London, March 4 (ANI): In an analysis of the nitrate content of an ice core drilled at Dome Fuji station in Antarctica, scientists have found what may be chemical traces of supernovae that exploded a thousand years ago.
According to a report in New Scientist, the analysis was done by Yuko Motizuki of the RIKEN research institute in Wako, Japan, and colleagues.
Nitrate is produced in the atmosphere by nitrogen oxides, which in turn should be created by the gamma radiation from a supernova.
Motizuki's group found high nitrate concentrations in three thin layers about 50 metres deep. Because snow gradually builds up into layers of ice, depth indicates age.
After calibrating this icy calendar using chemical markers laid down by known volcanic eruptions, the team found that one nitrate spike is close to the year 1054, when Chinese observers saw a bright supernova.
That explosion left behind the Crab Nebula and pulsar. Another spike is close to 1006, the year of an even brighter supernova.
The third nitrate spike is around the year 1060, when no supernova was reported.
The researchers suggest that it might be the result of a supernova in the less-well observed southern hemisphere or one that was hidden behind a dark interstellar cloud and therefore went unnoticed.
According to Gisela Dreschoff of the University of Kansas in Lawrence, she already had strong evidence that supernovae make their mark in Earth's icecaps.
Dreschoff and colleague Claude Laird, analysed three cores in 2004 - two pulled out of Antarctica and one from Greenland.
All three cores showed spikes marking both the 1054 and 1006 explosions.
Since those cores were taken from both the north and south polar regions, she said her team's detection of supernovae is firm; but she welcomes the Japanese core as further evidence.
Radioactive iron-60 in ocean sediment has previously been used to reveal a prehistoric supernova that occurred millions of years ago.
But, while sediment studies stretch back far in time, they may only reveal those supernovae that exploded relatively near the Sun, which would have left behind bigger isotope signals.
Because of the limited depth of the Antarctic ice, the new method would stretch back less than a million years.
But, if it does indeed record these subtle nitrate spikes, it could potentially reveal more distant supernovae.
If the new results are confirmed by more ice cores, "potentially you could get the supernova rate going back several thousand years", said Robert Rood of the University of Virginia in Charlottesville. (ANI)