In November 2012, a group of Japanese scientists made a remarkable announcement that much of the world’s media missed.
Fusa Miyake and pals at Nagoya University in Japan were studying tree ring records which can be resolved almost to the exact year. These guys found that the concentration of carbon-14 in Japanese cedar trees suddenly rose between 774 AD and 775 AD. Others have since found similar evidence and narrowed the date to 773 AD.
So what caused this sudden increase in carbon-14? Astronomers agree that the culprit must have come from space, beyond that they are at loggerheads. Now the battle is hotting up to correctly identify the extraterrestrial source of this medieval carbon spike and resulting scientific spat is becoming increasingly entertaining.
Carbon-14 is continually generated in the atmosphere by cosmic rays hitting nitrogen atoms, causing them to absorb a neutron. But because carbon-14 is radioactive, it naturally decays back into nitrogen with a half-life of about 5700 years.
This constant process of production and decay leaves the amount of carbon-14 in the atmosphere relatively constant. Pick up some carbon today and about one part in a trillion will be carbon-14.
Last month, a group of Chinese scientists pointed out that this process of carbon-14 production doesn’t just occur on Earth but all over the universe, wherever cosmic rays interact with nitrogen. For example, comets often contain ammonia (NH3) ice and so will contain a fraction of carbon-14 produced as they are bombarded by cosmic rays.
That raises an interesting possibility, they said. Perhaps Earth was struck by a comet in AD 773 and the impact showered the planet with carbon-14. Indeed
Today, Ilya Usoskin at the University of Oulu in Finland and Gennady Kovaltsov at the Ioﬀe Physical-Technical Institute in St Petersburg, Russia, firmly rule out this possibility.
They point out that the total amount of extra carbon-14 dumped into the atmosphere must have been equivalent to about 18 kilograms. That allows them to calculate the minimum size of any comet that could have been responsible.
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