Enceladus

Enceladus is the second-innermost of Saturn's large moons, after Mimas. Mimas and Enceladus were both discovered by William Herschel in 1789. Enceladus orbits Saturn at a mean distance of 148,000 miles (238,000 km), with an orbital period of 1.37 days.

Enceladus is quite similar to Mimas in size at 306 mi (494 km) in diameter, but has a smoother, brighter surface which reflects almost 99 percent of the sunlight that strikes it. Enceladus displays at least five different types of terrain. Parts of Enceladus show no craters larger than 35 km in diameter. Other areas show regions with no craters at all, indicating major resurfacing events in the geologically-recent past. There are fissures, plains, corrugated terrain, and other crustal deformations.

All of this indicates that the interior of Enceladus may be liquid today, even though it should have frozen solid aeons ago. Enceladus participates in an orbital resonance with its neighboring satellite, Dione, and may be heated by a tidal mechanism similar to Jupiter's moon Io.

The Cassini spacecraft observed active "ice volcanism" on Enceladus: warm fractures where evaporating ice escapes, and forms a huge cloud of water vapor over the south pole. This makes Enceladus (like Jupiter's moon Europa, and the Earth) one of the few places in the solar system where liquid water is known to exist.