Enceladus
Discovered by Fredrick William Herschel in 1789, tiny Enceladus is one of only four known bodies in the outer solar system with active volcanoes, the other three being the moons of Io, Triton and possibly Titan. In the case of Enceladus, these are cryovolcanoes: eruptions of water, nitrogen and ammonia, rather than molten rock.
Just 500 kilometres in diameter, Enceladus is far too small to generate its own heat. Instead, its geological activity is mostly likely due to its orbit around Saturn, with the gas giant constantly pulling and squeezing on the small satellite, warming the interior of the moon.
Inside this large body of warm water, Enceladus is conjectured to harbour life, while also being the wellspring for the ice crystal content of Saturn's E ring.
Mimas
Mimas was discovered by William Hershel the same year he first spotted Enceladus. The most remarkable feature of Mimas is its massive primary impact crater, which from certain angles causes the moon to look very much like the Death Star from Star Wars . The crater, named after its discoverer, is over 130 kilometres across: if a crater of equivalent scale was formed on Earth, it would be wider than Canada.
Mimas is also remarkable for the fact that it is the smallest object in the solar system to have formed into a sphere under the influence of its own self-gravitation.
Iapetus
Iapetus, the third largest moon of Saturn, is fascinating both tonally and morphologically: its stretched shape and equatorial ridge cause it to resemble nothing so much as a walnut, while the extreme difference in albedo between one hemisphere and the other bring to mind an Oreo cookie. It also has a significant crater, Turgis, that is nearly 40% of the moon's diameter, with a rim scarp nearly 15 kilometres high.
Titan
Saturn's largest moon is unique in several respects: it is the only known satellite in the solar system to have a dense atmosphere (one primarily composed of nitrogen), and the only known to have lakes on its surface (as it is far too cold to support liquid water, these bodies are liquid methane).
Titan was discovered in 1655 by the Dutch astronomer Christiaan Huygens. Today, the moon is considered by many scientists to be a possible habitat for non-terrestrial life. Its atmosphere is similar to that of the early Earth, and the potential of an "underground sea" of liquid water, along with tantalizing chemical imbalances in the atmosphere and crust, suggest that simple life may be present, operating in a methane, rather than water-based, environment.