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 RIKEN research institute 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 (www.arxiv.org/abs/0902.3446).

After calibrating this frozen calendar using chemical …