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

Abstract We retrieve number densities of molecular nitrogen (N 2 ) and methane (CH 4 ) from Titan’s 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 A multiplets, both produced directly from photofragmentation of N 2 . UVIS N 2 and CH 4 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 CH 4 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.