Titan's upper atmospheric structure and ionospheric composition

This Dissertation investigates the density structure of the neutral upper atmosphere and the composition of the ionosphere of Titan through Cassini observations. The highly extended atmosphere of Titan consists primarily of N2, CH4, and H2. The focus is on data extracted from the Ion and Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) instruments onboard Cassini. The INMS, which is fundamentally a quadrupole mass spectrometer, measures the abundance of neutral and ion components with masses of 1--8 and 12--99 Da. The CAPS instrument consists of three subsystems of which the Ion Beam Spectrometer (CAPS-IBS) is used in this study to derive mass spectra of thermal ions up to 400 Da. in mass in Titan's ionosphere.

From measurements of molecular nitrogen in Titan's upper atmosphere an atmospheric scale height is derived implying an effective temperature. From an analysis of 29 targeted flybys of Titan we find that the thermosphere is isothermal from an altitude of 1050 km to the exobase height with an average effective temperature of 153 K. The scale height, and hence the effective temperature, is found to be highly variable. We assess this variability against the relevant geospatial, solar, and magnetospheric parameters to determine which are highly correlated to the effective temperatures. Titan's thermospheric temperature is found to be controlled by variations in the magnetospheric plasma environment. No correlation is found to exist with respect to geospatial parameters (i.e., latitude or longitude) and anti-correlation is found with solar parameters implying that Titan's nightside is hotter than its dayside. Furthermore, Titan's thermosphere is found to respond to plasma forcings on timescales less than one Titan day.

To investigate the composition of Titan's ionosphere we present a 1D photochemical model of Titan's dayside ionosphere constrained by Cassini measurements. We show that the production of the primary products of photoionization match the INMS data to within 20%. The major ions, CH+5,C2H+ 5 , and HCNH+, are discussed at length and an investigation of the processes controlling their modeled densities is presented. We then present the ion density profiles for the major hydrocarbons in the C3--C6 groups and the major nitrogen-containing ions up to the C4 group. We find that significant chemistry in the nitrogen containing hydrocarbons is missing from previous models and suggest pathways for the growth of these molecules. We also find that the chemistry of Titan's ionosphere is not necessarily dominated by proton exchange processes and that significant molecular growth should be expected through associative ion-molecule reactions.

The composition of the ions observed by the Ion Beam Spectrometer (CAPS-IBS) are analyzed with a specific emphasis on those larger than benzene (C 6H6). The CAPS-IBS mass spectra are found to have several peaks corresponding to ions having up to 14 carbon atoms with significant densities and masses up to 400 Da. Fits to the high mass ion spectra determine that each observed peak grouping must contain more than one ion of substantial density possibly indicating some degree of nitrogen incorporation. We compare the high mass ion spectra to various laboratory experiments which have produced large hydrocarbons or tholins through plasma processing of N2, CH4, and various simple hydrocarbons. We conclude from these comparisons that it is likely that Titan's ionospheric chemistry proceeds to higher mass through the reactions of C2 hydrocarbons and nitrogen containing hydrocarbons. Density profiles of the C8--C13 groups are presented from the CAPS-IBS data which show a region of initiation at altitudes above 1050 km and below 1200 km followed by a stagnation and drop-off at the lowest altitudes. We present modeled density profiles of the ions in the C6 and larger groups using an empirical model.