Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission I. Hydrocarbons, nitriles and CO2 vertical mixing ratio profiles

Observations of the Composite InfraRed Spectrometer (CIRS) during the entire nominal Cassini mission (2004-2008) provide us with an accurate global view of composition and temperature in the middle atmosphere of Titan (between 100 and 500 km). We investigated limb spectra acquired at 0.5 cm(-1) resolution at nine different latitudes between 56 degrees S and 80 degrees N, with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations. From this limb data acquired between February 2005 and May 2008, we retrieved the vertical mixing ratio profiles of C2H2, C2H4, C2H6, C3H8, CH3C2H, C4H2, C6H6, HCN, HC3N and CO2. We present here for the first time, the latitudinal variations of the C2H6, C3H8, CO2, C2H4 and C6H6 vertical mixing ratios profiles. Some molecules, such as C2H6 or C3H8 present little variations above their condensation level. The other molecules (except CO2) show a significant enhancement of their mixing ratios poleward of 50 degrees N. C2H4 is the only molecule whose mixing ratio decreases with height at latitudes below 46 degrees N. Regions depleted in C2H2, HCN and C4H2 are observed around 400 km (0.01 mbar) and 55 degrees N. We also inferred a region enriched in CO2 located between 30 and 40 degrees N in the 2-0.7 mbar pressure range. At 80 degrees N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (similar to 0.07 mbar), which is in contradiction with Global Circulation Models that predict constant-with-height vertical profiles due to subsidence at the north pole. (C) 2009 Elsevier Inc. All rights reserved.