The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

• Published in: Icarus (New York, N.Y. 1962) Vol. 216; no. 2; pp. 507 - 534
• Main Authors: Koskinen, T.T., Yelle, R.V., Snowden, D.S., Lavvas, P., Sandel, B.R., Capalbo, F.J., Benilan, Y., West, R.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.12.2011
• Subjects: Atmospheres, Structure; Occultations; Titan; Occultations; Atmospheres, Structure; Titan
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2011.09.022

► We analyze stellar occultations by Titan observed by the Cassini/UVIS instrument. ► We derive density and temperature profiles for the upper atmosphere of Titan. ► We derive extinction profiles for Titan aerosols near their production region. ► We find evidence for the presence of low frequency gravity waves in the atmosphere. ► The results are compared with previous Voyager and Cassini measurements. Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH4, C2H2, C2H4, C4H2, HCN, HC3N and C6H6 between ∼400 and 1200km and extinction coefficients for aerosols between 400 and 900km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan.