Laboratory experiments as support to the built up of Titan's theoretical models and interpretation of Cassini-Huygens data

Abstract

To interpret the concentrations of the products measured in Titan's atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be:

i) experimental simulations: in our laboratory (LISA), representative Titan's simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH₃, CH₂, CH fragments, issued from electron dissociation of N₂ and photo-dissociation of CH₄ respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given.

ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes' laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C₄H⁺ hydrocarbons kinetic rate constants and VUV cross section of HC₃N and HC₅N will be detailed.