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Frozen cosmic fingerprints

Researchers claim to find evidence of 11th century supernovas and the solar cycle in an ice core

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12:01pm, March 9, 2009
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Analyzing the composition of an Antarctic ice core, Japanese researchers say they have found the chemical fingerprints of two well-known supernovas from the 11th century, as well as evidence of an 11-year solar cycle from the same century.  

“This is one of the first times that a distinct 11-year solar cycle has been observed for a period before the landmark studies of sunspots by Galileo Galilei with his telescope,” Yuko Motizuki of the RIKEN Nishina Center for Accelerator-Based Science in Wako, Japan, and his colleagues assert in an article posted online February 20 (http://arxiv.org/abs/0902.3446). The findings, the team adds, “have significant consequences” for dating ice cores, discovering previously unknown supernovas in the Milky Way and revealing the history of the solar cycle.

Motizuki told Science News that he and his colleagues would not speak with a reporter because they had submitted the article to Nature.

The team bases its findings on precision measurements of an ion known as nitrate (NO3-) in a 122-meter–long ice core drilled at Dome Fuji station in Antarctica in 2001. Two of the nitrate spikes recorded in the ice core coincide with known supernovas, SN 1006, named for the year in which this stellar explosion would have been seen from Earth, and the famous Crab Nebula supernova, which dates from A.D. 1054.

When a star relatively close to Earth explodes as a supernova, gamma rays from the outburst increase the concentration of nitrogen oxide above Earth’s stratosphere. This increase in atmospheric nitrogen oxide can boost the concentration of the nitrate ion in the ground layer. Similarly, fluctuations in solar energy during the sun’s 11-year activity cycle can also vary the amount of nitrate in soil and ice, previous studies have suggested.

Because energetic protons from solar flares can also raise the concentration of nitrates, researchers looking for supernova signals in ice cores must be careful to choose the portion of a core corresponding to an era when flare activity would be expected to be low. The interval between A.D. 1040 and A.D. 1060 is known to be quiescent, Motizuki and his colleagues note.

“The basic idea is an interesting one, but it’s way premature to accept these findings” at face value, comments Eric Wolff of the British Antarctic Survey in Cambridge, England. “If the authors could show convincingly that they had supernovas, this would be exciting, especially if the signals were unique enough that they could be used to diagnose the existence of supernovae in times before historical records. But I think we are a long way from that.”

The main problem, says Wolff, is that the amount of nitrate deposited in an ice core doesn’t directly reveal what’s happening at the surface or in the atmosphere. Sunlight striking the nitrate ion can either break it down into other compounds or convert it to nitric acid, which readily evaporates. Dust and sea salt can increase the ion’s deposition in ice cores. Last September in Atmospheric Chemistry and Physics, Wolff and his colleagues reported that at one coastal site in Antarctica, deposits of sea salt created nitrate spikes in ice cores that had nothing to do with solar or supernova activity.

In the new article, Motizuki’s team notes that the ice core lies inland, away from sea salt, and is therefore less likely to be contaminated by nitrates from nonastrophysical sources.

Wolff adds that a much longer section of the Dome Fuji core should be analyzed to determine whether the detected nitrate spikes truly stand out or are part of a recurrent pattern in the ice samples and therefore unlikely to come from a particular supernova or other celestial source. Other ice cores of similar vintage should also be examined to see if they show the same nitrate pattern, Wolff says.

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