Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2002-10, Vol.159 (2), p.505-517
• Main Authors: Lebonnois, Sébastien, Bakes, E.L.O., McKay, Christopher P.
• Format: Article
• Language: English
• Subjects: Earth Sciences; Planetology; Sciences of the Universe
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1006/icar.2002.6943

We investigate the chemical transition of simple molecules like C 2H 2 and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C 2H 2, HCN, HC 3N and other nitriles, and C 6H 6) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10 −14 g cm −2 s −1 and a C/N≃4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.