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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Electron and ion measurements made by the Voyager 1 plasma science instrument revealed a plasma wake surrounding Titan in Saturn's rotating magnetosphere. This wake is characterized by a plasma that is more dense and cooler than the surrounding subsonic magnetospheric plasma. The density enhancement is produced by the deflection of magnetospheric plasma around Titan and the addition of exospheric ions picked up by the rotating magnetosphere. By using simple models for ion pickup in the ion exosphere outside Titan's magnetic tail and ion flow within the boundaries of the tail, the interaction between Saturn's rotating magnetosphere and Titan is shown to resemble the interaction between the solar wind and Venus. Outside the magnetic tail of Titan, pickup of H+ formed by ionization of the H exosphere is indicated when synthetic and observed ion spectra are matched. Close to the boundary of the tail, a reduction in plasma flow speed is found, providing evidence for mass loading by the addition of N2+/H2CN+ and N+ to the flowing plasma. The boundary of the tail is indicated by a sharp reduction in the flux of high‐energy electrons, which are removed by inelastic scattering with the atmosphere and centrifugal drift produced when the electrons traverse the magnetic field draped around Titan. Within the tail the plasma is structured as the result of spatial and/or temporal variations. The ion mass cannot be determined uniquely in the tail; however, one measurement suggests the presence of a heavy ion with a mass of order 28 amu: One candidate is H2CN+, suggested as the dominant topside ion of the ionosphere, which may flow from the ionosphere into the tail.

The composition of the jovian atmosphere from 0.5 to 21 bars along the descent trajectory was determined by a quadrupole mass spectrometer on the Galileo probe. The mixing ratio of He (helium) to H2 (hydrogen), 0.156, is close to the solar ratio. The abundances of methane, water, argon, neon, and hydrogen sulfide were measured; krypton and xenon were detected. As measured in the jovian atmosphere, the amount of carbon is 2.9 times the solar abundance relative to H2, the amount of sulfur is greater than the solar abundance, and the amount of oxygen is much less than the solar abundance. The neon abundance compared with that of hydrogen is about an order of magnitude less than the solar abundance. Isotopic ratios of carbon and the noble gases are consistent with solar values. The measured ratio of deuterium to hydrogen (D/H) of (5 +/- 2) x 10(-5) indicates that this ratio is greater in solar-system hydrogen than in local interstellar hydrogen, and the 3He/4He ratio of (1.1 +/- 0.2) x 10(-4) provides a new value for protosolar (solar nebula) helium isotopes. Together, the D/H and 3He/4He ratios are consistent with conversion in the sun of protosolar deuterium to present-day 3He.

[1] We present results from the CAPS electron spectrometer obtained during the downstream flyby of Titan on 26 December 2005, which occurred during a period of enhanced plasma pressure inside the magnetosphere. The electron data show an unusual split signature with two principal intervals of interest outside the nominal corotation wake. Interval 1 shows direct evidence for ionospheric plasma escape at several R T in Titan's tail. Interval 2 shows a complex plasma structure, a mix between plasma of ionospheric and magnetospheric origin. We suggest a mechanism for plasma escape based on ambipolar electric fields set up by suprathermal ionospheric photoelectrons.

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager.

The Cassini Plasma Spectrometer (CAPS) instrument observed the plasma environment at Titan during the Cassini orbiter's TA encounter on October 26, 2004. Titan was in Saturn's magnetosphere during the Voyager 1 flyby and also during the TA encounter. CAPS measurements from this encounter are compared with measurements made by the Voyager 1 Plasma Science Instrument (PLS). The comparisons focus on the composition and nature of ambient and pickup ions. They lead to: A) the major ion components of Saturn's magnetosphere in the vicinity of Titan are H+, H2+ and O+/CH4+ ions; B) finite gyroradius effects are apparent in ambient O+ ions as the result of their absorption by Titan's extended atmosphere; C) the principal pickup ions are composed of H+, H2+, N+/CH2+, CH4+, and N2+; D) the pickup ions are in narrow energy ranges; and E) there is clear evidence of the slowing down of background ions due to pickup ion mass loading.

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