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Iron from ancient supernovae may still be raining down on Earth

A rare iron isotope produced by exploding stars has been found in Antarctic snow.

ByKatherine J. WuNOVA NextNOVA Next
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The Veil Nebula, debris from a massive star that exploded about 8,000 years ago. Supernovae like this tend to spray matter many light-years away, even sending radioactive atoms raining down on Earth. Image Credit: NASA/ESA/Hubble Heritage Team

Some 2 million years ago, the life of a nearby star ended—not with a whimper, but a bang.

Its death—a stellar explosion called a supernova—was so cataclysmic that it sent the star’s innards hurtling hundreds of light-years from their source, penetrating the magnetic field of our solar system and pelting its resident planets and moons with radioactive debris. Stellar shrapnel even found its way to Earth and its moon, settling deep beneath our planet’s oceans and locking itself in layers of lunar soil.

Many millennia have passed since that star met its maker. But in a way, our solar system might still be reeling from the impact of its neighbor’s demise.

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Reporting today in the journal Physical Review Letters, an international team of researchers has uncovered interstellar iron in a cache of recent Antarctic snow, suggesting that the dusty remnants from this ancient supernova—or others like it—are still infiltrating Earth today. Their tantalizing discovery hints that long after supernovae finish delivering their dust directly, they still leave traces of themselves behind, making it easy for sojourning solar systems like our own to gather the cosmic dust like Roombas vacuuming up old crumbs.

The findings underscore the lasting repercussions of stellar death—and may help researchers better understand the part of the galaxy our solar system is currently traversing.

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The Crab Nebula, a supernova remnant from a massive star that exploded about 1,000 years ago. Supernovae like this tend to spray matter many light-years away, even sending radioactive atoms raining down on Earth. Image Credit: NASA, ESA, J. Hester (Arizona State University)

Scouring the Earth for interstellar dust isn’t easy, in part because it has to be distinguished from, well, regular dust. Luckily, foreign matter has its tells: Its chemical makeup, for instance, usually includes elemental isotopes that aren’t naturally found on Earth. That’s the case for iron-60, a rare, unstable form of iron that’s commonly spewed from supernovae.

So far, only a few samples of Earthbound iron-60 have been discovered, and they’re all at least a couple million years old. The iron atoms in these deposits were probably all blasted to our planet straight from their stellar sources, says study author Dominik Koll, a physicist at Australian National University. Supernovae are violent affairs, capable of reshaping global climates and driving mass extinctions on nearby planets. Physicists are pretty sure nothing of this nature has happened recently in our neck of the cosmic woods, for one simple reason, Koll says: We would probably have noticed.

But that doesn’t mean Earth isn’t still encountering elemental refugees from supernovae past.

Astronomical observations suggest that our galaxy is currently traveling through a patch of the galaxy called the Local Interstellar Cloud (or the Local Fluff) that’s relatively thick with matter. It’s not entirely clear what comprises the Fluff, but there’s a good chance some of it is detritus from local stellar explosions, says study author Gunther Korschinek, an astroparticle physicist at Technical University Munich in Germany. If that’s the case, it could create the perfect opportunity for dust to enter the solar system as it hurtles through the Cloud.

To explore this possibility, the researchers focused their efforts on one of the most remote locales on Earth: the frigid landscape of Antarctica, where interstellar dust would have a good shot at being preserved in an uncontaminated state. Korschinek called in a favor with a colleague at Kohnen Station, who packed 1,100 pounds of still-frozen snow, all of which had fallen within the last two decades, into about 25 insulated Styrofoam packages and shipped them to Munich.

The researchers melted and filtered the snow to isolate the dust, then ran it through an extremely sensitive mass spectrometer—one of only two in the world—that was able to extract individual iron-60 atoms from a slurry of other bits of matter, including stable iron that naturally occurs on Earth.

Altogether, the team amassed just 73,000 iron-60 atoms—an extremely small fraction of the half ton of material they’d started with. But it was enough to help the researchers rule out the possibility that the dust had emerged from a more local source. Iron-60 can also make its way to Earth when cosmic rays—a form of high-energy radiation—cleave dust off of comets or asteroids within the solar system itself. This process, however, tends to create other rare isotopes as well, such as manganese-53, which the researchers deduced wasn’t present in the snow in high enough amounts to explain all the iron-60. The amount of iron-60 present also exceeded what could have been produced by nuclear weapons testing, which would have generated a lot of another isotope called iron-55.

In other words, most of the iron-60 in the snow seemed to have flown solo—which made it almost certain that the dust housing it hailed from outside the solar system.

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Our Solar System is currently moving through the Local Interstellar Cloud, a relatively dense patch of our local galactic neighborhood. Image Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

“This is challenging work, and I’m very impressed,” says Haolan Tang, an astrophysicist and iron-60 expert at the University of California, Los Angeles who was not involved in the study. “Iron-60 is normally undetectable...and this is the first detection that is recent.”

Though it’s difficult to pinpoint exactly where the iron-60 originated, Tang also thinks it has something to do with the Local Interstellar Cloud, which may have been shaped in part by nearby stars that went supernova eons ago. As our solar system meanders through this cosmic floof, it can’t help but gather the dust that’s still settling in the wake of ancient stellar eruptions.

If that’s the case, Tang says, continuing the search for iron-60 may give researchers a better sense of what’s going on in our solar system’s cosmic neighborhood du jour. “Now, we have good evidence from iron-60 that stellar activity is high in the local interstellar medium,” she says.

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Kohnen Station in Antarctica, where researchers collected half a ton of snow to search for atoms of iron-60. Image Credit: Sepp Kipfstuhl

This story is far from over, though, Korschinek says. A lot of work still needs to be done to confirm the team’s current theory. One key next step, he says, will be to dig deeper—literally.

The Local Interstellar Cloud is big, but not infinite. If the researchers’ hypothesis is correct, then there should be a clear drop-off in iron-60 in very old layers of Antarctic snow—around the time when our solar system is thought to have first entered the Fluff, between 40,000 and 150,000 years ago.

If that pans out, it would confirm iron-60’s identity as something of a cosmic clock, Tang says, and further bolster the case for future study of this rare isotope.

Our solar system will also eventually exit the Local Interstellar Cloud, an event that will likely cause Earth’s iron-60 levels to plummet, Korschinek says. Tracing that downtick would be great evidence, too. But since that departure isn’t expected to happen for another few thousand years, he says, our time might be better spent hunting for the iron-60 we’ve already got—wherever it might be hiding.

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