THE signature of ancient supernova explosions may be written into the ice of Antarctica.
The gamma radiation from supernovae is thought to produce nitrogen oxides in the atmosphere, which are then converted to nitrates. As the nitrates could be trapped in snowfall, it is possible that glaciers hold evidence of past supernovae.
Yuko Motizuki of the in Wako, Japan, and his colleagues took an ice core drilled at Dome Fuji station in Antarctica. They found high nitrate concentrations in three thin layers about 50 metres below the surface ().
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After calibrating this frozen calendar using chemical markers laid down by known volcanic eruptions, the team found that one nitrate spike was deposited close to the year 1054, when Chinese observers saw a bright supernova. That explosion left behind the Crab nebula and pulsar in the constellation of Orion. Another spike is close to 1006, the year of an even brighter supernova. The third spike, from around 1060, does not coincide with any observed supernova, but Motizuki suggests its light might have been blocked by a dark interstellar cloud.
If these results are confirmed by more ice cores, the method could recreate prehistoric skies. 鈥淧otentially you could get the supernova rate going back several thousand years,鈥� says of the University of Virginia, Charlottesville, who claimed a similar 鈥� but controversial 鈥� find in the 1970s. That鈥檚 important, because supernovae affect the workings of the whole galaxy, churning up interstellar gas and triggering star formation.