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Record of exploding stars 1,000 years ago may be written in polar ice

Ice cores drilled from the poles have provided valuable historical climate records, as the composition of the ice and the air bubbles trapped therein offers a relatively pristine glimpse of ancient conditions. Now a group of Japanese scientists says that the same technique may yield records of significant astronomical events as well.

In a paper posted recently to arxiv.org, Yuko Motizuki of the RIKEN Nishina Center for Accelerator-Based Science in Wako, Japan, and colleagues present evidence for an Antarctic ice-core record of supernovae, or stellar explosions, a millennium ago. A 400-foot (122-meter) core pulled up in 2001 at Dome Fuji station in East Antarctica shows spikes in the concentration of nitrate ion (NO3) that coincide with two known supernovae in the 11th century: supernova 1006, named for the year it was observed, and the Crab Nebula supernova of 1054. (Astronomers and astrologers in the Far East and the Middle East were already making detailed records of such events by that time.) Nearby supernovae, the researchers write, shower Earth with gamma rays, which can boost levels of nitrogen oxides in the atmosphere that might be recorded as nitrate spikes in the ice.

A third nitrate peak unearthed by the Japanese team, dated to 1060–1080, could be the mark of an unrecorded celestial event, the authors venture. An exotic stellar remnant called a soft gamma repeater might account for the mystery nitrate boost, as might a supernova obscured by clouds or simply not visible in the northern hemisphere, where the astronomers of the day were clustered.

The concept of drilling for astronomical records, the authors note, was proposed by astronomer Robert Rood of the University of Virginia and his colleagues in a 1979 paper in Nature, but the link between supernovae and nitrate spikes has remained speculative. Even the new research would be bolstered by additional correlations—to that end the authors note that a comprehensive review of ion levels and known supernovae from the past 2,000 years is now under way.

Photo of ice cores in storage courtesy of the National Ice Core Laboratory via Wikimedia Commons

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