University of Kansas

Cassini Studies

(DRAFT)

Model of Titan's Ionosphere with Detailed Hydrocarbon Ion Chemistry

C. N. Keller (Dept. of Physics, University of Jos, Jos, Nigeria)
V. G. Anicich (NASA Jet Propulsion Lab, 4800 Oak Grove Dr., Pasadena, CA 91109 USA)
T. E. Cravens (Dept. of Physics & Astronomy, University of Kansas, Lawrence, KS 66045, USA)

The final version of this paper was published in Planetary and Space Science, 46, 1157, 1998. Abstract, with link to full article, through ScienceDirect.

ABSTRACT. We have modified the previous (1992) model of Keller, Cravens, and Gan, of the ionosphere of Titan. The model is a one-dimensional photochemical model and takes into account newly measured gas phase kinetic rates, particularly those of the higher mass hydrocarbons and nitriles. The model neutral atmospheres developed by both Yung et al. (1984, 1987) and Toublanc et al. (1995) were used in our model. The current model includes more neutral and ion species and discriminates among the various higher mass hydrocarbon and nitrile ion species. Ion neutral chemistry produces HCNH+ as the single major ion species at the ionospheric peak (an altitude of 1055 km); however, the total density of all the higher mass hydrocarbons is more than that of HCNH+. The higher mass hydrocarbons include such species as: c-C3H3+, C5H5+, and C3H5+. Another important ion species is H2C3N+. Based on this model we expect the higher mass channels (e.g., 39 amu, 41 amu, 53 amu, 65 amu, 67 amu, and 69 amu) of the Cassini Ion-Neutral Mass Spectrometer to measure higher densities than previous models have predicted.

FIGURES
Figure 1a. Density profiles of neutral species in Titan's atmosphere. Taken from Yung (1984) and Yung et al. (1987).
Figure 1b. Density profiles of neutral species in Titan's atmosphere. Taken from Toublanc et al. (1995).
Figure 2a. More density profiles of neutral species in Titan's atmosphere. Taken from Yung (1984) and Yung et al. (1987).
Figure 2b. More density profiles of neutral species in Titan's atmosphere. Taken from Toublanc et al. (1995).
Figure 3a. More density profiles of neutral species in Titan's atmosphere. Taken from Yung (1984) and Yung et al. (1987).
Figure 3b. More density profiles of neutral species in Titan's atmosphere. Taken from Toublanc et al. (1995).
Figure 4a. Major ion density profiles from the ionospheric model using Yung's neutral atmospheric models. The solar zenith angle is 60 deg.
Figure 4b. Major ion density profiles from the ionospheric model using Toublanc's neutral atmospheric models. The solar zenith angle is 60 deg.
Figure 5a.Major hydrocarbon ion density profiles from the ionospheric model using Yung's atmospheric models. The solar zenith angle is 60 deg.
Figure 5b.Major hydrocarbon ion density profiles from the ionospheric model using Toublanc's atmospheric models. The solar zenith angle is 60 deg.
Figure 6.Flowchart representing the major ion chemistry in Titan's ionosphere.
Figure 7a.Computed ion mass spectrometer spectra at an altitude of 1055 km using Yung's atmospheric models.
Figure 7b.Computed ion mass spectrometer spectra at an altitude of 1055 km using Toublanc's atmospheric models.
Figure 8a.Computed ion mass spectrometer spectra at an altitude of 1655 km using Yung's atmospheric models.
Figure 8b.Computed ion mass spectrometer spectra at an altitude of 1655 km using Toublanc's atmospheric models.

Acknowledgments: The described research was supported at the University of Kansas by NASA grant number NAG5-4358 from the NASA Planetary Atmospheres Program. Some support from the NASA Cassini project (INMS Team) is also acknowledged, and NASA Planetary Atmospheres support at JPL is also acknowledged.

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Last modified March 9, 2007
T. Hunt-Ward
tizby@ku.edu