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Radioactive iron buried in Antarctic snow must have come to Earth from interstellar dust, according to a new study. This suggests that Antarctic snow and ice of different ages could be used to tell us about the patches of interstellar dust Earth has passed through, and so track its path across the Milky Way.

Dominik Koll at the Australian National University in Canberra and his colleagues collected 500 kilograms of Antarctic ice, melted it and studied what it contained. They found iron-60, a rare radioactive form of iron.

This sort of atom is only formed in special conditions. Koll and his team ruled out terrestrial sources such as nuclear power plants or weapons. This meant the iron must have been created during the explosion of a star. The supernova would have littered space with interstellar dust containing iron-60. This then eventually fell on Earth as the solar system passed through the cloud of dust.

鈥淚 was very excited when I saw the first counts of iron-60 appear in the data,鈥� says Koll. The presence of this isotope at detectable levels mean the interstellar dust was rich in remnants from supernovae.

The snow Koll studied was less than 20 years old, meaning the dust landed on Earth recently. But studying snow formed at different times could tell us more about the dust the solar system has travelled through in the past.

Local Fluff

At the moment, the solar system is moving through a region of the Milky Way called the local interstellar cloud, and has been for around 45,000 years. We know this cloud, also known as the Local Fluff, is 30 light years across but we don鈥檛 know much about its shape or density in different regions.

Looking at older ice could help us learn about the dust the Earth passed through long ago and so find out more about the Local Fluff. Some parts of it might be more dense if they had material injected by supernovae, for example. 鈥淭his finding is an important input for theoretical astrophysical models about how the solar neighbourhood and the larger structures around us were shaped by stellar explosions,鈥� says Koll.

鈥淚 really like this idea of using layers of Antarctic ice to get a tree-ring or ice-core like history of deposit of interstellar grains,鈥� says Angela Speck at the University of Missouri.

But the findings also allude to a controversy to do with interstellar dust, says Speck. After the big bang, there was only gas and no dust, and it is unclear whether all dust was formed after having been part of a star, or some could have condensed directly from the gas. Since Kolls鈥� dust contains material that could only have formed in a star, that adds weight to the former argument. 鈥淚 think this paper could feed important information into that discussion,鈥� she says.