University of Kansas
Cassini Studies
(DRAFT)
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University of KansasCassini Studies |
(DRAFT)
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Recent observations of the moons of Jupiter by the Galileo spacecraft have confirmed that the missions to the satellites of the outer solar system are treasure troves of scientific information. The moons of Saturn will undoubtedly prove this same point when they are visited by the Cassini probe early in the coming century. Chief among these Saturnian satellites is Titan, shrouded in haze and possessing an amazingly dense and compositionally complex atmosphere. Bombarded by both solar extreme ultraviolet (EUV) radiation and, much of the time, by Saturnian energetic magnetospheric electrons, this rich atmosphere gives rise to a complex ionosphere. Although several models of Titan's ionosphere have been developed over the past twenty years, the launch of the Cassini spacecraft coupled with continuing research on gas phase ion-molecule chemistry (Anicich and McEwan, 1997) have led us to develop a more comprehensive model of Titan's ionosphere.
Early models by Capone et al. (1976, 1980, 1981, 1983) modeled the lower ionosphere (altitudes of 50400 km) formed by ionization from galactic cosmic rays. Later models (Hunten et al., 1984; Strobel, 1985; Atreya, 1986; Bauer, 1987; and Ip, 1990) focused on the ionosphere formed due to solar EUV radiation and to impact ionization from Saturnian magnetospheric electrons. The peak ion density in these models was predicted to exist at an altitude of around 1000-1200 km.
Our previous work (Keller, Cravens, and Gan, 1992 - hereafter referred to as KCG) was a one-dimensional chemical equilibrium model of Titan's ionosphere using the neutral atmosphere model developed by Yung et al. (1984) and Yung (1987). The sources of ionization in KCG were both solar EUV photons and Saturnian magnetospheric electrons. The electron density (equal to total ion density) at the peak (1050 km altitude) was about 6000 cm-3 when both photons (at a solar zenith angle of 60 deg) and magnetospheric electrons were allowed to ionize the model atmosphere. For a solar zenith angle of 80 deg, KCG predicted a peak density of 3030 cm-3 at 1175 km. A recent re-analysis of Voyager 1 radio occultation data (Bird et al., 1997) found a maximum electron density of 2400 plus/minus 1100 cm-3 at an altitude of 1180 plus/minus 150 km at the terminator. The model in KCG as well as many of the later models predicted that the major ion species near Titan's ionospheric peak was HCNH+. This species predominates near the ionospheric peak because it is unreactive with any of the neutral species included in the earlier models, leaving its only loss channel to be relatively slow electron dissociative recombination. Recently, Fox and Yelle (1995, 1997) and Fox (1996a, b) were the first to recognize that significant ion-neutral loss reactions can exist for HCNH+ and have important implications for the identity of the major ion species in Titan's ionosphere. In particular, Fox and Yelle suggested that HCNH+ is not the major ion species in Titan's ionosphere. The measured gas phase ion-neutral reaction rates for these reactions are listed by Anicich and McEwan (1997). These loss channels are:
| HCNH+ + HC3N --> H2C3N+ + HCN | k1a=3.4 x 10-9 cm3 s-1 | (1a) | HCNH+ + H2O --> H3O+ + HCN | k1b=8.8 x 10-10 cm3 s-1 | (1b) |
| HCNH+ + C4H2 --> C4H3+ + HCN | k1c=1.6 x 10-9 cm3 s-1 | (1c) |
Fox and Yelle (1995, 1997) pointed out the importance of reactions 1a and 1c. Even though these neutral reactants are only minor constituents of Titan's atmosphere (and were neglected in our previous model) they provide significant competition to electron dissociative recombination in destroying HCNH+. Note that in some of the figures and tables we will also refer to HCNH+ as H2CN+. Very deep in the atmosphere (i.e., an altitude of 150 km or so) ion-molecule clustering reactions of HCNH+ with N2 or CH4 will take place (Vacher et al., 1997), but this type of reaction is not important near the ionospheric peak.
The KCG model treated the hydrocarbon ion species (C2Hm+ where m is an integer) as distinct species, but did not discriminate as well for the higher mass hydrocarbons (CnHm+ where integer n > 2). Kinetic measurements (some recently reported by Anicich and McEwan, 1997) have allowed us to further specify these higher mass hydrocarbon ion species, thus giving us a more complete picture of the composition of Titan's ionosphere.
Recently Toublanc et al. (1995) developed a new model of Titan's neutral atmosphere. While the major species (N2, CH4, and H2) are almost identical in density with the model of Yung et al. (1984; 1987), the minor species differ significantly in a number of cases. With our updated model we study how using these two different neutral atmospheric compositions would affect the resulting ionospheric composition.
Dynamical effects in Titan's ionosphere have been considered by Keller et al. (1994) (1 dimensional MHD model) and by Cravens et al. (1998) (2-dimensional MHD model). We realize that plasma dynamics (especially horizontal motion as the plasma from the Saturnian magnetosphere flows around Titan) become more important than chemical effects at altitudes greater than about 1700 km. This is well above the ionospheric peak region; therefore the chemical model developed here should produce valid information around the ionospheric peak.
In section 2 we discuss the parameters used in our revised model, with most of the discussion centering on the two different neutral atmosphere models employed. Section 3 presents the results of this revised model and compares these newer results with those from our previous work. The chemistry of the major ion species is presented in Section 4, and suggestions are given for which of the yet unknown reaction rates might be the most useful to measure for future modeling. Section 5 presents some predicted ion mass spectra based on results of this model.
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T. Hunt-Ward tizby@ku.edu |