Xenon occurs in Nature as a constituent element of the atmosphere, in an amount of 8,6 x 10-6 % per volume of dry air. This gas is heavier than krypton and less abundant than its precessores in the group of the noble gases. Its abundance is considered to be about 2,9 x 10-9 % in the terrestrial crust (including oceans and atmosphere).
Most of existing atmospheric xenon has primordial origin. However, it is considered that the amount induced by the uranium and thorium fission is about 0,5%.