University of Kansas
Cassini Studies
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University of KansasCassini Studies |
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Figure 6 is a flow chart illustrating the major production and loss channels in the chemistry of Titan's ionosphere. The production of the major ion HCNH+ is shown by the heavier, shaded line. Each of the molecular ion species can also recombine dissociatively, but this is not shown in this flow chart.
4.1 HCNH+
The formation of the major ion species near the peak, HCNH+, follows the same pathways as indicated in KCG. Note that even though we call HCNH+ the "major" ion species it accounts for only 35% of the peak electron density for our new models. Those reactions responsible for 70% of the HCNH+ production are:
| N2 + hn --> N2+ + e- | Peak Production Rate = 1.7 x 101 cm-3 s-1 | (4a) |
| N2+ + CH4 --> CH3+ + N2 + H | k4b = 9.12 x 10-10 cm3 s-1 | (4b) |
| CH3+ + CH4 --> C2H5+ + H2 | k4c = 1.10 x 10-9 cm3 s-1 | (4c) |
| C2H5+ + HCN --> HCNH+ + C2H4 | k4d = 2.70 x 10-9 cm3 s-1 | (4d) |
The remainder of the HCNH+ production is largely via the reactions:
| HCN+ + CH4 --> HCNH+ + CH3 | k5a = 1.14 x 10-9 cm3 s-1 | (5a) |
| N+ + CH4 --> HCNH+ + H2 | k5b = 3.8 x 10-10 cm3 s-1 | (5b) |
| C2H3+ + HCN --> HCNH+ + C2H2 | k5c = 2.3 x 10-9 cm3 s-1 | (5c) |
Since the scale height of HCN is less than that of methane in this region, the production of HCNH+ falls off around 1500 km and C2H5+ becomes the major ion species. The HCNH+ density peaks at a slightly lower altitude, and C2H5+ becomes the major ion species at a considerably lower altitude when using Toublanc's atmosphere rather than Yung's because of the diminished HCN density and enhanced methane density in his model over those values in Yung's model.
One new feature in the current ionospheric model is the addition of three loss channels for HCNH+ via equations 1-3 (whose importance was first appreciated by Fox and Yelle (1995, 1997)). However, because of the low density of the neutral species involved in these reactions, the electron dissociative recombination reaction:
| HCNH+ + e- --> HCN + H | k6 = 6.4 x 10-7 300/Te cm3 s-1 | (6) |
still accounts for greater than 60% (Yung's atmosphere) and greater than 90% (with Toublanc's atmosphere) of the loss of HCNH+.
4.2 C2H5+ and CH5+
The major production (~80% at the peak and ~90% at 1650 km) of C2H5+ is from CH3+ via reaction 4c. Because of its production link to methane with its larger scale height C2H5+ becomes the major ion at altitudes slightly above the ionospheric peak. However, at altitudes above 1700 km (assuming that chemical processes still dominate over dynamical processes) CH5+ becomes the major ion species. This is because CH5+ production is more closely linked to the direct ionization of methane and because of increased production of N2H+ (the major precursor of CH5+). N2H+ production is in turn increased at increasing altitudes due to the growing mixing ratio of H2 at higher altitudes (cf. KCG).
4.3 C3Hm+ Species
With the exception of C3H5+ and c-C3H3+ (the cyclic isomer) the densities of the C3Hm+ species remain of the order of 10-1 to 100 cm-3. This is because these species are produced from relatively low density precursors (CNC+-->C3H+; CH+-->C3H2+; C2H2+-->C3H4+; C3H+-->l-C3H3+ (the linear isomer)) and are lost mostly via fast reactions with methane and acetylene to produce higher mass hydrocarbons. An exception is the minor species C3H7+ which might be expected to be significant in Titan's ionosphere since it is produced from the major ion species C2H5+ and CH5+ reacting with C2H6 and methane, and is lost mostly through electron dissociative recombination. The fact that C3H7+ is not found in large quantities is due to the small reaction rates (~1% of the rates of competing reactions) of these major ions when they are producing C3H7+.
