And every winter, when it looks like the Earth has been washed clean once again, I feel happy. But I’m even more delighted this year because I learned that there’s something far more amazing about snow; this time, it’s the snow in Antarctica. Last year, a team of scientists examined several hundred pounds of it and discovered a surprise: it’s sprinkled with stardust.
That stardust is composed of significant amounts of a form of iron that isn’t naturally produced on Earth. Called iron-60 (60Fe), it has four more neutrons than Earth’s most common form of the element. While iron-60 had previously been found in deep-ocean crusts, there it had likely settled on the Earth’s surface millions of years ago. What was found in the Antarctic snow accumulated over the past two decades. According to scientists, who published their findings in the journal Physical Review Letters on August 12, 2019, it’s the very first detection of a recent influx of iron-60 onto Earth.
It fell to Earth
Outer-space objects ranging from dust to meteors regularly fall to Earth, but they are generally made of the same materials as our planet since everything in the solar system, including the sun itself, assembled from the same building blocks billions of years ago. Because iron-60 is not among those common materials, it must have arrived from somewhere beyond our solar system.
Stars fling out a variety of tiny particles during their lifetimes, in addition to all the heat and light. When stars are younger, they generally emit lighter metals (astronomers tend to refer to everything bigger than helium as a metal), such as carbon and oxygen. But aging, massive stars and a certain type of supernova explosion—which makes for very beautiful Hubble Space Telescope photos—result in either a black hole or an incredibly small, dense star that no longer generates heat.
A supernova, having spent many millennia fusing big nuclei into even bigger ones, can spew out particles of heavier metals, including iron-60 and its stable cousin, iron-56, causing them to travel through the universe, where they occasionally come into contact with other stars and planets.
Only stars tens of times more massive than our sun could build iron isotopes, however, which means that the iron-60 found in Antarctica originated from outside the solar system. Scientists believe it must have been a supernova in the neighborhood of our sun; not so near as to kill us but not too far as to be diluted in space.
It came down to counting
As you can imagine, picking out any kind of dust particles from snow is a daunting task. Determining which ones are stardust from the myriad other particles swirling around in the snow is even more challenging.
For the 2019 study, the researchers collected 1,100 pounds of fresh, Antarctica surface snow (falling no more than 20 years ago), shipped it in a frozen state to the Technical University of Munich in Germany, melted it, filtered it and chemically analyzed it. The measurement of iron-60 was then done at a lab in Garching, Germany, with accelerator mass spectrometry, which is single-atom counting.
The snow contained substantial quantities of dust enriched with the rare iron-60—which isn’t naturally formed on our planet—that had accumulated over the past two decades or so.
This radioactive isotope has a half-life of around 2.6 million years, which means that this is the time it takes for one-half of the atomic nuclei in a sample of the substance to decay. In other words, it is very long-lasting compared to many other radioactive molecules with short half-lives, enabling scientists to detect it.
At the South Pole, though, researchers need to account for possible Earthly sources of the isotopes before they can confirm they are interstellar. The science team consulted data on nuclear weapons from the 1950s and ’60s, the movement of fallout from nuclear power plants and reactors, and tests and accidents such as the 2011 disaster in Fukushima. They calculated that none of the terrestrial sources, only a minuscule amount, could have generated the quantity of iron-60 buried in the Antarctic snow. By studying additional isotopes, such as manganese-53, they also ruled out any significant contributions from cosmic rays, which generate iron-60 when they interact with dust and meteorites. In the end, what was left was hundreds of times more of the interstellar iron than the researchers expected.
It could be cloud connected
The research team was able to make a relatively precise determination as to when the iron-60 was deposited on Earth because the snow layer that was analyzed was not older than 20 years, and the iron-60 would have dissipated too much throughout the universe if it had come from a particularly distant stellar explosion—based on the half-life of iron-60, any atoms originating from the formation of Earth would have completely decayed by now. That implies that our planet was showered with the iron-60 particles at some point in our solar system’s travels through a region of our galaxy known as the Local Interstellar Cloud. This region—which spans about 30 light-years across—is thought to have formed after massive stars exploded as supernovae, ejecting vast quantities of hot gases in their outer layers into space.
Our solar system entered the Local Interstellar Cloud approximately 40,000 years ago and will exit it in a few thousand years. If the gas cloud hypothesis is correct, then material from ice cores older than 40,000 years would not contain interstellar iron-60. Examining those ice cores could help verify the transition of the solar system into the gas cloud, which would be a groundbreaking discovery for scientists working on creating a clearer time line of our solar system.
It is all from the stars
Astrochemist and astronomer Dr. Carl Sagan once said, “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.”
I like knowing that some of our snow is, too.
Here’s to finding your true places and natural habitats,