Prof. Patrick Irwin

Abstract:

Observations of Titan's far infra-red spectra by the Cassini orbiter's Composite InfraRed Spectrometer have been used to determine the latitude distribution of HCN at 1 mbar by fitting the HCN and CO rotational lines in the 18-60 cm(-1) (160-550 microm) spectral range. Results confirm the north polar HCN enrichment previously observed using mid-IR data and support the conclusion that Titan's nitrile species are significantly more enriched than hydrocarbons species with similar predicted photochemical lifetimes. This suggests Titan's photochemical cycle includes an additional sink for nitrogen bearing species. The abundance of CO was also determined, and had a mean value of 55 +/- 6 ppm at 20 mbar. However, it was not possible to reliably determine the CO latitude variation due to unconstrained temperatures in the north polar lower stratosphere.

Abstract:

In this paper we describe the first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which has been observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25 degrees S and 75 degrees N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.