The Earth's atmosphere contains excesses of 129Xe and 136Xe from decay of the short-lived radionuclides 129I and 244Pu, respectively. The inferred abundances of these radionuclides are substantially less than those in meteorites, nominally indicating that Earth as a planet did not form until more than 100 Myr later than the solar system as a whole, an interval rather longer than that suggested by other lines of evidence. From the comparison of elemental abundance ratios between atmospheric (assumed to approximate the total terrestrial noble gas inventory) and solar noble gases, we conclude that ~90% of Earth's primordial Xe is missing from the atmosphere. When account is taken of the radiogenic Xe that is likely missing as well, the 129I-244Pu formation interval of Earth may be reduced significantly, down to about 50--70 Myr. ¿ 1999 American Geophysical Union