Isotopes of argon
has 26 known isotopes, from Ar to Ar, of which three are stable. On Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 302 years, Ar with a half-life of 32.9 years, and Ar with a half-life of 35.01 days. All other isotopes have half-lives of less than two hours, and most less than one minute. Isotopes lighter than 38Ar decay to chlorine or lighter elements, while heavier ones beta decay to potassium.
The naturally occurring K, with a half-life of 1.248 years, decays to stable Ar by electron capture and by positron emission, and also to stable Ca via beta decay. These properties and ratios are used to determine the age of rocks through potassium–argon dating.
Despite the trapping of Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of K decay, since 99.6% of Earth's atmospheric argon is Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of ~85% Ar, ~15% Ar and only trace Ar. Similarly, the ratio of the isotopes Ar:Ar:Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.
In the Earth's atmosphere, radioactive Ar is made by cosmic ray activity, primarily from Ar. In the subsurface environment, Ar is also produced through neutron capture by K or 42Ca, with proton or alpha emission respectively; 37Ar was created in subsurface nuclear explosions similarly from 40Ca. The content of Ar in natural argon is measured to be of ×10 g/g, or Bq/kg weight.
The content of 42Ar in the Earth's atmosphere, though it had previously been reported as a cosmogenic isotope, is lower than 6×10−21 of the element. Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon and this may be obtained from underground sources that have been isolated from the atmosphere long enough for these isotopes to decay.
36Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013. This was the first time a noble molecule was detected in outer space.