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As an integral part of the Earth-Moon system, the Moon has been a witness to more than 4.5 thousand million years of solar system history, and it is the only planetary body except Earth for which we have samples from known locations. The Moon is thought to be the product of an early planetary collision of a Mars-sized body with Earth that took place on the order of 4.4 – 4.5 thousand million years ago. The Moon’s simple composition and its restricted geological activity provide insight into elementary planetary processes.
Small, mid-sized and planet-sized moons in orbit around the four giant planets Jupiter, Saturn, Uranus and Neptune are abundant in the outer Solar System. These bodies are different from the terrestrial planets and are therefore grouped into a class of planetary objects termed icy satellites because their surfaces are dominated by the presence of water ice, indicated by H2O absorptions in the near infrared. Further, their average densities are much lower than those of the terrestrial planets, on the order of 2 gcm-3 or less.
Titan, Saturn’s largest moon, has been the subject of intensive research since the arrival of the Cassini-Huygens spacecraft at Saturn in 2004. Titan is especially interesting because it is the only satellite in the Solar System with a dense atmosphere. Despite its long distance to the Sun and, consequently the low temperature of the atmosphere and surface, Titan is a very interesting target for geological studies. It features the same geological processes found on airless moons. However, due to the presence of the atmosphere, additional processes play a role that we know from planets with an atmosphere, like Earth and Mars. For example, aeolian processes involve erosion, transport, and deposition of surface materials through the action of wind, while fluvial and lacustrine processes involve erosion, transport, and deposition by liquids present on the surface.
The strong gravitational attraction of Jupiter prevented the formation of planets in the region that is currently occupied by the main asteroid belt. But the drama of the arrested planetary development turns out to have fortunate consequences: The asteroids in the main belt preserve the original state of the young solar system.
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