Titan's aerosols I. Laboratory investigations of shapes, size distributions, and aggregation of particles produced by UV photolysis of model Titan atmospheres

Titan's aerosols are believed to have significant effects on the physical and radiative properties of its atmosphere. To investigate the physical properties of model Titan aerosols, experiments in which acetylene, ethylene, and hydrogen cyanide were photolyzed separately and as a mixture by ultraviolet light have been performed. In general, the individual particles formed were spherical, apparently amorphous, and quite sticky. When 1 Torr of C 2H 2 (in 55 Torr N 2) was photolyzed, the average diameter of the individual particles was about 0.6 μm and most ( ≈ 4 5 ) of the particles were found in nonspherical near-linear aggregates. The mean diameter of the particles decreased to 0.4 μm for 0.1 Torr C 2H 2 and increased to 0.8 μm for 10 Torr C 2H 2. Aerosols formed from photolysis of C 2H 4 were physically similar to those formed from C 2H 2. Photolysis of HCN rapidly produced particles that apparently did not grow to sizes (>0.09 μm) large enough to be collected and imaged. The formation of particles from acetylene was observed within minutes in our experiments, but was slowed by about a factor of 4 when ethylene and hydrogen cyanide were added.