Our solar system contains several terrestrial-sized bodies with permanent atmospheres, such as Earth, Mars, Venus and Titan. Terrestrial-sized extrasolar planets are also increasingly discovered and investigated as possible venues for other life forms in the universe.

Aim of the ISSI International Team is to examine in comparative manner the unmagnetised terrestrial sized bodies in our solar system, Titan, Venus and Mars. This will give us a deeper understanding of the influence of external factors (e.g., distance from Sun, spin rate, obliquity, atmospheric composition and column density, gravity, space environment) on the properties of these atmospheres, including their long term evolution, and to draw possible conclusions for extrasolar terrestrial sized planets which to-date have received little attention.

Key science questions are:

1. (1)What are the characteristics and roles of waves and other smaller scale perturbations as momentum drivers in the upper atmospheres?
   

2. (2)What are the characteristics and origins of global diurnal, seasonal and other temporal variations and horizontal structures in the upper atmosphere?
   

3. (3)What are the chemical and dynamical processes shaping the ionospheres of terrestrial sized bodies?
   

4. (4)What lessons are learnt for extrasolar terrestrial sized planets?