M. Cabane et E. Chassefiere, LABORATORY SIMULATIONS OF TITANS ATMOSPHERE - ORGANIC GASES AND AEROSOLS, Planetary and space science, 43(1-2), 1995, pp. 47-65
Titan, the main satellite of Saturn, has been observed by remote sensi
ng for many years, both from interplanetary probes (Pioneer and Voyage
r's flybys) and from the Earth. Its N-2 atmosphere, containing a small
fraction of CH4 (approximate to 2%), with T approximate to 90 K and P
approximate to 1.5 bar at the ground level, is irradiated by solar UV
photons and deeply bombarded by energetic particles, i.e. Saturn magn
etospheric electrons and protons, interplanetary electrons and cosmic
rays. The resulting energy deposition, which takes place mainly below
1000 km, initiates chemical reactions which yield gaseous hydrocarbons
and nitriles and, through polymerisation processes, solid aerosol par
ticles which grow by coagulation and settle down to the ground. At the
present time, photochemical models strongly require the results of sp
ecific laboratory studies. Chemical rate constants are not well known
at low temperatures, charged-particle-induced reactions are difficult
to model and laboratory simulations of atmospheric processes are there
fore of great interest. Moreover, the synthesis of organic compounds w
hich have not been detected to date provides valuable information for
future observations. The origin and chemical composition of aerosols d
epend on the nature of chemical and energy sources. Their production f
rom gaseous species may be monitored in laboratory chambers and their
optical or microphysical properties compared to those deduced from the
observations of Titan's atmosphere. The development of simulation cha
mbers of Titan's extreme conditions is necessary for a better understa
nding of past and future observations. Space probes will sound Titan's
atmosphere by remote sensing and in situ analysis in the near future
(Cassini-Huygens mission). It appears necessary, as a preliminary step
, to test on-board experiments in such chambers, and as a final step,
when new space data have been acquired, to use them for more general s
cientific purposes.