Comparison of Saturn and Titan Upper Atmospheres in the Cassini Era: TGCM Simulations

Of the solar system bodies with permanent atmospheres, Saturn and its moon Titan warrant a particularly interesting comparison due to their identical distances from the Sun. In spite of this, their upper atmospheres differ considerably, with Saturn's main gases H, H_2 and He reaching exospheric temperatures of 400 K, while N_2 and CH_4 in Titan's atmosphere reach around 180 K. Differences in gravity, rotation rate and size create further important differences between Saturn and Titan which affect the horizontal distribution of temperatures and composition as well as global dynamics. Due to different neutral gases, Saturn's and Titan's ionospheres also differ considerably. Furthermore, Saturn has an intrinsic magnetic field and magnetosphere which considerably affects the upper atmosphere, particularly at high latitudes, whereas Titan probably has no intrinsic magnetic field, but is exposed to the magnetosphere of Saturn, leading to complex coupling processes. The forthcoming observations of the Cassini spacecraft provide an ideal opportunity for comparing Saturn's and Titan's upper atmospheres and improving our understanding of their differences. In preparation for these comprehensive observations we have over the past years developed general circulation models to simulate globally the dynamics, energetics and composition of Saturn's and Titan's thermospheres and ionospheres. These models solve fundamental physical gas equations under the influence of boundary conditions such as solar heating and thus form ideal "laboratories" not only for trying to reproduce observations, but more importantly for understanding the physical processes causing the observed and modeled behavior. In this talk we will give an overview of our modeling and an insight into the physics of Saturn's and Titan's upper atmospheres and will predict phenomena which may be of particular interest in the forthcoming Cassini observations.