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.