Today’s interesting fact has to do with the element carbon.

There are three common isotopes of carbon that are found on earth, carbon-12, carbon-13 and carbon-14. Around 99% is carbon-12, about 1% is carbon-13 and carbon-14 is found in trace amounts.

Carbon-14 is radioactive and decays into nitrogen-14. You might think that this is why carbon-14 is so rare and that it might eventually disappear, but carbon-14 is produced when cosmic rays enter the atmospheres, produce neutrons, through a reaction convert the nitrogen-14 into a carbon-14 atom and a hydrogen atom.

Carbon-14 is constantly replenished, so that in the there is always about the same amount in the atmosphere at any time. Carbon-14 gets picked up by plants and through herbivores enters the animal food chain. Once an animal or plant dies, the ratio of carbon-14 to the other carbon isotopes doesn’t remain constant, as the carbon decays.

Because we know the half life of carbon-14, and the amount of carbon-14 is thought to remain constant (which remains true as long as the amount of cosmic rays remains constant), by determining how much carbon-14 remains in a sample, can determine how old the sample is, which is how radio-carbon dating works.