Aging of Titan's Aerosols

Titan's haze is composed of aerosols containing long chain polymers of acetylene with some hydrogen cyanide. These polymers have alternating double/single and triple/single bonds, which can open spontaneously or under the action of UV radiation or particle impact. Once opened, they can induce the opening of a double or triple bond in an adjacent chain and link to it. This cross-linking and chain elongation hardens or “ages” the polymer particles, making them less sticky. As observed experimentally and calculated theoretically, newly formed polymer particles grow by collecting other polymer chains and by complete merging into symmetrical spheres. However, when aged, they merely adhere to each other and do not merge. Eventually, when hard enough, they do not even adhere to each other. In this paper we calculate the spontaneous aging process as applied to Titan's atmospheric conditions and find that the surface tension and viscosity of the aerosols below H∼570 km are one order of magnitude harder than when the aerosols formed. Furthermore, UV irradiation and particle impacts reduce both viscosity and surface tension by an additional factor of 10-100. Thus, the aerosol particles expected to be encountered by the descending Huygens probe will, most likely, be quite hard.