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Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..644v&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Other
Scientific paper
In the upper part of atmospheres lies the ionosphere, a region of particular interest for planetary science, because it provides the link between the neutral atmosphere, and the ionizing processes from outer space. On Titan, it is created by the interaction of solar ultraviolet radiation and magnetospheric electrons with the main atmospheric constituents, N2 and CH4. Cassini has revealed that an extremely complex chemistry occurs in Titan's ionosphere. The INMS mass spectrometer detected positively charged hydrocarbons and nitrogen-bearing species with a charge-to-mass ratio (m/z) up to 100 amu [1]. In 2007, the Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan's upper atmosphere [2]. The data showed evidence for negatively charged ions with m/z up to 10,000 amu and at lower m/z for two distinct peaks below 50 amu, corresponding to a total density of ~200 cm-3, giving an anion to cation ratio of ~0.1. This detection happened almost simultaneously with the surprising discovery of four negative ions in the interstellar medium: C4H-, C6H-, C8H- and C3N- [3; 4; 5; 6; 7]. The possible presence of negative ions in Titan's upper atmosphere had only been briefly discussed before the Cassini-Huygens mission. Three-body electron attachment to radicals or collisional charging of aerosols had been suggested as a source of negatively charged species. Because the first process is negligible at high altitude (neutral densities lower than 1015 cm-3) and because aerosols were not expected above ~500 km, ionospheric models considered the presence of negatively charged species to be highly unlikely. However, the observations clearly show that Titan has the most complex ionosphere of the Solar System with an intense chemistry, leading to an increase of molecular size. By analyzing the optical properties of the detached haze layer observed at 520 km in Titan's mesosphere, Lavvas et al. provided the first quantitative evidence that thermospheric chemistry is the main source of haze on Titan [8]. The negative ions observed by ELS are very likely hydrocarbon and nitrogen-bearing species but their stoichiometry and structure are largely unknown because of the poor mass resolution of the spectrometer. In order to interpret the data, it is therefore necessary to develop kinetic models of the ionosphere of Titan and confront them with repeated measurements. In order to determine the processes controlling the formation of negative ions in Titan's atmosphere we use the photochemical model developed by Vuitton et al. [9; 10]. This model was used to successfully explain the processes controlling the positive ion formation in Titan's ionosphere. Furthermore, it was used for the investigation of the ion-neutral chemical processes controlling the formation of the observed thermospheric benzene abundance [11]. In order to properly describe the negative ion chemistry, eleven negative ions and about a hundred reactions involving negatively charged species have been added to the original model. The model solves the continuity equation in onedimension at altitudes between 700 and 1500 km, assuming local chemical equilibrium. It takes into account production and loss processes that include photoionization, photodetachement, energetic electron impact, and chemical reactions between ions and neutrals and between positively and negatively charged species. Due to the small chemical lifetime of ions compared with the characteristic time for diffusion, the latter is not included in the calculations. The photoelectron flux that leads to the production of negative ions is calculated by solving the Boltzmann transfer equation that provides a stationary solution for the intensity (cm-2 s-1 eV-1 sr-1) of electrons at different energies, angles and altitudes within the atmosphere ([12] and references therein). The ion densities depend closely upon the composition of the neutral atmosphere. The density of the main atmospheric constituents, N2, CH4 and H2 are well established by the INMS neutral measurements [13; 14], but minor neutral species can still have a strong, even controlling, effect on the ion composition and few of these have been measured accurately. The density vertical profiles for the neutral species are then based on the predictions of photochemical models [11; 15; 16] scaled to the densities inferred at 1100 km from the INMS ion measurements [10] when available. The model results indicate that CN- and C3N- are most probably responsible for the two features seen at low mass in the CAPS spectra. Throughout the ionosphere, CN- is exclusively produced by dissociative electron attachment on HCN, as shown in Fig. 1. Dissociative electron attachment to other nitriles, radiative attachment to CN and ion-pair formation are largely negligible. Associative detachment with H and CH3 is responsible for most of the CN- destruction. Proton transfer of HC3N to CN- also contributes at lower altitude while photodetachment dominates above 1425 km. Recombination with positive ions is small for all altitudes. For C3N-, proton transfer to CN- and to a lesser extend, radiative attachment to C3N are the dominant formation processes, as presented in Fig. 2. While dissociative electron attachment is responsible for most of the CN- formation, it is a minor mechanism for C3N- because of the lower cross section for HC3N by comparison to HCN. The negative ions reaction list contains about 30 ionization and electron attachment reactions and 70 ion-neutral reactions. The rate coefficients have been compiled from a comprehensive literature search. The list includes the source of the data, accuracies for the rate coefficients, and the temperature dependence of the rate coefficient, when available. Determination of a precise reaction list is an iterative process. Many reactions were considered important in the ionosphere of Titan, but no data were available. For example, the following proton transfer reaction: CN- + HC3N → C3N- + HCN is responsible for most of the C3N- production but has never been studied. To proceed with the analysis we estimated rates for the missing reactions based on analogy with similar reactions for other molecules or Fig. 2 Production and destruction rates of C3N- as a function of altitude. from their collision rate. Clearly, more accurate information is needed, from which we can derive more precise estimates of negative ion densities. This analysis also identifies numerous other deficiencies in currently available rate coefficient data thereby providing guidance for future laboratory measurements.
Coates Allison
Dutuit Odile
Lavvas Panayiotis
Lewis Gethyn R.
Thissen Roland
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