Methane Group Ions in Saturn’s Outer Magnetosphere?

Details Methane Group Ions in Saturn’s Outer Magnetosphere? Methane Group Ions in Saturn’s Outer Magnetosphere?

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p11b1232s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P11B-1232

Physics

[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [2756] Magnetospheric Physics / Planetary Magnetospheres, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6281] Planetary Sciences: Solar System Objects / Titan

Scientific paper

Yelle et al. [2008] have estimated from Cassini Ion Neutral Mass Spectrometer (INMS) measurements that methane is escaping from Titan’s upper atmosphere at the rate of 2.5-3.0×109 mol/cm2/s and in order to explain this loss rate Strobel [2008] has proposed a hydrodynamic escape model to explain such high loss rates. This translates to loss of 2.8×1027 methane mol/s. The consequence of this work is the formation of a methane torus around Saturn which will dissociate to CH3 and other fragments of methane. The CH3 will then become ionized to form CH3+ with pickup energies ≈ keV after which it can be detected by the Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS). Up till now the ion composition within Saturn’s outer magnetosphere in the vicinity of Titan’s orbit have yielded negative results with water group ions W+ dominating. The water group ions probably result from the emission of fast neutrals from the Enceladus torus via charge exchange reactions but still gravitationally bound to Saturn [see Johnson et al., 2005 and Sittler et al. 2006] and then become ionized in the outer magnetosphere as ~≈keV pickup ions. The CAPS IMS produces two ion composition data products, one called Straight Through (ST) and the other Linear Electric Field (LEF). The first has a higher sensitivity, while the latter has a greater discrimination in time-of-flight (TOF). For ST data O+ and CH4+ have similar TOF with the primary discriminator being the O- fragment which appears at a different TOF than for mass 16 ions. One can also look for other discriminators called ghost peaks. In case of LEF W+ ions produce TOF peak close to that for atomic O+ and the methane will produce TOF close to that for atomic C+ which has a significantly different(shorter) TOF than O+. We will be reporting on our continual search for methane ions within Saturn’s outer magnetosphere. References: 1. Yelle, R. V., J. Cui and I.C.F. Müller-Wodarg, JGR, 2008. 2. Strobel, D. F., Icarus, 193, 588, [2008] 3. Johnson. R.E., et. Astrophys. J. Letts, 644, L137-L139, 2005 4. Sittler, E. C., Jr., et al., JGR, 111, A09223, 2006

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