Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission: I. Hydrocarbons, nitriles and CO 2 vertical mixing ratio profiles

Abstract

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°S and 80°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 C 2H 2, C 2H 4, C 2H 6, C 3H 8, CH 3C 2H, C 4H 2, C 6H 6, HCN, HC 3N and CO 2. We present here for the first time, the latitudinal variations of the C 2H 6, C 3H 8, CO 2, C 2H 4 and C 6H 6 vertical mixing ratios profiles. Some molecules, such as C 2H 6 or C 3H 8 present little variations above their condensation level. The other molecules (except CO 2) show a significant enhancement of their mixing ratios poleward of 50°N. C 2H 4 is the only molecule whose mixing ratio decreases with height at latitudes below 46°N. Regions depleted in C 2H 2, HCN and C 4H 2 are observed around 400 km (0.01 mbar) and 55°N. We also inferred a region enriched in CO 2 located between 30 and 40°N in the 2–0.7 mbar pressure range. At 80°N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (∼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.