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

Abstract

We present a spectral analysis of the far ultraviolet (FUV: 1150–1900 Å) 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 Π g → X 1 Σ g + ), N I multiplets from solar photodissociative excitation of N 2 , resonantly scattered solar H Ly‐ α and sunlight reflected by N 2 in the mesosphere‐stratosphere and modified by aerosols (e.g., tholins) and hydrocarbon absorption. Below 1450 Å, the strongest emissions arise from H Ly‐ α 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 Å, most of the UVIS signal is due to reflected sunlight. Mixing ratios of tholins, C 2 H 2 , C 2 H 4 and C 4 H 2 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 Å occurs near 250 km (Wilson and Atreya, 2004). We also present the first geometric albedo measurement of Titan from 1500–1900 Å.