University of Kansas Cassini Studies

(DRAFT) Model of Titan's Ionosphere with Detailed Hydrocarbon Ion Chemistry C. N. Keller, V. G. Anicich, and T. E. Cravens

Figures 7a and 7b display density versus mass number (i.e., ion mass spectrometer spectra) at an altitude of 1055 km (the ionospheric peak region) computed from the results of our model using both Yung's (Figure 7a) and Toublanc's (Figure 7b) atmospheres. The highest mass peak at 28 amu corresponds to the large amount of HCNH+ in our models (even though this mass peak also contains contributions from N2+ and C2H4+). Using Toublanc's atmosphere leads to a much higher abundance of ions with mass 29 amu (mostly C2H5+). In this peak region of the ionosphere our model predicts significant amounts (densities ~100 cm-3) at masses of 39 amu (C3H3+), 41 amu (C3H5+), 51 amu (C4H3+; Yung's model only), 52 amu (C3H2N+), 53 amu (C4H5+), 65 amu (C5H5+), 67 amu (C5H7+), 77 amu (C6H5+), 79 amu (C5H5N+; Yung's model only), and 91 amu (C7H7+). Note the pattern of mass peaks with clumps of species separated by a mass of about 12 (carbon).

Figures 8a and 8b present ion mass spectrometer spectra computed at an altitude of 1655 km (the upper limit of the chemically controlled ionosphere). Because of decreased ion production at higher altitudes, the absolute densities are, of course, much lower than near the peak. The lower mass (m < 30 amu) species dominate the ion mass spectra. The peak at 17 amu reflects the increasing density of CH5+ at higher altitudes. The peak at 28 amu is still composed of 75% HCNH+ ion species with the remainder being contributed by N2+. The peak at 29 amu is mostly C2H5+. The only other significant ion peak at 41 amu corresponds to C3H5+ still present in appreciable amounts at this altitude.

Although the addition of a new neutral atmosphere and some new chemical reactions does not produce drastically different conclusions from previous models of Titan's ionosphere, the more careful discrimination of the higher mass hydrocarbon and nitrile ion species does suggest that higher mass species (m > 30 amu) may produce significant activity in the higher mass channels of the INMS of the Cassini spacecraft.

Let us re-examine Figure 4, which shows the principle ionic species expected in Titan's ionosphere. What one observes is a set of curves showing the build up of ion abundances which peak at an altitude of about 1000 km. Since the curves result from a set of consecutive reactions, the peaks can be seen to have a staggered distribution. The ions that peak at the higher altitudes are formed earlier in the sequence than those that peak at lower altitudes. The bottom of the ionosphere (around the peak region) is a virtual chemical factory. The extent of the chemistry that is occurring in this region is not well understood because of a lack of laboratory data on the ion chemistry of the more complicated hydrocarbons and nitriles.

REFERENCES

Return to Titan Chemistry Abstract and Index Page.
Return to Space Physics Main Cassini Page
Return to Titan Studies Main Page.

Last modified January 30, 2004 T. Hunt-Ward tizby@ku.edu