Latitudinal variations of HCN, HC3N, and C2N2 in Titan’s stratosphere derived from Cassini CIRS data

Midand far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan’s atmosphere. HCN, HC3N, C2H2, and temperature were derived from 2.5 cm −1 spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60◦ N. Three 0.5 cm−1 spectral resolution farIR observations were used to retrieve C2N2 and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5–5 mbar for temperature and 0.1–10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, HC3N, and C2N2 show a marked increase in abundance towards the north, whereas C2H2 remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90◦–20◦ S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 × 10−7 at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 × 10−7 at 60◦ N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 × 10−4 m s−1. HC3N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 × 10−8 at 60◦ N at the 0.1-mbar level. The difference in gradient for the HCN and HC3N latitude variations can be explained by HC3N’s much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of C2N2, giving a value of around 9× 10−10 at 50◦ N at the 3-mbar level. © 2005 Elsevier Inc. All rights reserved.