Proving to be both varied and fascinating, moons are far more common than planets in our Solar System. Our own Moon has had a profound influence on Earth, not only through tidal effects, but even on the behaviour of some marine animals. Many remarkable things have been discovered about the moons of the giant outer planets from Voyager, Galileo, Cassini, and other spacecraft. Scientists have glimpsed volcanic activity on Io, found oceans of water on Titan, and captured photos of icy geysers bursting from Enceladus. It looks likely that microbial life beyond the Earth may be discovered on a...