Similarly, the ratio of the isotopes 36Ar:38Ar:40Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.
[5] In the Earth's atmosphere, radioactive 39Ar (half-life 268(8) years) is made by cosmic ray activity, primarily from 40Ar.
In the subsurface environment, it is also produced through neutron capture by 39>K or alpha emission by calcium.
36Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013.
[9][10] 37Ar is a synthetic radionuclide that is created via neutron capture of 40Ca followed by alpha particle emission, as a result of subsurface nuclear explosions.