Molecular nitrogen and methane density retrievals from Cassini UVIS dayglow observations of Titan’s upper atmosphere

Abstract

We retrieve number densities of molecular nitrogen (N2) and methane (CH4) from Titan’s upper atmosphere using the UV dayglow. We use Cassini Ultraviolet Imaging Spectrograph (UVIS) limb observations from 800 to 1300km of the N I 1493Å and N II 1085Å multiplets, both produced directly from photofragmentation of N2. UVIS N2 and CH4 densities are in agreement with measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) from the same flyby if INMS densities are scaled up by a factor of 3.0 as reported in previous studies. Analysis of three Cassini flybys of Titan shows that (1) the CH4 homopause on Titan is between 900 and 1100km, (2) upper atmospheric temperatures vary by less than 10K over 6h at the same geographic location and (3) from 1100 to 1700 local solar time temperatures also vary by less than 10K. The capability of retrieving the global-scale composition from these data complements existing techniques and significantly advances the study of upper atmospheric variability at Titan and for any other atmosphere with a detectable UV dayglow.