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

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: H-2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C-2 and C-3 nonmethane hydrocarbons, and possibly PH3 and Cl. He-4/H-2 in the Jovian atmosphere was measured to be 0.157 +/- 0.030. C-13/C-12 was found to be 0.0108 +/- 0.0005, and D/H and He-3/He-4 were measured. Ne was depleted, less than or equal to 0.13 times solar, Ar less than or equal to 1.7 solar, Kr less than or equal to 5 solar, and Xe less than or equal to 5 solar. CH4 has a constant mixing ratio of (2.1 +/- 0.4) x 10(-3) (C-12, 2.9 solar), where the mixing ratio is relative to H-2. Upper limits to the H2O mixing ratio rose from 8 x 10(-7) at pressures <3.8 bars to (5.6 +/- 2.5) x 10(-5) (O-16, 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) (S-32, 2.5 solar) above 15 bars. Only very large upper limits to the NH3 mixing ratio have been set at present. If PH, and Cl were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C-2 and C-3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of He-4, (2) a local, altitude-dependent 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 + He-3)/H in this part of the galaxy during the past 4.6 Gyr.