GROWTH OF AEROSOLS IN TITANS ATMOSPHERE AND RELATED TIME SCALES - A STOCHASTIC APPROACH

The evolution of Titan's aerosols is studied from their production alt itude down to the ground using a stochastic approach. A background aer osol distribution is assumed, obtained from previous Eulerian modellin g, and the evolution of a ''tagged'' particle, released near the forma tion altitude, is followed by simulating in a random way its growth th rough coagulation with particles of the background distribution. The t wo distinct growth stages, proposed by Cabane et al. (1992) to explain the formation of monomers and subsequent aggregates, are confirmed. T he first stage may be divided into two parts. Firstly, within almost-e qual-to 1 terrestrial day, particles grow mainly through collisions wi th larger particles. They reach their size of monomer (r(m) almost-equ al-to 0.09 mum) through typically one to five such collisions. Secondl y, within a few terrestrial days to almost-equal-to 1 terrestrial mont h, particles evolve mainly by collisions with continuously created sma ll particles and acquire their compact spherical structure. In the sec ond stage, whose duration is almost-equal-to 30 terrestrial years, or one Titan's seasonal cycle, particles grow by cluster-cluster aggregat ion during their fall through the atmosphere and reach, at low stratos pheric levels, a typical radius of 0.4 - 0.5 mum.