 In a similar way , xenon isotopic ratios such as 129Xe / <unk> and <unk> / <unk> are also a powerful tool for understanding planetary differentiation and early outgassing . For example , The atmosphere of Mars shows a xenon abundance similar to that of Earth : 0 @.@ 08 parts per million , however Mars shows a higher proportion of 129Xe than the Earth or the Sun . As this isotope is generated by radioactive decay , the result may indicate that Mars lost most of its primordial atmosphere , possibly within the first 100 million years after the planet was formed . In another example , excess 129Xe found in carbon dioxide well gases from New Mexico was believed to be from the decay of mantle @-@ derived gases soon after Earth 's formation . 
