• CAPS



• INMS



• Related Links



• Cassini Acronyms




• Home

J. H. Waite, Jr.,¹ R. Thorpe,¹ W. Lewis,¹ T. E. Cravens,² W.H. Ip,³ W. T. Kasprzak,⁴ J. G. Luhmann,⁵ R. L. McNutt,⁶ and R. V. Yelle^7
1Southwest Research Institute, San Antonio TX 78228-0510 USA 
²University of Kansas, Lawrence KS 66045 USA
³Max-Planck-Institut fuer Aeronomie, D3411 Katlenburg-Lindau, Germany
⁴NASA Goddard Space Flight Center, Greenbelt MD 20771 USA
⁵University of California, Berkeley CA 94720 USA
⁶Applied Physics Laboratory, Laurel MD 20723 USA
⁷Boston University, Boston MA 02215

Introduction

    During its four-year tour of the Saturn system, the Cassini Orbiter will make over forty flybys of Titan, dipping to altitudes as low as 950 km above the moon’s surface. The Orbiter’s Ion and Neutral Mass Spectrometer (INMS) will make extensive in-situ measurements in Titan’s neutral atmosphere and ionosphere, gathering data on the identities and abundances of constituent neutral and ion species at different locations in Titan’s orbit around Saturn. These measurements, together with complementary data from Cassini investigations such as the Cassini Plasma Spectrometer, the Radio Science Subsystem, the UV Imaging Spectrograph, and others, will yield the first detailed picture of the chemistry, composition, and structure of Titan’s neutral and ionized upper atmosphere as they vary with changes in solar EUV irradiance and magnetospheric charged particle flux (both functions of the moon’s orbital position) and with latitude. INMS data will also help to characterize Titan’s interaction with Saturn’s co-rotating magnetospheric plasma and to measure the outflow of neutral gas and plasma from the moon’s upper atmosphere into the magnetosphere. 

Titan’s atmosphere is composed predominantly of nitrogen; the next most important constituent is methane. The dissociation of these two species by solar EUV photons and magnetospheric electrons initiates the formation of a variety of non-methane hydrocarbons and nitriles in the thermosphere and mesosphere. Of the former, acetylene (C₂H₂) is particularly important, because of its postulated role in the formation of Titan’s haze layers as well as its crucial role as catalyst in the photosensitized destruction of methane in the stratosphere. Polyacetylenes (C_2nH₂), created through insertion reactions involving ethynyl (C₂H) and butadiynyl (C₄H) radicals (from the photolysis of C₂H₂ and C₄H₂, respectively), have been proposed as one source of the aerosols that constitute Titan’s detached haze layer [Chassefiere and Cabane, 1995]. Maximum production of polyacetylenes occurs at an altitude of ~800 km. In the stratosphere, however, where C₂H₂ photolysis peaks and acetylene is substantially less abundant relative to methane than at higher altitudes, the key radical in the polymerization of acetylene, C₂H, reacts preferentially with CH₄ to produce methyl (CH₃) and re-form acetylene; C₂H₂ polymerization is thus inhibited. The role of acetylene at these altitudes is to catalyze, through its photolytic product C₂H (and also, via another pathway, C₂), the destruction of methane. This catalytic process is responsible for the bulk of the methane destruction in Titan’s atmosphere. The resulting methyl combines with itself to form ethane, the most abundant stratospheric hydrocarbon. 

