University of Kansas
Cassini Studies
(DRAFT)
|
|
University of KansasCassini Studies |
(DRAFT)
|
4.5 C5Hm+ Species
Even though it has no ion-neutral loss channel, C5H3+ does not appear in any great amount in Titan's ionosphere because its major source is from the minor ion species C3H2+. The other three C5Hm+ species in our model appear at densities of the order of 10 - 100 cm-3 in the peak region.
C5H5+ is produced mostly via the reactions:
| C3H5+ + C2H2 --> C5H5+ + H2 | k12a = 3.8 x 10-10 cm3 s-1 | (12a) |
| C3H4+ + C2H2 --> C5H5+ + H | k12b = 4.9 x 10-10 cm3 s-1 | (12b) |
and lost mostly (~80% with Yung's atmosphere and ~90% with Toublanc's atmosphere) to electron dissociative recombination. The remainder of the C5H5+ reacts with C2H2 and C4H2 to produce C7H7+ and other higher mass hydrocarbons.
C5H7+ is produced via the reaction:
| C3H5+ + C2H4 --> C5H7+ + H2 | k13 = 1.19 x 10-10 cm3 s-1 | (13) |
and lost only to electron dissociative recombination.
Similarly, C5H9+ is produced in our model solely via the radiative association reaction:
| C3H5+ + C2H4 --> C5H9+ + hn | k14 = 5.1 x 10-11 cm3 s-1 | (14) |
and is lost via electron dissociative recombination. The density of C5H9+ is less than that of C5H7+ due to the slower reaction rate of the radiative association reaction (equation 14) compared to the association reaction with release of hydrogen (equation 13).
4.6 C6,7Hm+ Species
The only major ions produced in this group are C6H5+, C6H7+, and C7H7+. Some other C6,7Hm+ species are produced, but at such low abundances that they are included in the generic CnHm+ category.
C6H5+ is produced mostly via the reactions:
| C4H3+ + C2H2 --> C6H5+ + hn | k15a = 2.2 x 10-10 cm3 s-1 | (15a) |
| C4H5+ + C4H2 --> C6H5+ + C2H2 | k15b = 4.0 x 10-10 cm3 s-1 | (15b) |
and lost to reactions producing C6H7+, C7H7+, and C8,9Hm+. However about half of the loss of C6H5+ is due to electron dissociative recombination.
C6H7+ is produced mostly from the reactions:
| C6H5+ + C2H4 --> C6H7+ + C2H2 | k16a = 8.5 x 10-11 cm3 s-1 | (16a) |
| C5H5+ + C3H4 --> C6H7+ + C2H2 | k16b = 5.6 x 10-10 cm3 s-1 | (16b) |
and lost almost entirely to electron dissociative recombination with a minor loss channel to production of C7H7+.
C7H7+ is produced in our model from the reactions:
| C5H5+ + C2H2 --> C7H7+ + hn | k17a = 3.1 x 10-11 cm3 s-1 | (17a) |
| C6H5+ + C2H6 --> C7H7+ + CH4 | k17b = 3.9 x 10-12 cm3 s-1 | (17b) |
| C4H5+ + C3H4 --> C7H7+ + H2 | k17c = 6 x 10-10 cm3 s-1 | (17c) |
| C6H5+ + CH4 --> C7H7+ + H2 | k17d = 7.5 x 10-11 cm3 s-1 | (17d) |
and lost mostly to electron dissociative recombination with minor loss channels to production of C9H9+ and C11H9+.
4.7 CnHm+ Species (n>7) and Discussion of Total Hydrocarbon Ion Density
Even though our current model discriminates more than KCG did among the higher mass hydrocarbons, there is still a group of undiscriminated higher mass hydrocarbons which still make an appreciable contribution to the ionosphere in our model. The major species in this category are the C8Hm+ and C9Hm+ species formed from C6Hm+ and C7Hm+ precursors. In the KCG model at 1055 km the "specified" hydrocarbon ion species had densities which added up to 2100 cm-3, giving a total hydrocarbon ion density of close to 2700 cm-3. For our new model with the Yung neutral atmosphere, the "specified" hydrocarbon ion densities at 1055 km sum to 2663 cm-3, and the generic species, CnHm+, has a density of only 270 cm-3, giving a total hydrocarbon ion density close to 2930 cm-3. This total hydrocarbon ion density exceeds the HCNH+ density (about 2100 cm-3) at this altitude and is 50% of the electron density.
It is likely that even C8Hm+ and C9Hm+ species would also react with C2H2 (and perhaps other neutral species) to produce even higher mass hydrocarbon ion species. Based on our current limited knowledge of reaction rates of these hydrocarbons, we have stopped the reaction chain at this generic classification CnHm+.
4.8 Higher Mass Nitrile Species
C3HN+ is formed by the reaction of N2+ and N+ with HC3N. Since this neutral species is present at low densities in both atmospheric models, the production of C3HN+ is not large. Also, most of what is produced quickly reacts with methane to produce C3H2N+ and C2H3N+ (included in the ZHi+ species designation in our model).
C3H2N+ is formed via the reactions:
| HCNH+ + HC3N --> C3H2N+ + HCN | k18a = 3.4 x 10-9 cm3 s-1 | (18a) |
| C2H5+ + HC3N --> C3H2N+ + C2H4 | k18b = 3.55 x 10-9 cm3 s-1 | (18b) |
and lost mostly in the rapid reaction:
| C3H2N+ + C2H4 --> C5H5N+ + H | k19 = 1.3 x 10-9 cm3 s-1 | (19) |
and secondarily to electron dissociative recombination.
In our model, reaction 19 is the sole source of C5H5N+. Currently we have no neutral species which is believed to react with C5H5N+, leaving it no loss channel except through electron dissociative recombination, hence causing it to accumulate to densities around 500-1000 cm-3 when Yung's atmosphere is used. As expected, the densities of all the nitrile species are diminished when using Toublanc's atmosphere, because of the greatly reduced density of HCN and HC3N in his model.
4.9 Water Species
Although there is considerable difference between Yung's model and that of Toublanc concerning the amount of water present, in neither model do water ion species reach significant amounts.
H2O+, formed from N2+ and N+ reacting with water, is quickly converted to H3O+ by reaction with methane and hydrogen. H3O+ is also formed by reaction of HCNH+, C2H5+, and CH5+ with water. In other space plasma environments (e.g. cometary comae) H3O+ is a terminal ion and the dominant ion. However, in Titan's ionosphere H3O+ reacts with HCN to form more of the major ion HCNH+. This ion neutral loss channel keeps the density of H3O+ of order 10 cm-3 even in the relatively water abundant Toublanc atmosphere.
Next: Implications for the Cassini Mission
Return to
Titan Chemistry Abstract and Index Page.
Return to Space Physics Main Cassini
Page
Return to Titan Studies Main Page.
|
T. Hunt-Ward tizby@ku.edu |