The snow arrived at the laboratory in Munich inside Styrofoam boxes. It came from a German research station in Antarctica, where summer made the snow, whisked around in the wind like sand on a beach, easy to lift. A pair of scientists had shoveled in thousands of pounds of the stuff. The boxes, kept under freezing conditions, traveled by plane to the ice shelf and then by ship to South Africa and Germany. In the lab, researchers melted the snow, sifted out any buried solids, and chucked them into an incinerator.
When the scientists analyzed the ash, they found something unusual: a radioactive form of iron. The isotope, known as iron-60, is rare on Earth. But it is produced in abundance in space, when a star, having exhausted the fuel that makes it shine, explodes.
The burst releases newly forged chemical elements into the universe like tufted dandelion seeds. The radioactive iron, carried inside microscopic particles of dust, glides across space and can settle on whatever it encounters. Sometimes, it ends up here.
“It’s actual stardust,” says Dominik Koll, a physicist at the Australian National University who analyzed the snow. “And we find it on our planet.”
Scientists have found it more than once. A part of the team behind the recent discovery first detected iron-60 in 1999, on the floor of the Pacific Ocean. The researchers studied the rocky sediment like tree rings, looking for clues about the environment that shaped it. The radioactive iron showed up in sediment that formed about 2.8 million years ago, suggesting the dying star that produced the isotope exploded around the same time. The supernova went off at a distance far enough that it didn’t decimate Earth, but close enough to shower the planet with cosmic debris.
Scientists knew there had to be more interstellar dust hidden elsewhere on Earth. “If the supernova’s raining debris on the Earth, you shouldn’t just find it in one place. It should be all over the Earth,” says Brian Fields, an astrophysicist at the University of Illinois, and one of the first people who proposed searching for stellar leftovers in seafloor sediment. “In fact, it should cover the solar system.”
Twenty years passed before scientists found evidence of interstellar dust again and proved Fields’s prediction right. Several studies in 2016 reported that radioactive iron showed up in seafloor sediment excavated from the Pacific, Atlantic, and Indian Oceans, and in lunar soil collected during the Apollo missions. The iron appeared in sediment dating back more than 8 million years. The variety of the findings, in location and timestamp, suggested that the iron-60 came from not one supernova, but a flurry of them.
Antarctica seemed like the right pick for the next dig site in what Fields calls “supernova archaeology.” Astronomers have observed the aftermath of many supernovas with telescopes capable of peering deep into the universe, but this new field has offered much more local materials to study. Koll and his colleagues believed that the icy environment, remote and relatively pristine, would allow any concentrations of iron-60 to remain undisturbed.
But the Antarctic ice is far more exposed than the bottom of the ocean, so scientists had to rule out earthly sources of radioactive isotopes. Koll and his colleagues consulted data on nuclear weapons from the 1950s and ’60s and the movement of fallout from nuclear power plants. Neither source, they determined, generated the iron-60 buried in the Antarctic snow. “They clearly demonstrate that terrestrial sources can be excluded,” says Anton Wallner, a physicist at the Australian National University who was not involved in this study, but led several of the discoveries in 2016.
This new relic of supernova activity is different from previous findings in one tantalizing respect. In the other research, the iron-60 vanished from the sea sediment after about 2 million years, which suggested that interstellar dust stopped raining down on Earth long ago. The iron in Antarctica was found in snow that had accumulated in the past 20 years.“There wasn’t just a single downpour,” Fields says. “It continues as a drizzle today.”
How exactly the cosmic droplets got here is a bit murky. The iron-60 may have traveled toward Earth in a straight shot, cruising on the winds from a stellar blast. Or it could have joined with the clouds of dust that hover all around the universe, including right near our solar system. The radioactive particles may have ricocheted around these clouds and bounced toward Earth, or become swept up in the planet’s wake as it moved through the clouds.
The dust grains would become attracted to the contours of the Earth’s magnetic field, says Priscilla Frisch, an astrophysicist at the University of Chicago, who was not involved in the latest research. “The terrestrial magnetic field lines approach closest to the surface of the Earth near the North and South polar regions,” Frisch says. It would be an easy drop from there to the frosty landscape of Antarctica, waiting for a couple of researchers and their shovels to come by.
There’s something almost ridiculous about the thought of fitting supernova dust in a Styrofoam box. The Styrofoam, like nearly everything else, owes much of its existence to dying stars. Over billions of years, supernova explosions, fiery and radiant, manufactured the elements that would help construct other stars, planets, and eventually, life. “We are made of stardust,” Carl Sagan once said. The maxim seemed to encapsulate something truly ancient, a cosmic crest of formation that was left far behind us. But the same kind of stardust still surrounds us. We’re still in the midst of the storm.
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