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

The Galileo probe mass spectrometer determined the composition of the Jovia n 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 constitue nts detected: H-2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C-2 and C-3 nonm ethane hydrocarbons, and possibly PH3 and Cl. He-4/H-2 in the Jovian atmosp here 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 t han or equal to 5 solar, and Xe less than or equal to 5 solar. CH4 has a co nstant 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, 0.0 33 +/- 0.015 solar) at 11.7 bars and, provisionally, about an order of magn itude 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 t o 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 a t 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 e ntered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a modera te 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.