkable finding derived from just a handful of atoms embedded within a block of ice.
The ice core data suggests that the Solar System may have been traversing these clouds for at least 80,000 years, initially passing through a sparser region before entering the denser one that we are currently moving through.
The precise genesis of the Local Interstellar Cloud remains unknown to us. The team’s findings align with the theory that it originated from a supernova and opens the possibility of mapping the cloud’s structure using tangible material right here on Earth.
“These findings suggest that the Local Interstellar Cloud functions as a cosmic archive of iron-60 synthesized by supernova explosions,” the researchers state. “The imprint of hexavalent iron across the temporal profile points to a variation in the local interstellar medium over the last 80,000 years.”
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