Titan's stratospheric C 2N 2, C 3H 4, and C 4H 2 abundances from Cassini/CIRS far-infrared spectra

Abstract

Far-IR (25–50 μm, 200–400 cm −1) nadir and limb spectra measured during Cassini's four year prime mission by the Composite InfraRed Spectrometer (CIRS) instrument have been used to determine the abundances of cyanogen (C 2N 2), methylacetylene (C 3H 4), and diacetylene (C 4H 2) in Titan's stratosphere as a function of latitude. All three gases are enriched at northern latitudes, consistent with north polar subsidence. C 4H 2 abundances agree with those derived previously from mid-IR data, but C 3H 4 abundances are about 2 times lower, suggesting a vertical gradient or incorrect band intensities in the C 3H 4 spectroscopic data. For the first time C 2N 2 was detected at southern and equatorial latitudes with an average volume mixing ratio of 5.5 ± 1.4 × 10 − 11 derived from limb data ( > 3 - σ significance). This limb result is also corroborated by nadir data, which give a C 2N 2 volume mixing ratio of 6 ± 3 × 10 − 11 (2- σ significance) or alternatively a 3- σ upper limit of 17 × 10 − 11 . Comparing these figures with photochemical models suggests that galactic cosmic rays may be an important source of N 2 dissociation in Titan's stratosphere. Like other nitriles (HCN, HC 3N), C 2N 2 displays greater north polar relative enrichment than hydrocarbons with similar photochemical lifetimes, suggesting an additional loss mechanism for all three of Titan's main nitrile species. Previous studies have suggested that HCN requires an additional sink process such as incorporation into hazes. This study suggests that such a sink may also be required for Titan's other nitrile species.