Molecular nitrogen and methane density retrievals from Cassini UVIS dayglow observations of Titan\u27s upper atmosphere

We retrieve number densities of molecular nitrogen (N-2) and methane (CH4) from Titan\u27s upper atmosphere using the UV dayglow. We use Cassini Ultraviolet Imaging Spectrograph (UVIS) limb observations from 800 to 1300 km of the N I 1493 A and N II 1085 angstrom multiplets, both produced directly from photo-fragmentation of N-2. UVIS N-2 and CH4 densities are in agreement with measurements from Cassini\u27s 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 1100 km, (2) upper atmospheric temperatures vary by less than 10 K over 6 h at the same geographic location and (3) from 1100 to 1700 local solar time temperatures also vary by less than 10 K. 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. Published by Elsevier Inc