University of Kansas
Cassini Studies
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University of KansasCassini Studies |
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Figures 4a and 4b present density profiles of the major ions in both models of Titan's ionosphere, and Figures 5a and 5b focus only on the major hydrocarbon species. Numerical data is tabulated for all the species at altitudes of 1055 km and 1655 km in Tables 3a and 3b.
Near the ionospheric peak region (1055 km) one notes that there are only a few noticeable changes (from KCG) in the ionospheric composition resulting from the new chemistry and the extended Yung atmosphere in this model. The density of the major ion HCNH+ has decreased by about 37% from KCG through the addition of the new loss channels (equations 1a-1c). The ions that formerly ended up as HCNH+ are now found as C3H2N+, C5H5N+, C4H3+, and some of the CnHm+ ions. In the KCG model we assumed that the reaction C2H5+ + C2H2 --> C3H3+ + CH4 formed the linear isomer of C3H3+, which further reacted to form higher mass hydrocarbons CnHm+. We now believe that almost all of the C3H3+ produced is in the form of the cyclic isomer which is unreactive with other neutral species, thus becoming a more prominent ion in this model. The other significant difference between the KCG model and the current model (with Yung neutral atmosphere) is that the previous model did not discriminate among many of the higher mass hydrocarbon species. They were all included under the designation CnHm+. The current model separates these ion species into the C3Hm+ and C4Hm+ and higher mass species.
The species labeled ZLo+ (m < 30 amu) and ZHi+ (m 3 >= 30 amu) refer to products from ion-neutral reactions which either could not be determined from the gas phase experimental data, or were not included as distinct ions in our model. ZLo+ is most likely made up largely of NH2+. ZHi+ is made up mostly of higher mass nitrile species (e.g. C4H6N+, C2HN+, and C2H4N+). The CnHm+ designation refers to still higher mass hydrocarbons which have not yet been included as distinct species in our model. Most of this designation consists of the C8Hn+ and C9Hn+ species.
Noticeable differences between our models using Yung's atmosphere or Toublanc's atmosphere occur mostly in the higher mass hydrocarbon and nitrile species. This is because the chemistry producing the higher mass hydrocarbons is driven mostly by CH3+ and C2H5+ reacting with C2H2 and C2H4. Both of these neutral species are found in much lower quantities in Toublanc's atmosphere than in Yung's atmosphere. The lower concentrations of HCN in Toublanc's atmosphere reduce the production of HCNH+ from C2H5+, thus increasing the C2H5+ density from Toublanc's model by a factor of about three times over the density from Yung's model. However, the loss of HCNH+ via reaction with C4H2 and HC3N in Toublanc's atmosphere is also decreased so that the density of the major ion in both models is about the same. The lower density of HC3N in Toublanc's atmosphere ensures that the higher mass nitriles (C3H2N+ and C5H5N+) are reduced in density by factors of 5.5 and 8.7 respectively from their values in Yung's atmospheric model. Because of the large amount of water in Toublanc's atmosphere, production of H3O+ is greatly enhanced (factor of 28 times), but never becomes a major ion species even in Toublanc's atmosphere. This is due to its rapid loss reaction with HCN to reproduce the major ion.
Fox and Yelle (1997) have recently constructed a model of Titan's ionosphere which includes 67 species (mostly ion species but a few minor neutral species) and 626 chemical reactions. The electron density profiles of their model and ours agree rather well, and both models indicate that hydrocarbon ions are very important near the peak. The Fox and Yelle model (like KCG) considers a generic heavy hydrocarbon ion species and does not distinguish most individual species. The chief difference between the two models is that the HCNH+ density near the peak is about 600 cm-3 in their model and about 2000 cm-3 in our model. The loss rate of this ion is mainly via dissociative recombination and reaction 1a and 1c in both models and is comparable for the first two of these loss channels (for the Yung et al. case in our model). However, the production rate of HCNH+ is about three times greater in our model (for the Yung et al. case) due to an HCN density which is several times greater than it is in Fox and Yelle's model. Their model also includes much higher losses of HCNH+ to higher mass hydrocarbons (via reaction 1c) due to their greatly enhanced C4H2 density near the peak. The C4H2 density near 1050 km in their model is about six times greater than that in Yung et al., and almost a factor 70 greater than the corresponding density in Toublanc et al. Our model with the Toublanc neutral atmosphere has a lower HCNH+ production rate than our model with the Yung HCN density due to the lower Toublanc HCN density.
At higher altitudes (e.g. 1655 km) there are very few differences between KCG and our new model using either of the atmospheric models. This is because at the higher altitudes, methane dominates the neutral atmosphere, and the ion chemistry of the major species here (C2H5+, CH5+, and CH4+) was not changed significantly in this new model.
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T. Hunt-Ward tizby@ku.edu |