| Popis: |
Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N 2 and CH4 ,a nd including many organics. This atmosphere also partly consists of hazes and aerosol particles which shroud the surface of this satellite, giving it a reddish appearance. The aerosols observed in Titan's atmosphere are thought to be synthesized at high altitudes ( 300 km) and fall to the surface. Varying with temperature profiles, condensation phenomena take place in the lower atmosphere, about 100 km below. These solid particles, often called 'tholins', have been currently investigated for many years by laboratory scientists and physics modellers. This paper assesses past research and results in different fields (elemental composition, optical constants, pyrolysis, particle size), highlighting interests and questions aroused by these studies. It also presents the latest results and advances, and concludes with existing problems and future pathways. Among the several approaches for the study of the chemical evolution of a planetary atmosphere, benchwork experimental simulation is a powerful tool. The study of the gaseous phase generated during simulation can provide information on the nature and relative abundances of the minor trace species which are expected, given the main composition of the atmosphere. The study of solid phase products can also yield insights on the chemical composition of the solid particles constituting atmospheric hazes. Such information can then be used as a precious guide for observational campaigns, and for theoretical modelings of the studied atmosphere. Moreover, such experimental aerosol analogues are useful for developing and calibrating instruments for in situ analysis of extraterrestrial atmospheres. Much has been documented about the gaseous phase products ever since experimental simulations were started. Some of these were specifically devoted to Titan, the largest satellite of Saturn (Chang et al., 1979; Raulin et al., 1982; Sagan et al., 1992; Cabane et al., 1995). It turns out that very little work has been published about the solid phase. Hence a need for systematic studies of these solid products synthetised during simulation experiments. This is particularly important in the case of Titan's simulations, due to a host of new observational data available about its atmosphere or which will be presently available in a near future, from ground or Earth-orbit observations, and, at the start of the new millennium, from the Cassini-Huygens mission. |
Full text from SpringerLink