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P. R. Mahaffy, H. B. Niemann (NASA/GSFC), A. Alpert (NASA Academy, GSFC), S. K. Atreya, T. M. Donahue (Dept. Atmos. Oceanic, and Space Sci., U. Michigan), T.C. Owen (IfA, U. Hawaii)
Measurement of noble gas abundances and isotopic distributions in the atmospheres of the giant planets is necessary to constrain models of planetary formation. The mixing ratios and isotopic abundances of He, Ne and Ar at Jupiter were obtained by direct sampling of the atmosphere by the Galileo Probe Mass Spectrometer (GPMS). The measurement at Jupiter of the trace heavy noble gases Kr and Xe, on the other hand, was enabled through use of chemical enrichment cells. In these experiments, samples of the Jovian atmosphere were passed over high surface area adsorbants that trapped a portion of the gas. Subsequent thermal release of the gas substantially increased the ratio of heavy to light species that could be introduced to the ion source of the mass spectrometer. This procedure allows even parts per billion mixing ratios of Kr and Xe to be sampled with good counting statistics. Recent post-encounter studies on the GPMS Engineering Unit refurbished to employ nearly identical enrichment cells to the flight unit have allowed the preliminary estimates of the mixing ratios and isotopic distributions of Kr and Xe to be refined. We report on progress in these studies.