HR: 16:00h
AN: P34A-03    [Abstracts]
TI: Cassini Ultraviolet Imaging Spectrograph Observations of Titan's Atmosphere
AU: Shemansky, D
EM: dons@hippolyta.usc.edu
AF: Dept. of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA United States
AU: Stewart, I
EM: ian.stewart@lasp.colorado.edu
AF: LASP, University of Colorado, Boulder, CO United States
AU: * Larsen, K W
EM: larsen@lasp.colorado.edu
AF: LASP, University of Colorado, Boulder, CO United States
AU: Esposito, L
EM: larry.esposito@lasp.colorado.edu
AF: LASP, University of Colorado, Boulder, CO United States
AB: During the third Cassini encounter with Titan (TB), on December 13, 2004, the Ultraviolet Imaging Spectrograph (UVIS) conducted a series of observations of Titan's atmospheric structure and composition. Included were two stellar occultations (Shaula and Spica) and a series of imaging scans of the day and night side of the moon. Analysis of the far ultraviolet (125 - 190 nm) occultation of Shaula (Lambda Sco) has provided details of the vertical distribution of hydrocarbons in the range 450-1600 km, compared to that observed by Voyager (900-1200 km). The occultation probed the night side of Titan in the southern hemisphere. Six hydrocarbon species were identified from their absorptions in the stellar spectrum, CH₄, C₂H₂, C₂H₄, C₂H₆, HCN, and C₄H₂. An iterative forward synthesis model of the Titan atmosphere was constructed including these absorbers, assuming a hemispherically symmetrical atmosphere, to deduce their distribution and an atmospheric temperature profile. We find a maximum temperature of 173 K at 450 km, a minimum of 114 K at 615 km and an isothermal region at 150 K above 1100 km. The UVIS derived temperature at the top of the atmosphere agrees within about 10% with the results of the in situ Ion Neutral Mass Spectrometer obtained during the second encounter with Titan (TA) on October 26, 2004. The transition from convective to diffusive separation occurs between 800 km and 1100 km. Additionally, we find a previously unobserved 80 km layer at 1350 km, showing abundances of CH₄ and C₂H₄ a factor of ~5 above the ambient underlying distribution.
DE: 0360 Transmission and scattering of radiation
DE: 5405 Atmospheres--composition and chemistry
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2005 Joint Assembly