Xenon is unique among the Noble Gases in that it has an isotope, 129Xe, that is the fossil daughter of an extinct nuclide. Iodine-129, its precursor, decays to 129Xe with a half-life of about sixteen million years, and since the earth is four and a half billion years old (and since all the elements on earth were created in stars before the earth accreted), there is no 129I on earth today; after the first hundred million years of earth’s existence there would have been less than 2 percent left, after a billion years there would have been too little to measure, and by today we can safely say there is “none” left. But now let’s go back to the very creation of the solar system. We know that the elements that exist today were created earlier in stars and blown out into space, and somehow they accreted into the sun and planets. We know roughly how and in which types of stars the elements were created, but we still don’t know the details of their synthesis, and we know even less about their accretion into the sun and planets, and until the xenon studies we had absolutely no idea when they were created. Suppose that the creation of the elements took place billions of years before solar system formation (after all, the universe is nearly ten billion years older than we are). Then all the 129I would have decayed into xenon long before the sun and planets formed, the 129Xe would have mixed with all the other xenon isotopes, and upon its incorporation into the solid particles of the solar system the xenon would be isotopically homogeneous. The sun, the earth, the meteorites, and the planets and moons would have incorporated differing amounts of xenon, according to their mode of formation and evolution, but they would all have the same mix of xenon isotopes (with perhaps some easily recognized mass fractionation). But suppose instead that the elements were created just previous to solar system formation; that is, within a few half-lives of 129I—say, less than a hundred million years.
Comets as Tracers of Solar System Formation and Evolution
