[Previous] | [Session 12] | [Next]
P. R. Mahaffy, H. B. Niemann (NASA Goddard Space Flight Center), S. K. Atreya, M. H. Wong (Univ. of Michigan), T. C. Owen (Univ. of Hawaii)
Galileo Probe Mass Spectrometer data have been used to derive the 15 N/14 N isotope ratio in ammonia at Jupiter. Although the mass spectral interference from the water contribution to 18 amu makes an accurate derivation of the 15 NH3 /14 NH3 ratio difficult from measurements of the singly ionized signals at 18 and 17 amu, this interference is not present in the doubly charged 8.5 and 9.0 amu signals from 14 NH3 ++ and 15 NH3 ++ respectively. Although the count rate from the 9 amu signal is low during the direct sampling of the atmosphere, the ammonia signal was considerably enhanced during the first enrichment cell (EC1) experiment that measured gas sampled between 0.8 and 2.8 bar. Count rates at 9 amu in the EC1 experiment reach 60/second. In the EC1 measurements the 8.5 amu signal is not measured directly, but can be calculated from the ammonia contribution to 17 amu and the ratio of NH3 ions of double to single charge observed during a 1/8 amu resolution mass scan taken near the end of the descent. This high resolution mass scan also gives this isotope ratio from ammonia sampled much deeper in the atmosphere. These results are described and compared with ISO-SWS observations (T. Fouchet et al., 2000) that give this ratio at 400 mbar.
R. Fouchet, E. Lellouch, B. Bezard, T. Encrenaz and P. Drossart, Icarus 143, 223-243 (2000).