Titan airglow spectra from the Cassini Ultraviolet Imaging Spectrograph: FUV disk analysis

We present a spectral analysis of the far ultraviolet (FUV: 1150-1900 angstrom) disk airglow observations of Titan's atmosphere by the Cassini Ultraviolet Imaging Spectrograph (UVIS). The FUV spectrum consists of emissions from the Lyman-Birge-Hopfield (LBH) band system of N-2 excited by photoelectrons (a (1)Pi(g) -> X (1)Sigma(+)(g)), N I multiplets from solar photodissociative excitation of N-2, resonantly scattered solar H Ly-alpha and sunlight reflected by N-2 in the mesosphere-stratosphere and modified by aerosols (e. g., tholins) and hydrocarbon absorption. Below 1450 angstrom, the strongest emissions arise from H Ly-a with an intensity of 208 Rayleighs (R), LBH bands with an intensity of 43 R, and the N I multiplets with a combined intensity of 16 R. Above 1450 angstrom, most of the UVIS signal is due to reflected sunlight. Mixing ratios of tholins, C2H2, C2H4 and C4H2 have been derived from the reflected sunlight using a Rayleigh scattering model. The derived mixing ratios are in good agreement with Voyager infrared observations and with FUV photochemical models, assuming solar energy deposition above 1450 angstrom occurs near 250 km (Wilson and Atreya, 2004). We also present the first geometric albedo measurement of Titan from 1500-1900 angstrom.