Nitriles produced in Titan’s upper atmosphere through reactions of odd nitrogen radicals such as N, N(²D) and CN with hydrocarbons [Yung et al., 1984; Yung, 1987] are also believed to play an important role in the 

formation of aerosols. The dominant nitrile species is HCN, which is created through insertion reactions of N atoms with methyl (CH₃) and methylene (CH₂) radicals. Other nitriles are HC₃N, C₂N₂, C₄N₂, and CH₃CN. Chassefiere and Cabane [1995] suggest that C-N oligomers synthesized from dicyanoacetylene (C₄N₂) may be a source of aerosols for the detached haze layer comparable to the polyacetylenes, which are formed in the same altitude range. An additional contribution to Titan’s detached haze layer may come from C-H-N oligomers created at higher altitudes by energetic electron bombardment of Titan’s upper atmosphere. According to the Chassefiere and Cabane model, such C-H-N oligomers--formed lower in the atmosphere, between 350 and 400 km, by energetic electron bombardment of N₂, CH₄, C₂H₂, C₂H₆, and HCN--are the dominant constituent of Titan’s main haze layer. A significant contribution to the main haze layer from photochemically produced polyacetylenes, as suggested by Bar-Nun et al. [1988], is considered by Chassefiere and Cabane to be unlikely. 

INMS will not sample Titan’s atmosphere below 950 km. However, the information that INMS can provide--both through direct measurement of neutral densities and through measurement of ion densities from which neutral densities can in some cases be deduced--on species such as C₂H₂, C₄H₂, HCN, HC₃N, and CH₃ is of direct relevance to questions of aerosol formation and stratospheric chemistry. INMS measurements in Titan’s thermosphere will thus provide a valuable and necessary complement to data on Titan’s lower atmosphere from the Probe ACP and GCMS and the Orbiter UVIS, VIMS, and CIRS investigations. 

Figures
Fig 1. Titan haze formation according to the model of Chassefiere and Cabane... 
Fig 2. Key Photochemical Reactions in Titan's Thermosphere and Mesosphere
Fig 3. Altitude profiles of mixing ratios for key hydrocarbons... 
Fig 4. Number densities and mixing ratios for key neutral species at 950 km... 
Fig 5. The Cassini Ion and Neutral Mass Spectrometer (INMS). 
Fig 6. Simulated spectra for a closed source (top) and open source (bottom) INMS measurement. 

References
Bar-Nun, A., et al., Shape and optical properties of aerosols formed by photolysis of acetylene, ethylene, and hydrogen cyanide, J. Geophys. Res., 93, 8383, 1988.
Chassefiere, E., and M. Cabane, Two formation regions for Titan’s hazes: indirect clues and possible synthesis mechanisms, Planet. Space Sci., 43, 91, 1995.
Harrison, A. G., et al., Total cross sections for ionization by electron impact, Canadian J. Chem., 44, 1967, 1966
Kanik, I., et al., Total electron scattering and electronic state excitation cross sections for O₂, CO, and CH₄, J. Geophys. Res., 98, 7447, 1993.
Krishnakumar, E., and S. K. Srivastava, Ionization cross sections of rare-gas atoms by electron impact, J. Phys. B: At. Mol. Opt. Phys., 21, 1055, 1988.
Krishnakumar, E., and S. K. Srivastava, Cross sections for the production of N₂⁺, N⁺ + N₂²⁺ and N₂⁺ by electron impact on N₂, J. Phys. B: At. Mol. Opt. Phys., 23, 1893, 1990.
Lara, L. M., et al., Vertical distribution of Titan’s atmospheric neutral constituents, J. Geophys. Res., 101, 23261, 1996.
Tawara, H, and T. Kato, Total and partial ionization cross sections of atoms and ions by electron impact, Atomic Data and Nuclear Data Tables, 36, 167, 1987.
Thompson, W. R., et al., The Titan haze revisited: magnetospheric energy sources and quantitative tholin yields, Icarus, 112, 376, 1994.
Toublanc, D., et al., Photochemical modeling of Titan’s atmosphere, Icarus, 113, 2, 1995.
Yung, Y. L., M. Allen, and J. P. Pinto, Photochemistry of the atmosphere of Titan: comparison between model and observations, Astrophys. J. Supp., 55, 465, 1984.
Yung, Y. L, An update of nitrile photochemistry on Titan, Icarus, 72, 468, 1987.

Return to the INMS homepage