Formation of Organic Aerosols in Titan's Upper Atmosphere
11 May 2007The most striking visual feature of Titan is the thick haze layer that envelopes the entire Saturnian moon and is thought to consist of complex organic molecules. Reported today in Science, new Cassini results point to a possible source for formation of the organic haze particles.
J.H. Waite, Jr. et al.  present observations made by three instruments on board Cassini during six Titan flybys, with closest approach altitudes between 950 and 1030 km above the moon's surface (see also Titan flyby pages on the right-hand navigation).
The Ion Neutral Mass Spectrometer (INMS) has confirmed the presence of benzene (C6H6) in the upper atmosphere.
The Ion Beam Spectrometer (IBS) and the Electron Spectrometer (ELS) are two sensors of the Cassini Plasma Spectrometer (CAPS) instrument. Data gathered by these two sensors reveal the presence of heavy positive ions and even more massive negative ions in Titan's upper atmosphere. The ions are identified with large hydrocarbon-nitriles.
Waite, Jr. et al. propose that these ions play an important role in the formation process of organic aerosols, by means of ion-neutral chemical processes.
A schematic of the processes in Titan's upper atmosphere is depicted on the right. The two main constituents of Titan's atmosphere - nitrogen and methane - under the influence of solar UV radiation and energetic particles are ionised and dissociated. Subsequent chemical reactions lead to the formation of heavy and large hydrocarbon-nitriles. When these organic molecules become big enough, aerosols are formed that contribute to the haze layers in the upper atmosphere.
J. H. Waite, Jr. et al., "The Process of Tholin Formation in Titan's Upper Atmosphere", Science, 11 May 2007, Vol. 316. no. 5826, pp. 870 - 875, DOI: 10.1126/science.1139727