Abstract. The Galileo Probe Mass Spectrometer provided the first data on the noble gas mixing and isotope ratios in the Jovian atmosphere. These measurements and the comparison with solar values constrain models of Jupiter's formation• Significant refinements to the initially reported abundances of argon, krypton, and xenon have been enabled through post-encounter laboratory calibrations using a refurbished engineering unit mass spectrometer nearly identical to the flight unit. The abundances relative to hydrogen for argon, krypton, and xenon are respectively 2.5 + 0.5, 2.7 + 0.5, and 2.6 + 0.5 times the solar ratios. The mixing ratios of He and Ne found in these studies are consistent with previously reported values of 0.8 and 0.1 times solar respectively. The Jovian 36Ar/3aAr ratio is 5.6 + 0.25 and the 2øNe/22Ne ratio is 13 _+ 2, consistent with the solar values of 5.77 and 13.81, respectively, that are derived from lunar mineral grain analysis. The distribution of xenon isotopes at Jupiter also resembles the solar distribution. IntroductionThe abundance distribution of the various elements in the atmosphere of Jupiter provides insight into the mechanism of the formation of this planet. Fractionation of the different noble gases during the evolution of the atmosphere of a planet can be used to test theories of various mechanisms of atmospheric loss from the terrestrial planets [Pepin, 1991 ]. The ratios found in the Sun and in the giant planets are expected to represent the values for these ratios in the protosolar nebula. Differences in the noble gas ratios between the Sun and Jupiter, however, would suggest a fractionation process in the formation of the grains and planetesimals that formed Jupiter.This work describes the derivation of the noble gas mixing ratios from the GPMS data. Refinements are also given to noble gas isotope ratios previously reported [Niemann et al., , 1996. These results were enabled by laboratory calibrations of the engineering unit (EU) GPMS. This unit was refurbished to make its performance as identical as possible to the flight unit (FU). The heavy noble gases, krypton and xenon, were detected in enrichment cell experiments involving trapping on a high surface area adsorbant. The EU experiments duplicated the variation of pressure with time encountered by the probe during its descent and the temporal variation of the temperature of the enrichment cells during the adsorption of these trace species. The Extraction of the Jovian noble gas isotope ratios from the GPMS data requires a detailed consideration of instrumental effects. For example, the use of a highly miniaturized ion pump system operating throughout the descent from 0.4 to 22 bar ambient Jupiter pressure gave rise to a bias in the noble gas isotope ratios obtained for a portion of these measurements. The EU descent simulations allow these effects to be further quantified and the instrument pressure regimes established from which these ratios are most reliably obtained. 15,06115,062 MAHAFFY ET AL; NOBLE GAS ABUNDANCE AND ...

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