Titan's Stratospheric Chemistry from Cassini-Huygens Infrared Measurements
Abstract
Titan's atmosphere exhibits a complex interplay between photochemistry, aerosol microphysics, radiative forcing and dynamics, with marked seasonal variations. Methane photolysis in the upper atmosphere produces radical species which, along with the nitrogen atoms produced by N2 dissociation, react to form a suite of hydrocarbons and nitriles. A thick organic haze is also generated through mechanisms that are still largely unknown. These constituents are then transported by the general circulation and by eddy mixing. The Cassini-Huygens mission is harvesting valuable data on the chemistry at work in Titan's atmosphere. The instruments aboard Huygens characterized in situ the vertical profile of aerosols from 140 km down to the surface (with some information on their chemical composition) and measured the abundances of some important gases. The Cassini Infrared Spectrometer (CIRS) is currently mapping the abundances of various hydrocarbons, nitriles and oxygen compounds in the lower stratosphere. In addition, dedicated limb-observing sequences allow us to retrieve gas vertical profiles in the stratosphere and lower mesosphere at specific locations. The Visible and In- frared Mapping Spectrometer (VIMS) is capable of probing down to Titan's surface and allows an investigation of the methane cycle and formation of clouds in the lower atmosphere. Information on the vertical and horizontal distribution of haze opacity is also available from CIRS and VIMS spectra. The analysis of these data in terms of abundances relies on radiative transfer models that in turn require precise spectroscopic information on the relevant absorbers. Important information is still missing in this regard and calls for laboratory measurements and spectroscopic analyses. The horizontal and vertical distributions of aerosol particles and gases can be used as a probe of the general circulation and eddy mixing in the atmosphere. This however requires a precise knowledge of the chemical schemes involving the species and of the formation and growth of aerosol particles. But many questions regarding Titan's chemistry remain unanswered despite the development of more and more sophisticated photochemical (-dynamical) models. Key photolysis and reaction rates are poorly known even as concerns the most abundant species. Low-temperature laboratory measurements are needed not only for the rates themselves but also for the branching ratios. Finally, the chemical schemes that lead from the gas phase to the formation of the organic haze are quite uncertain. Study of kinetics and products among 1 possible PAH and nitrile polymerization (as well as copolymerisation) pathways are crucially needed. 2
- Publication:
-
European Planetary Science Congress 2006
- Pub Date:
- 2006
- Bibcode:
- 2006epsc.conf..274B