 Between locations in the Solar System, the isotopic composition of argon varies greatly. Where the major source of argon is the decay of 40K in rocks, 40Ar will be the dominant isotope, as it is on Earth. Argon produced directly by stellar nucleosynthesis, is dominated by the alpha process nuclide, 36Ar. Correspondingly, solar argon contains 84.6 % 36Ar (according to solar wind measurements), and the ratio of the three isotopes 36Ar: 38Ar: 40Ar in the atmospheres of the outer planets is 8400: 1600: 1. This contrasts with the abundance of primordial 36Ar in Earth's atmosphere, which is only 31.5 ppmv (= 9340 ppmv × 0.337 %), comparable with that of neon (18.18 ppmv) on Earth and with interplanetary gasses, measured by probes.
