The importance of being ice

The inner planets and moons with which we are familiar (Earth, Mars, the Moon, etc.) consist of rock-forming minerals that are dominantly silicates. Silicates are composed of silicon and oxygen bonded together, usually in combination with aluminum, magnesium, iron, calcium, sodium, and other metals. On the Galilean satellites (Io excepted), the crust-forming rock is water ice, where hydrogen substitutes for silicon in its bonds with oxygen. Other ices are present in small quantities, such as methane, ammonia, solid nitrogen, carbon monoxide, carbon dioxide or sulfur dioxide, and other more complex molecules. Silicates and carbonaceous material are also present in large quantities but, except for dirtying the surface ices, they tend to reside deep in the interior, the less dense ices in the outer layers. These ices are volatile under average Earth conditions, but at surface temperatures of 100 K (–285 °F) or so, ice is much harder and more durable on Europa than the crunchable ice cubes in our drinks. Thus, the outer layers of these bodies behave much like the terrestrial planets, and are subject to many of the same processes, as well as some peculiar to the icy satellites. The dominance of ice as a crust-forming rock has several important consequences, however, for the geologic processes that occur within these planetary bodies.