The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer

Abstract

Abstract. The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H20 , H2S , C 2 and C3 nonmethane hydrocarbons, and possibly PH 3 and C1.4He/H2 in the Jovian atmosphere was measured to be 0.157 _+ 0.030. 13C/12C was found to be 0.0108 +_ 0.0005, and D/H and 3He/4He were measured. Ne was depleted, -<0.13 times solar, Ar -<1.7 solar, Kr -<5 solar, and Xe -<5 solar. CH 4 has a constant mixing ratio of (2.1 _+ 0.4) x 10 -3 (•2C, 2.9 solar), where the mixing ratio is relative to H 2. Upper limits to the H20 mixing ratio rose from 8 x 10 -7 at pressures <3.8 bars to (5.6 _+ 2.5) x 10 -5 (•60, 0.033 _+ 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10 -6 at pressures less than 3.8 bars but rose from about 0.7 x 10 -5 at 8.7 bars to about 7.7 x 10 -5 (328, 2.5 solar) above 15 bars. Only very large upper limits to the NH 3 mixing ratio have been set at present. If PH 3 and CI were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitudedependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr. IntroductionThe Galileo probe mass spectrometer (GPMS) obtained useful data concerning the composition of the Jovian atmosphere along the probe trajectory between pressure levels of 0.51 and 21.1 bars. Among species detected were H 2 and HD, 3He and 4He; the isotopes of the noble gases Ne, Ar, Kr, and Xe; the volatiles CH4, NH 3, H20 , H2S; a chlorine compound which may have been HC1, and a large number of C2 and C3 nonmethane hydrocarbons (NMHCs). Mixing ratios, or data capable of being translated into mixing ratios, have been obtained from the mass spectra acquired by direct sampling of the atmosphere and information provided by two enrichment cells for numerous species. Striking aspects of the abundance profiles were (1) the very low mixing ratios of condensable volatiles such as H2S and H20 at pressures <8 bars and their gradual increase at higher pressures along the probe trajectory, InstrumentThe GPMS has been described in detail [Niemann et al., 1992]. A quadrupole mass filter scanning in integral mass steps from 2 to 150 atomic mass units (amu) provided mass analysis. A one half-second integration time was allotted to each mass step. A secondary electron multiplier detected the ions transmitted by the mass filter. The ...

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