Titan's stratospheric composition driven by condensation and dynamics

Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation (the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.