Titan’s Topside Ionospheric Composition: Cassini Plasma Spectrometer Ion Mass spectrometer Measurements

Abstract:

In [1] the first quantitative evidence of ionospheric outflows (r > 10,000 km) coming from Titan was given using the Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer (IMS) data for the T9 flyby. Later in [2] similar outflows were shown for T63 and T75. In [3] evidence for ionospheric outflows for T15 was given and [4] showed evidence of outflows for T41. Normally, the CAPS IMS cannot be used to measure Titan’s relatively dense ionosphere because the IMS has high sensitivity to measure the more tenuous plasmas of Saturn’s magnetosphere and its detectors will experience count rates beyond their maximum allowed rates, therefore the IMS is configured not to measure the ionospheric plasma. But, whenever there are high altitude Titan wake flybys the ion densities are low enough so the CAPS IMS can measure these ionospheric outflows and their corresponding composition characteristic of the topside ionosphere (i.e., composition freezes in above the exobase) using its unique compositional capabilities. For example, the IMS can distinguish against specific ion types such as hydrocarbon, nitrile and water group ions due fragmentation of molecular ions within the instrument (i.e., incident ions strikes ultra-thin carbon foils at 14.6 kV or higher with exiting fragments such C^+,0,-, N^+,0, O^+,0,-1). The other ionospheric instruments only measure the ion mass-per-charge (M/Q), while the CAPS IMS measures both the ion M/Q and its fragments. Specific attention will be given to such ions as NH₄⁺, N⁺, O⁺, CH₄⁺, C₂H₅⁺, HCNH⁺ and C₃H₇⁺. These results may impose important constraints upon Titan’s ionospheric water group, hydrocarbon and nitrile ion chemistry. Are NH₄⁺ ions present as indicated by INMS at 1100 km altitude and/or water group ions? Our work has concentrated on the T15 flyby. Estimates of the NH₄⁺, N⁺ and O⁺ abundances presently have upper values < 20% of the total ion density with actual abundances and their uncertainties to be given.

[1] Sittler, E.C. Jr., et al., (2010), Planet. Space Sci., 58, 327-350.

[2] Coates, A.J., et al., (2012), J. Geophys. Res., 117, A05324, doi:10.1029/2012JA017595.

[3] Sillanpää et al., (2011), J. Geophys. Res., 116, A07223, doi:10.1029/2011JA016443.

[4] Sittler, E. C., Jr., et al., (2013), Fall AGU Abstract P53C-1880, San Francisco, CA.