The major sources of c-C3H3+ are via the reactions:
| CH3+ + C2H2 --> c-C3H3+ + H2 | k7a = 1.15 x 10-9 cm3 s-1 | (7a) |
| C2H5+ + C2H2 --> c-C3H3+ + CH4 | k7b = 6.80 x 10-11 cm3 s-1 | (7b) |
| C2H4+ + C2H2 --> c-C3H3+ + CH3 | k7c = 6.47 x 10-10 cm3 s-1 | (7c) |
The cyclic isomer c-C3H3+ is quite unreactive with the neutral species in Titan's atmosphere. It is lost only through the slower electron dissociative recombination process, and hence its prominence in the peak region of the ionosphere. However, one should note that the reactions of c-C3H3+ have not been measured well enough in the laboratory. More detailed studies are called for.
C3H5+ is produced largely via the reactions:
| C2H5+ + C2H4 --> C3H5+ + CH4 | k8a = 3.55 x 10-10 cm3 s-1 | (8a) |
| C2H3+ + CH4 --> C3H5+ + H2 | k8b = 1.90 x 10-10 cm3 s-1 | (8b) |
| C2H2+ + CH4 --> C3H5+ + H | k8c = 7.03 x 10-10 cm3 s-1 | (8c) |
Reaction of this ion with acetylene and ethylene converts most of the C3H5+ to C5Hm+ species. Even though these ion-neutral reaction channels are active, a significant amount (25% with Yung's atmosphere and 38% with Toublanc's atmosphere) of C3H5+ is lost to electron dissociative recombination. Table 3a shows that there is more c-C3H3+ then C3H5+ when using Yung's atmosphere, while the reverse is true when using Toublanc's atmospheric model. C3H5+ is produced from C2H5+ at a higher rate than c-C3H3+ (cf. equations 7b and 8a) in both models. However, Toublanc's model atmosphere contains more methane than does Yung's model at this altitude, hence increasing the production of C3H5+. Also, since Toublanc's atmosphere has less acetylene and ethylene for C3H5+ to react with (and be lost), the net result is a larger C3H5+ density when using Toublanc's atmosphere.
4.4 C4Hm+ Species
Of the four species of this class included in our models only C4H3+ and C4H5+ accumulate in appreciable amounts. C4H5+ is formed mostly via the reactions:
| C2H5+ + C2H2 --> C4H5+ + H2 | k9a = 1.2 x 10-10 cm3 s-1 | (9a) |
| C2H4+ + C2H2 --> C4H5+ + H | k9b = 1.93 x 10-10 cm3 s-1 | (9b) |
| C2H2+ + C2H4 --> C4H5+ + H | k9c = 3.17 x 10-10 cm3 s-1 | (9c) |
Even though C4H5+ has some ion-neutral loss channels (to form C7H7+ and C6H5+), over 80% of its loss (over 95% with Toublanc's atmosphere) is due to electron dissociative recombination. The ion-neutral loss channels contribute so little to its loss because they involve the low density species C3H4 and C4H2. In Table 3a one can observe that C4H5+ has a slightly greater density in the model using Toublanc's atmosphere. His atmosphere has about half as much acetylene present to help produce C4H5+, but it has dramatically less C3H4 and C4H2 to form loss modes for C4H5+.
There are several isomers of C4H5+. The experimental data we have in hand does not discriminate between these isomers. From thermodynamic considerations, one would predict that the cyclic isomer would be the predominant isomer formed. This seems to be the case for most other higher mass hydrocarbons (e.g., C5H5+ and C6H7+ -- an aromatic-like structure has the lowest heat of formation).
In our model with Yung's atmosphere, C4H3+ is formed primarily via reaction 1c in which the major ion reacts with C4H2. Secondary formation of C4H3+ is via the reactions:
| C2H3+ + C2H2 --> C4H3+ + H2 | k10a = 2.40 x 10-10 cm3 s-1 | (10a) |
| C2H2+ + C2H2 --> C4H3+ + H | k10b = 9.52 x 10-10 cm3 s-1 | (10b) |
For Toublanc's atmosphere, which has much less C4H2, the production of C4H3+ is diminished by 85%, and occurs almost equally via reactions 10a and 1c. In both models electron dissociative recombination accounts for over 40% of the loss of C4H3+, while the radiative association reaction:
| C4H3+ + C2H2 --> C6H5+ + hn | k11 = 2.2 x 10-10 cm3 s-1 | (11) |
accounts for much of the remainder of the loss of C4H3+.
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T. Hunt-Ward tizby@ku.edu |